Grayscale image

From Rosetta Code
Task
Grayscale image
You are encouraged to solve this task according to the task description, using any language you may know.

Many image processing algorithms are defined for grayscale (or else monochromatic) images.


Task

Extend the data storage type defined on this page to support grayscale images.

Define two operations, one to convert a color image to a grayscale image and one for the backward conversion.

To get luminance of a color use the formula recommended by CIE:

 L  =  0.2126 × R   +   0.7152 × G   +   0.0722 × B 

When using floating-point arithmetic make sure that rounding errors would not cause run-time problems or else distorted results when calculated luminance is stored as an unsigned integer.

Ada[edit]

type Grayscale_Image is array (Positive range <>, Positive range <>) of Luminance;

Conversion to a grayscale image:

function Grayscale (Picture : Image) return Grayscale_Image is
type Extended_Luminance is range 0..10_000_000;
Result : Grayscale_Image (Picture'Range (1), Picture'Range (2));
Color  : Pixel;
begin
for I in Picture'Range (1) loop
for J in Picture'Range (2) loop
Color := Picture (I, J);
Result (I, J) :=
Luminance
( ( 2_126 * Extended_Luminance (Color.R)
+ 7_152 * Extended_Luminance (Color.G)
+ 722 * Extended_Luminance (Color.B)
)
/ 10_000
);
end loop;
end loop;
return Result;
end Grayscale;

Conversion to a color image:

function Color (Picture : Grayscale_Image) return Image is
Result : Image (Picture'Range (1), Picture'Range (2));
begin
for I in Picture'Range (1) loop
for J in Picture'Range (2) loop
Result (I, J) := (others => Picture (I, J));
end loop;
end loop;
return Result;
end Color;

BASIC256[edit]

BASIC256 greyscale Mona Lisa.jpg
BASIC256 greysacle Grey Mona lisa.jpg
w = 143
h = 188
name$ = "Mona_Lisa.jpg"
graphsize w,h
imgload w/2, h/2, name$
fastgraphics
 
for x = 0 to w-1
for y = 0 to h-1
p = pixel(x,y)
b = p % 256
p = p \256
g = p % 256
p = p \ 256
r = p % 256
l = 0.2126*r + 0.7152*g + 0.0722*b
color rgb(l,l,l)
plot x,y
next y
refresh
next x
 
imgsave "Grey_"+name$,"jpg"

BBC BASIC[edit]

This uses the formula for gamma-corrected images, which is more appropriate to this task (see discussion page).

Original bbc.jpg
Greyscale bbc.jpg
      Width% = 200
Height% = 200
 
VDU 23,22,Width%;Height%;8,16,16,128
*display c:\lena
 
FOR y% = 0 TO Height%-1
FOR x% = 0 TO Width%-1
rgb% = FNgetpixel(x%,y%)
r% = rgb% >> 16
g% = (rgb% >> 8) AND &FF
b% = rgb% AND &FF
l% = INT(0.3*r% + 0.59*g% + 0.11*b% + 0.5)
PROCsetpixel(x%,y%,l%,l%,l%)
NEXT
NEXT y%
END
 
DEF PROCsetpixel(x%,y%,r%,g%,b%)
COLOUR 1,r%,g%,b%
GCOL 1
LINE x%*2,y%*2,x%*2,y%*2
ENDPROC
 
DEF FNgetpixel(x%,y%)
LOCAL col%
col% = TINT(x%*2,y%*2)
SWAP ?^col%,?(^col%+2)
= col%

C[edit]

Definition/interface for a grayscale image.

typedef unsigned char luminance;
typedef luminance pixel1[1];
typedef struct {
unsigned int width;
unsigned int height;
luminance *buf;
} grayimage_t;
typedef grayimage_t *grayimage;
 
grayimage alloc_grayimg(unsigned int, unsigned int);
grayimage tograyscale(image);
image tocolor(grayimage);

The same as alloc_img, but for grayscale images.

grayimage alloc_grayimg(unsigned int width, unsigned int height)
{
grayimage img;
img = malloc(sizeof(grayimage_t));
img->buf = malloc(width*height*sizeof(pixel1));
img->width = width;
img->height = height;
return img;
}

Convert from color image to grayscale image.

grayimage tograyscale(image img)
{
unsigned int x, y;
grayimage timg;
double rc, gc, bc, l;
unsigned int ofs;
 
timg = alloc_grayimg(img->width, img->height);
 
for(x=0; x < img->width; x++)
{
for(y=0; y < img->height; y++)
{
ofs = (y * img->width) + x;
rc = (double) img->buf[ofs][0];
gc = (double) img->buf[ofs][1];
bc = (double) img->buf[ofs][2];
l = 0.2126*rc + 0.7152*gc + 0.0722*bc;
timg->buf[ofs][0] = (luminance) (l+0.5);
}
}
return timg;
}

And back from a grayscale image to a color image.

image tocolor(grayimage img)
{
unsigned int x, y;
image timg;
luminance l;
unsigned int ofs;
 
timg = alloc_img(img->width, img->height);
 
for(x=0; x < img->width; x++)
{
for(y=0; y < img->height; y++)
{
ofs = (y * img->width) + x;
l = img->buf[ofs][0];
timg->buf[ofs][0] = l;
timg->buf[ofs][1] = l;
timg->buf[ofs][2] = l;
}
}
return timg;
}

Notes

  • tocolor and tograyscale do not free the previous image, so it must be freed normally calling free_img. With a cast we can use the same function also for grayscale images, or we can define something like
#define free_grayimg(IMG) free_img((image)(IMG))
  • Luminance is rounded. Since the C implementation is based on unsigned char (256 possible values per components), L can be at most 255.0 and rounding gives 255, as we expect. Changing the color_component type would only change 256, 255.0 and 255 values here written in something else, the code would work the same.

C#[edit]

To convert TO grayscale:

 
Bitmap tImage = new Bitmap("spectrum.bmp");
 
for (int x = 0; x < tImage.Width; x++)
{
for (int y = 0; y < tImage.Height; y++)
{
Color tCol = tImage.GetPixel(x, y);
 
// L = 0.2126·R + 0.7152·G + 0.0722·B
double L = 0.2126 * tCol.R + 0.7152 * tCol.G + 0.0722 * tCol.B;
tImage.SetPixel(x, y, Color.FromArgb(Convert.ToInt32(L), Convert.ToInt32(L), Convert.ToInt32(L)));
}
}
 
// Save
tImage.Save("spectrum2.bmp");
 

Clojure[edit]

 
(import '[java.io File]
'[javax.imageio ImageIO]
'[java.awt Color]
'[java.awt.image BufferedImage]))
 
(defn rgb-to-gray [color-image]
(let [width (.getWidth color-image)]
(partition width
(for [x (range width)
y (range (.getHeight color-image))]
(let [rgb (.getRGB color-image x y)
rgb-object (new Color rgb)
r (.getRed rgb-object)
g (.getGreen rgb-object)
b (.getBlue rgb-object)
a (.getAlpha rgb-object)]
;Compute the grayscale value an return it: L = 0.2126·R + 0.7152·G + 0.0722·B
(+ (* r 0.2126) (* g 0.7152) (* b 0.0722)))))))
 
 
(defn write-matrix-to-image [matrix filename]
(ImageIO/write
(let [height (count matrix)
width (count (first matrix))
output-image (new BufferedImage width height BufferedImage/TYPE_BYTE_GRAY)]
(doseq [row-index (range height)
column-index (range width)]
(.setRGB output-image column-index row-index (.intValue (nth (nth matrix row-index) column-index))))
output-image)
"png"
(new File filename)))
 
(println
(write-matrix-to-image
(rgb-to-gray
(ImageIO/read (new File "test.jpg")))
"test-gray-cloj.png"))
 
 

Common Lisp[edit]

Use the function rgb-to-gray-image to convert a rgb-image as loaded by the function defined Bitmap/Read a PPM file#Common Lisp. The package identifier assumes that you have the package as defined in Basic bitmap storage#Common Lisp. With the function grayscale-image-to-pgm-file it is possible to write out the gray image as pgm file which can then be further processed.

 
(in-package #:rgb-pixel-buffer)
 
(defun rgb-to-gray-image (rgb-image)
(flet ((rgb-to-gray (rgb-value)
(round (+ (* 0.2126 (rgb-pixel-red rgb-value))
(* 0.7152 (rgb-pixel-green rgb-value))
(* 0.0722 (rgb-pixel-blue rgb-value))))))
(let ((gray-image (make-array (array-dimensions rgb-image) :element-type '(unsigned-byte 8))))
(dotimes (i (array-total-size rgb-image))
(setf (row-major-aref gray-image i) (rgb-to-gray (row-major-aref rgb-image i))))
gray-image)))
 
(export 'rgb-to-gray-image)
 
 
(defun grayscale-image-to-pgm-file (image file-name &optional (max-value 255))
(with-open-file (p file-name :direction :output
:if-exists :supersede)
(format p "P2 ~&~A ~A ~&~A" (array-dimension image 1) (array-dimension image 0) max-value)
(dotimes (i (array-total-size image))
(print (row-major-aref image i) p))))
 
(export 'grayscale-image-to-pgm-file)
 
 

D[edit]

This example uses the bitmap module defined in the Bitmap Task page.

module grayscale_image;
 
import core.stdc.stdio, std.array, std.algorithm, std.string, std.ascii;
public import bitmap;
 
struct Gray {
ubyte c;
enum black = typeof(this)(0);
enum white = typeof(this)(255);
alias c this;
}
 
 
Image!Color loadPGM(Color)(Image!Color img, in string fileName) {
static int readNum(FILE* f) nothrow @nogc {
int n;
while (!fscanf(f, "%d ", &n)) {
if ((n = fgetc(f)) == '#') {
while ((n = fgetc(f)) != '\n')
if (n == EOF)
return 0;
} else
return 0;
}
return n;
}
 
if (img is null)
img = new Image!Color();
 
auto fin = fopen(fileName.toStringz(), "rb");
scope(exit) if (fin) fclose(fin);
if (!fin)
throw new Exception("Can't open input file.");
 
if (fgetc(fin) != 'P' ||
fgetc(fin) != '5' ||
!isWhite(fgetc(fin)))
throw new Exception("Not a PGM (PPM P5) image.");
 
immutable int nc = readNum(fin);
immutable int nr = readNum(fin);
immutable int maxVal = readNum(fin);
if (nc <= 0 || nr <= 0 || maxVal <= 0)
throw new Exception("Wrong input image sizes.");
img.allocate(nc, nr);
auto pix = new ubyte[img.image.length];
 
immutable count = fread(pix.ptr, 1, nc * nr, fin);
if (count != nc * nr)
throw new Exception("Wrong number of items read.");
 
pix.copy(img.image);
return img;
}
 
 
void savePGM(Color)(in Image!Color img, in string fileName)
in {
assert(img !is null);
assert(!fileName.empty);
assert(img.nx > 0 && img.ny > 0 &&
img.image.length == img.nx * img.ny,
"Wrong image.");
} body {
auto fout = fopen(fileName.toStringz(), "wb");
if (fout == null)
throw new Exception("File can't be opened.");
fprintf(fout, "P5\n%d %d\n255\n", img.nx, img.ny);
auto pix = new ubyte[img.image.length];
foreach (i, ref p; pix)
p = cast(typeof(pix[0]))img.image[i];
immutable count = fwrite(pix.ptr, ubyte.sizeof,
img.nx * img.ny, fout);
if (count != img.nx * img.ny)
new Exception("Wrong number of items written.");
fclose(fout);
}
 
 
Gray lumCIE(in RGB c) pure nothrow @nogc {
return Gray(cast(ubyte)(0.2126 * c.r +
0.7152 * c.g +
0.0722 * c.b + 0.5));
}
 
Gray lumAVG(in RGB c) pure nothrow @nogc {
return Gray(cast(ubyte)(0.3333 * c.r +
0.3333 * c.g +
0.3333 * c.b + 0.5));
}
 
Image!Gray rgb2grayImage(alias Conv=lumCIE)(in Image!RGB im) nothrow {
auto result = new typeof(return)(im.nx, im.ny);
foreach (immutable i, immutable rgb; im.image)
result.image[i] = Conv(rgb);
return result;
}
 
Image!RGB gray2rgbImage(in Image!Gray im) nothrow {
auto result = new typeof(return)(im.nx, im.ny);
foreach (immutable i, immutable gr; im.image)
result.image[i] = RGB(gr, gr, gr);
return result;
}
 
version (grayscale_image_main) {
void main() {
auto im1 = new Image!Gray;
im1.loadPGM("lena.pgm");
gray2rgbImage(im1).savePPM6("lena_rgb.ppm");
 
auto img2 = new Image!RGB;
img2.loadPPM6("quantum_frog.ppm");
img2.rgb2grayImage.savePGM("quantum_frog_grey.pgm");
}
}

Euler Math Toolbox[edit]

 
>A=loadrgb("mona.jpg");
>insrgb(A);
>function grayscale (A) ...
${r,g,b}=getrgb(A);
$c=0.2126*r+0.7152*g+0.0722*b;
$return rgb(c,c,c);
$endfunction
>insrgb(grayscale(A));
>insrgb(A|grayscale(A));
 

Erlang[edit]

The code below extends the erlang module on Bitmap task. This module supports RGB and grayscale modes. RGB colors are specified as {rgb, R, G, B} and saved as bytes into an array. Grayscale colors are likewise specified as {gray, L} where L is luminance.

-module(ros_bitmap).
 
-export([new/2, fill/2, set_pixel/3, get_pixel/2, convert/2]).
 
-record(bitmap, {
mode = rgb,
pixels = nil,
shape = {0, 0}
}).
 
tuple_to_bytes({rgb, R, G, B}) ->
<<R:8, G:8, B:8>>;
tuple_to_bytes({gray, L}) ->
<<L:8>>.
 
bytes_to_tuple(rgb, Bytes) ->
<<R:8, G:8, B:8>> = Bytes,
{rgb, R, G, B};
bytes_to_tuple(gray, Bytes) ->
<<L:8>> = Bytes,
{gray, L}.
 
new(Width, Height) ->
new(Width, Height, {rgb, 0, 0, 0}).
 
new(Width, Height, rgb) ->
new(Width, Height, {rgb, 0, 0, 0});
 
new(Width, Height, gray) ->
new(Width, Height, {gray, 0, 0, 0});
 
new(Width, Height, ColorTuple) when is_tuple(ColorTuple) ->
[Mode|Components] = tuple_to_list(ColorTuple),
Bytes = list_to_binary(Components),
#bitmap{
pixels=array:new(Width * Height, {default, Bytes}),
shape={Width, Height},
mode=Mode}.
 
fill(#bitmap{shape={Width, Height}, mode=Mode}, ColorTuple)
when element(1, ColorTuple) =:= Mode ->
new(Width, Height, ColorTuple).
 
set_pixel(#bitmap{pixels=Pixels, shape={Width, _Height}, mode=Mode}=Bitmap,
{at, X, Y}, ColorTuple) when element(1, ColorTuple) =:= Mode ->
Index = X + Y * Width,
Bitmap#bitmap{pixels=array:set(Index, tuple_to_bytes(ColorTuple), Pixels)}.
 
get_pixel(#bitmap{pixels=Pixels, shape={Width, _Height}, mode=Mode},
{at, X, Y}) ->
Index = X + Y * Width,
Bytes = array:get(Index, Pixels),
bytes_to_tuple(Mode, Bytes).
 
luminance(<<R:8, G:8, B:8>>) ->
<<(trunc(R * 0.2126 + G * 0.7152 + B * 0.0722))>>.
 
%% convert from rgb to grayscale
convert(#bitmap{pixels=Pixels, mode=rgb}=Bitmap, gray) ->
Bitmap#bitmap{
pixels=array:map(fun(_I, Pixel) ->
luminance(Pixel) end, Pixels),
mode=gray};
 
%% convert from grayscale to rgb
convert(#bitmap{pixels=Pixels, mode=gray}=Bitmap, rgb)->
Bitmap#bitmap{
pixels=array:map(fun(_I, <<L:8>>) -> <<L:8, L:8, L:8>> end, Pixels),
mode=rgb};
 
%% no conversion if the mode is the same with the bitmap.
convert(#bitmap{mode=Mode}=Bitmap, Mode) ->
Bitmap.
 

Euphoria[edit]

function to_gray(sequence image)
sequence color
for i = 1 to length(image) do
for j = 1 to length(image[i]) do
color = and_bits(image[i][j], {#FF0000,#FF00,#FF}) /
{#010000,#0100,#01} -- unpack color triple
image[i][j] = floor(0.2126*color[1] + 0.7152*color[2] + 0.0722*color[3])
end for
end for
return image
end function
 
function to_color(sequence image)
for i = 1 to length(image) do
for j = 1 to length(image[i]) do
image[i][j] = image[i][j]*#010101
end for
end for
return image
end function

FBSL[edit]

24-bpp BMP-format P.O.T.-size image solution:

FBSLLena.png
DIM colored = ".\LenaClr.bmp", grayscale = ".\LenaGry.bmp"
DIM head, tail, r, g, b, l, ptr, blobsize = 54 ' sizeof BMP file headers
 
FILEGET(FILEOPEN(colored, BINARY), FILELEN(colored)): FILECLOSE(FILEOPEN) ' load buffer
head = @FILEGET + blobsize: tail = @FILEGET + FILELEN ' set loop bounds
 
FOR ptr = head TO tail STEP 3 ' transform color triplets
b = PEEK(ptr + 0, 1) ' read Windows colors stored in BGR order
g = PEEK(ptr + 1, 1)
r = PEEK(ptr + 2, 1)
l = 0.2126 * r + 0.7152 * g + 0.0722 * b ' derive luminance
SETMEM(FILEGET, RGB(l, l, l), ptr - head + blobsize, 3) ' write grayscale
NEXT
 
FILEPUT(FILEOPEN(grayscale, BINARY_NEW), FILEGET): FILECLOSE(FILEOPEN) ' save buffer

Forth[edit]

\ grayscale bitmap (without word-alignment for scan lines)
 
\ bdim, bwidth, bdata all work with graymaps
 
: graymap ( w h -- gmp )
2dup * bdata allocate throw
dup >r 2! r> ;
 
: gxy ( x y gmp -- addr )
dup bwidth rot * rot + swap bdata + ;
 
: [email protected] ( x y gmp -- c ) gxy [email protected] ;
: g! ( c x y bmp -- ) gxy c! ;
 
: gfill ( c gmp -- )
dup bdata swap bdim * rot fill ;
 
: gshow ( gmp -- )
dup bdim
0 do cr
dup 0 do
over i j rot [email protected] if [char] * emit else space then
loop
loop
2drop ;
 
\ RGB <-> Grayscale
: lum>rgb ( 0..255 -- pixel )
dup 8 lshift or
dup 8 lshift or ;
 
: pixel>rgb ( pixel -- r g b )
256 /mod 256 /mod ;
: rgb>lum ( pixel -- 0..255 )
pixel>rgb
722 * swap
7152 * + swap
2126 * + 10000 / ;
 
: bitmap>graymap ( bmp -- gmp )
dup bdim graymap
dup bdim nip 0 do
dup bwidth 0 do
over i j rot [email protected] rgb>lum
over i j rot g!
loop
loop nip ;
 
: graymap>bitmap ( gmp -- bmp )
dup bdim bitmap
dup bdim nip 0 do
dup bwidth 0 do
over i j rot [email protected] lum>rgb
over i j rot b!
loop
loop nip ;

Fortran[edit]

(These fragments should be added to RCImageBasic module, see Basic bitmap storage)

First let's define a new type; the sc stands for Single Channel, which can be luminance (as it is here).

type scimage
integer, dimension(:,:), pointer :: channel
integer :: width, height
end type scimage

In order to allow proper overloading, the following subroutines of the storage should be renamed appending the _rgb suffix: valid_image, inside_image, alloc_img, free_img, fill_img, get_pixel, put_pixel, init_img. The single channel version would be named with the _sc suffix, then we should define the proper interfaces to use the already written code as before. Here there are only the interfaces and subroutines needed for the task.

interface alloc_img
module procedure alloc_img_rgb, alloc_img_sc
end interface
 
interface free_img
module procedure free_img_rgb, free_img_sc
end interface

Now we can define useful interfaces and subroutines more task-related:

interface assignment(=)
module procedure rgbtosc, sctorgb
end interface
subroutine alloc_img_sc(img, w, h)
type(scimage) :: img
integer, intent(in) :: w, h
 
allocate(img%channel(w, h))
img%width = w
img%height = h
end subroutine alloc_img_sc
 
subroutine free_img_sc(img)
type(scimage) :: img
 
if ( associated(img%channel) ) deallocate(img%channel)
end subroutine free_img_sc
 
subroutine rgbtosc(sc, colored)
type(rgbimage), intent(in) :: colored
type(scimage), intent(inout) :: sc
 
if ( ( .not. valid_image(sc) ) .and. valid_image(colored) ) then
call alloc_img(sc, colored%width, colored%height)
end if
 
if ( valid_image(sc) .and. valid_image(colored) ) then
sc%channel = floor(0.2126*colored%red + 0.7152*colored%green + &
0.0722*colored%blue)
end if
 
end subroutine rgbtosc
 
subroutine sctorgb(colored, sc)
type(scimage), intent(in) :: sc
type(rgbimage), intent(inout) :: colored
 
if ( ( .not. valid_image(colored) ) .and. valid_image(sc) ) then
call alloc_img_rgb(colored, sc%width, sc%height)
end if
 
if ( valid_image(sc) .and. valid_image(colored) ) then
colored%red = sc%channel
colored%green = sc%channel
colored%blue = sc%channel
end if
 
end subroutine sctorgb

Usage example (fragment) which can be used to convert from rgb image to grayscale image and back (since we only can output the rgb kind):

type(scimage) :: gray
type(rgbimage) :: animage
! ... here we "load" or create animage
! while gray must be created or initialized to null
! or errors can arise...
call init_img(gray)
gray = animage
animage = gray
call output_ppm(an_unit, animage)

Go[edit]

package raster
 
import (
"math"
"math/rand"
)
 
// Grmap parallels Bitmap, but with an element type of uint16
// in place of Pixel.
type Grmap struct {
Comments []string
rows, cols int
px []uint16
pxRow [][]uint16
}
 
// NewGrmap constructor.
func NewGrmap(x, y int) (b *Grmap) {
g := &Grmap{
Comments: []string{creator}, // creator a const in bitmap source file
rows: y,
cols: x,
px: make([]uint16, x*y),
pxRow: make([][]uint16, y),
}
x0, x1 := 0, x
for i := range g.pxRow {
g.pxRow[i] = g.px[x0:x1]
x0, x1 = x1, x1+x
}
return g
}
 
func (b *Grmap) Extent() (cols, rows int) {
return b.cols, b.rows
}
 
func (g *Grmap) Fill(c uint16) {
for i := range g.px {
g.px[i] = c
}
}
 
func (g *Grmap) SetPx(x, y int, c uint16) bool {
defer func() { recover() }()
g.pxRow[y][x] = c
return true
}
 
func (g *Grmap) GetPx(x, y int) (uint16, bool) {
defer func() { recover() }()
return g.pxRow[y][x], true
}
 
// Grmap method of Bitmap, converts (color) Bitmap to (grayscale) Grmap
func (b *Bitmap) Grmap() *Grmap {
g := NewGrmap(b.cols, b.rows)
g.Comments = append([]string{}, b.Comments...)
for i, p := range b.px {
g.px[i] = uint16((int64(p.R)*2126 + int64(p.G)*7152 + int64(p.B)*722) *
math.MaxUint16 / (math.MaxUint8 * 10000))
}
return g
}
 
// Bitmap method Grmap, converts Grmap to Bitmap. All pixels in the resulting
// color Bitmap will be (very nearly) shades of gray.
func (g *Grmap) Bitmap() *Bitmap {
b := NewBitmap(g.cols, g.rows)
b.Comments = append([]string{}, g.Comments...)
for i, p := range g.px {
roundedSum := int(p) * 3 * math.MaxUint8 / math.MaxUint16
rounded := uint8(roundedSum / 3)
remainder := roundedSum % 3
b.px[i].R = rounded
b.px[i].G = rounded
b.px[i].B = rounded
if remainder > 0 {
odd := rand.Intn(3)
switch odd + (remainder * 3) {
case 3:
b.px[i].R++
case 4:
b.px[i].G++
case 5:
b.px[i].B++
case 6:
b.px[i].G++
b.px[i].B++
case 7:
b.px[i].R++
b.px[i].B++
case 8:
b.px[i].R++
b.px[i].G++
}
}
}
return b
}

For demonstration program see task Bitmap/Read a PPM file.

Haskell[edit]

module Bitmap.Gray(module Bitmap.Gray) where
 
import Bitmap
import Control.Monad.ST
 
newtype Gray = Gray Int deriving (Eq, Ord)
 
instance Color Gray where
luminance (Gray x) = x
black = Gray 0
white = Gray 255
toNetpbm = map $ toEnum . luminance
fromNetpbm = map $ Gray . fromEnum
netpbmMagicNumber _ = "P5"
netpbmMaxval _ = "255"
 
toGrayImage :: Color c => Image s c -> ST s (Image s Gray)
toGrayImage = mapImage $ Gray . luminance

A Gray image can be converted to an RGB image with Bitmap.RGB.toRGBImage, defined here.

J[edit]

Color bitmap structure and basic functions for manipulations with it are described here.

Grayscale image is stored as two-dimensional array of luminance values. Allowed luminance scale is the same as for the color bitmap; the functions below are neutral to scale.

NB. converts the image to grayscale according to formula
NB. L = 0.2126*R + 0.7152*G + 0.0722*B
toGray=: [: <. +/ .*"1&0.2126 0.7152 0.0722
 
NB. converts grayscale image to the color image, with all channels equal
toColor=: 3 & $"0

Example:

viewRGB toColor toGray myimg

Java[edit]

void convertToGrayscale(final BufferedImage image){
for(int i=0; i<image.getWidth(); i++){
for(int j=0; j<image.getHeight(); j++){
int color = image.getRGB(i,j);
 
int alpha = (color >> 24) & 255;
int red = (color >> 16) & 255;
int green = (color >> 8) & 255;
int blue = (color) & 255;
 
final int lum = (int)(0.2126 * red + 0.7152 * green + 0.0722 * blue);
 
alpha = (alpha << 24);
red = (lum << 16);
green = (lum << 8);
blue = lum;
 
color = alpha + red + green + blue;
 
image.setRGB(i,j,color);
}
}
}
 

JavaScript[edit]

HTML 5 Demonstration: https://repl.it/repls/NiceFaroffRockrat

 
function toGray(img) {
let cnv = document.getElementById("canvas");
let ctx = cnv.getContext('2d');
let imgW = img.width;
let imgH = img.height;
cnv.width = imgW;
cnv.height = imgH;
 
ctx.drawImage(img, 0, 0);
let pixels = ctx.getImageData(0, 0, imgW, imgH);
for (let y = 0; y < pixels.height; y ++) {
for (let x = 0; x < pixels.width; x ++) {
let i = (y * 4) * pixels.width + x * 4;
let avg = (pixels.data[i] + pixels.data[i + 1] + pixels.data[i + 2]) / 3;
 
pixels.data[i] = avg;
pixels.data[i + 1] = avg;
pixels.data[i + 2] = avg;
}
}
ctx.putImageData(pixels, 0, 0, 0, 0, pixels.width, pixels.height);
return cnv.toDataURL();
}
 

Julia[edit]

Adhering to the Task Description

 
using Color, Images, FixedPointNumbers
 
const M_RGB_Y = reshape(Color.M_RGB_XYZ[2,:], 3)
 
function rgb2gray(img::Image)
g = red(img)*M_RGB_Y[1] + green(img)*M_RGB_Y[2] + blue(img)*M_RGB_Y[3]
g = clamp(g, 0.0, 1.0)
return grayim(g)
end
 
function gray2rgb(img::Image)
colorspace(img) == "Gray" || return img
g = map((x)->RGB{Ufixed8}(x, x, x), img.data)
return Image(g, spatialorder=spatialorder(img))
end
 
ima = imread("grayscale_image_color.png")
imb = rgb2gray(ima)
imc = gray2rgb(imb)
imwrite(imc, "grayscale_image_rc.png")
 

Rounding errors are unlikely to be an issue for rgb2gray. The calculation of g promotes it to the literal float type (typically Float64).

A More Idiomatic Approach

 
using Color, Images, FixedPointNumbers
 
ima = imread("grayscale_image_color.png")
imb = convert(Image{Gray{Ufixed8}}, ima)
imwrite(imb, "grayscale_image_julia.png")
 
Output:

I didn't find a colorful image that I was comfortable modifying and sharing, so I'm omitting the image files from my solution to this task. Try out these images for something to work with. Although these images are intended for image processing testing and development and are said to be available for unrestricted use, I could find no clear and definitive statement of their usage rights.

The results of the two approaches (according to task, rc, and idiomatic, julia) are indistinguishable except perhaps by close examination. The julia file is native grayscale, and the rc file is RGB that shows only grays.

The task description is silent on the issue of companded sRGB versus linear RGB. Most images are actually sRGB, and strictly speaking, the transformation to get Y from RGB is applicable to linear RGB. I imagine that, unlike the rc version, the julia version reverses compansion prior to applying the CIE transformation to extract luminance from RGB.

Lingo[edit]

on rgbToGrayscaleImageFast (img)
res = image(img.width, img.height, 8)
res.paletteRef = #grayScale
res.copyPixels(img, img.rect, img.rect)
return res
end
 
on rgbToGrayscaleImageCustom (img)
res = image(img.width, img.height, 8)
res.paletteRef = #grayScale
repeat with x = 0 to img.width-1
repeat with y = 0 to img.height-1
c = img.getPixel(x,y)
n = c.red*0.2126 + c.green*0.7152 + c.blue*0.0722
res.setPixel(x,y, color(256-n))
end repeat
end repeat
return res
end

Lua[edit]

function ConvertToGrayscaleImage( bitmap )
local size_x, size_y = #bitmap, #bitmap[1]
local gray_im = {}
 
for i = 1, size_x do
gray_im[i] = {}
for j = 1, size_y do
gray_im[i][j] = math.floor( 0.2126*bitmap[i][j][1] + 0.7152*bitmap[i][j][2] + 0.0722*bitmap[i][j][3] )
end
end
 
return gray_im
end
 
function ConvertToColorImage( gray_im )
local size_x, size_y = #gray_im, #gray_im[1]
local bitmap = Allocate_Bitmap( size_x, size_y ) -- this function is defined at http://rosettacode.org/wiki/Basic_bitmap_storage#Lua
 
for i = 1, size_x do
for j = 1, size_y do
bitmap[i][j] = { gray_im[i][j], gray_im[i][j], gray_im[i][j] }
end
end
 
return bitmap
end

Mathematica / Wolfram Language[edit]

Mathematica has a built-in grayscale conversion function called "ColorConvert". This example does not use it since it appears the luminance calculation is different from the CIE spec. Grayscale to RGB "conversion" just changes the single channel grayscale image to a triple channel image.

toGrayscale[rgb_Image] := ImageApply[#.{0.2126, 0.7152, 0.0722}&, rgb]
toFakeRGB[L_Image] := ImageApply[{#, #, #}&, L]

MATLAB[edit]

Built in colour to grayscale converter uses the following forumula: 0.2989*R + 0.5870*G + 0.1140*B

function [grayImage] = colortograyscale(inputImage)
grayImage = rgb2gray(inputImage);

OCaml[edit]

Conversion to a grayscale image:

let to_grayscale ~img:(_, r_channel, g_channel, b_channel) =
let width = Bigarray.Array2.dim1 r_channel
and height = Bigarray.Array2.dim2 r_channel in
 
let gray_channel =
let kind = Bigarray.int8_unsigned
and layout = Bigarray.c_layout
in
(Bigarray.Array2.create kind layout width height)
in
for y = 0 to pred height do
for x = 0 to pred width do
let r = r_channel.{x,y}
and g = g_channel.{x,y}
and b = b_channel.{x,y} in
let v = (2_126 * r + 7_152 * g + 722 * b) / 10_000 in
gray_channel.{x,y} <- v;
done;
done;
(gray_channel)

Conversion to a color image:

let to_color ~img:gray_channel =
let width = Bigarray.Array2.dim1 gray_channel
and height = Bigarray.Array2.dim2 gray_channel in
let all_channels =
let kind = Bigarray.int8_unsigned
and layout = Bigarray.c_layout
in
Bigarray.Array3.create kind layout 3 width height
in
let r_channel = Bigarray.Array3.slice_left_2 all_channels 0
and g_channel = Bigarray.Array3.slice_left_2 all_channels 1
and b_channel = Bigarray.Array3.slice_left_2 all_channels 2
in
Bigarray.Array2.blit gray_channel r_channel;
Bigarray.Array2.blit gray_channel g_channel;
Bigarray.Array2.blit gray_channel b_channel;
(all_channels,
r_channel,
g_channel,
b_channel)

and functions to get/set a pixel:

let gray_get_pixel_unsafe (gray_channel) =
(fun x y -> gray_channel.{x,y})
 
let gray_put_pixel_unsafe (gray_channel) v =
(fun x y -> gray_channel.{x,y} <- v)

Octave[edit]

Use package: image

function [grayImage] = colortograyscale(inputImage)
grayImage = rgb2gray(inputImage);

Differently from MATLAB, the grayscale is computed as mean of the three RGB values. Changing this non-optimal behaviour is a matter of fixing three lines in the rgb2gray.m file; since it's a GPL-ed code, here it is a semplified version (error checking, usage help, argument checking removed)

function gray = rgb2gray (rgb)
switch(class(rgb))
case "double"
gray = luminance(rgb);
case "uint8"
gray = function/uint8.html">uint8(luminance(rgb));
case "uint16"
gray = function/uint16.html">uint16(luminance(rgb));
endswitch
endfunction
 
function lum = luminance(rgb)
lum = 0.2126*rgb(:,:,1) + 0.7152*rgb(:,:,2) + 0.0722*rgb(:,:,3);
endfunction

Original code of the rgb2gray.m in the image package version 1.0.8 is by Kai Habel (under the GNU General Public License)

Oz[edit]

We define a "graymap" as a two-dimensional array of floats. In module "Grayscale.oz", we implement conversion functions from and to bitmaps:

functor
import
Array2D
export
ToGraymap
FromGraymap
define
fun {ToGraymap bitmap(Arr)}
graymap({Array2D.map Arr Luminance})
end
 
fun {Luminance Color}
F = {Record.map Color Int.toFloat}
in
0.2126*F.1 + 0.7152*F.2 + 0.0722*F.3
end
 
fun {FromGraymap graymap(Arr)}
bitmap({Array2D.map Arr ToColor})
end
 
fun {ToColor Lum}
L = {Float.toInt Lum}
in
color(L L L)
end
end

Perl[edit]

Library: Imlib2

Since we are using Imlib2, this one does not implement really a gray-scale (single channel) storage; it only converts an RGB image to an RGB image with the same three colour components for each pixel (which result in a gray-scale-like image)

#! /usr/bin/perl
 
use strict;
use Image::Imlib2;
 
sub tograyscale
{
my $img = shift;
my $gimg = Image::Imlib2->new($img->width, $img->height);
for ( my $x = 0; $x < $gimg->width; $x++ ) {
for ( my $y = 0; $y < $gimg->height; $y++ ) {
my ( $r, $g, $b, $a ) = $img->query_pixel($x, $y);
my $gray = int(0.2126 * $r + 0.7152 * $g + 0.0722 * $b);
# discard alpha info...
$gimg->set_color($gray, $gray, $gray, 255);
$gimg->draw_point($x, $y);
}
}
return $gimg;
}
 
my $animage = Image::Imlib2->load("Lenna100.jpg");
my $gscale = tograyscale($animage);
$gscale->set_quality(80);
$gscale->save("Lennagray.jpg");
 
exit 0;

Perl 6[edit]

This script expects to be fed a P6 .ppm file name at the command line. It will convert it to grey scale and save it as a binary portable grey map (P5 .pgm) file.

sub MAIN ($filename = 'default.ppm') {
 
my $in = open($filename, :r, :enc<iso-8859-1>);
 
my ($type, $dim, $depth) = $in.lines[^3];
 
my $outfile = $filename.subst('.ppm', '.pgm');
my $out = open($outfile, :w, :enc<iso-8859-1>);
 
$out.say("P5\n$dim\n$depth");
 
for $in.lines.ords -> $r, $g, $b {
my $gs = $r * 0.2126 + $g * 0.7152 + $b * 0.0722;
$out.print: chr($gs min 255);
}
 
$in.close;
$out.close;
}

Using the .ppm file from the Write a PPM file task:

Original: Ppm-perl6.png Grey Scale: Pgm-g2-perl6.png

Phix[edit]

Translation of: Euphoria

Requires read_ppm() from Read_a_PPM_file, see Write_a_PPM_file for actual use. Included as demo\rosetta\Bitmap_Greyscale.exw

function to_gray(sequence image)
sequence color
for i=1 to length(image) do
for j=1 to length(image[i]) do
-- unpack color triple
color = sq_div(sq_and_bits(image[i][j], {#FF0000,#FF00,#FF}),
{#010000,#0100,#01})
image[i][j] = floor(0.2126*color[1] + 0.7152*color[2] + 0.0722*color[3])*#010101
end for
end for
return image
end function
 
sequence img = read_ppm("Lena.ppm")
img = to_gray(img)

PHP[edit]

Uses the Bitmap class defined for writing a PPM file

class BitmapGrayscale extends Bitmap {
public function toGrayscale(){
for ($i = 0; $i < $this->h; $i++){
for ($j = 0; $j < $this->w; $j++){
$l = ($this->data[$j][$i][0] * 0.2126)
+ ($this->data[$j][$i][1] * 0.7152)
+ ($this->data[$j][$i][2] * 0.0722);
$l = round($l);
$this->data[$j][$i] = array($l,$l,$l);
}
}
}
}
 
$b = new BitmapGrayscale(16,16);
$b->fill(0,0,null,null, array(255,255,0));
$b->setPixel(0, 15, array(255,0,0));
$b->setPixel(0, 14, array(0,255,0));
$b->setPixel(0, 13, array(0,0,255));
$b->toGrayscale();
$b->writeP6('p6-grayscale.ppm');

PL/I[edit]

 
do j = 1 to hbound(image,1);
do i = 0 to hbound(image,2);
color = image(i,j);
R = substr(color, 17, 8);
G = substr(color, 9, 8);
B = substr(color, 1, 8);
grey = trunc(0.2126*R + 0.7152*G + 0.0722*B);
greybits = grey;
image(i,j) = substr(greybits, length(greybits)-7, 8);
end;
end;
 

PicoLisp[edit]

# Convert color image (PPM) to greyscale image (PGM)
(de ppm->pgm (Ppm)
(mapcar
'((Y)
(mapcar
'((C)
(/
(+
(* (car C) 2126) # Red
(* (cadr C) 7152) # Green
(* (caddr C) 722) ) # Blue
10000 ) )
Y ) )
Ppm ) )
 
# Convert greyscale image (PGM) to color image (PPM)
(de pgm->ppm (Pgm)
(mapcar
'((Y)
(mapcar
'((G) (list G G G))
Y ) )
Pgm ) )
# Write greyscale image (PGM) to file
(de pgmWrite (Pgm File)
(out File
(prinl "P5")
(prinl (length (car Pgm)) " " (length Pgm))
(prinl 255)
(for Y Pgm (apply wr Y)) ) )
 
# Create an empty image of 120 x 90 pixels
(setq *Ppm (make (do 90 (link (need 120)))))
 
# Fill background with green color
(ppmFill *Ppm 0 255 0)
 
# Draw a diagonal line
(for I 80 (ppmSetPixel *Ppm I I 0 0 0))
 
 
# Convert to greyscale image (PGM)
(setq *Pgm (ppm->pgm *Ppm))
 
# Write greyscale image to .pgm file
(pgmWrite *Pgm "img.pgm")
 
# Convert to color image and write to .ppm file
(ppmWrite (pgm->ppm *Pgm) "img.ppm")

PureBasic[edit]

Procedure ImageGrayout(image)
Protected w, h, x, y, r, g, b, gray, color
 
w = ImageWidth(image)
h = ImageHeight(image)
StartDrawing(ImageOutput(image))
For x = 0 To w - 1
For y = 0 To h - 1
color = Point(x, y)
r = Red(color)
g = Green(color)
b = Blue(color)
gray = 0.2126*r + 0.7152*g + 0.0722*b
Plot(x, y, RGB(gray, gray, gray)
Next
Next
StopDrawing()
EndProcedure
 
Procedure ImageToColor(image)
Protected w, h, x, y, v, gray
 
w = ImageWidth(image)
h = ImageHeight(image)
StartDrawing(ImageOutput(image))
For x = 0 To w - 1
For y = 0 To h - 1
gray = Point(x, y)
v = Red(gray) ;for gray, each of the color's components is the same
;color = RGB(0.2126*v, 0.7152*v, 0.0722*v)
Plot(x, y, RGB(v, v, v))
Next
Next
StopDrawing()
EndProcedure

Python[edit]

Works with: Python version 3.1

Extending the example given here

# String masquerading as ppm file (version P3)
import io
ppmfileout = io.StringIO('')
 
def togreyscale(self):
for h in range(self.height):
for w in range(self.width):
r, g, b = self.get(w, h)
l = int(0.2126 * r + 0.7152 * g + 0.0722 * b)
self.set(w, h, Colour(l, l, l))
 
Bitmap.togreyscale = togreyscale
 
 
# Draw something simple
bitmap = Bitmap(4, 4, white)
bitmap.fillrect(1, 0, 1, 2, Colour(127, 0, 63))
bitmap.set(3, 3, Colour(0, 127, 31))
print('Colour:')
# Write to the open 'file' handle
bitmap.writeppmp3(ppmfileout)
print(ppmfileout.getvalue())
print('Grey:')
bitmap.togreyscale()
ppmfileout = io.StringIO('')
bitmap.writeppmp3(ppmfileout)
print(ppmfileout.getvalue())
 
 
'''
The print statement above produces the following output :
 
Colour:
P3
# generated from Bitmap.writeppmp3
4 4
255
255 255 255 255 255 255 255 255 255 0 127 31
255 255 255 255 255 255 255 255 255 255 255 255
255 255 255 127 0 63 255 255 255 255 255 255
255 255 255 127 0 63 255 255 255 255 255 255
 
Grey:
P3
# generated from Bitmap.writeppmp3
4 4
254
254 254 254 254 254 254 254 254 254 93 93 93
254 254 254 254 254 254 254 254 254 254 254 254
254 254 254 31 31 31 254 254 254 254 254 254
254 254 254 31 31 31 254 254 254 254 254 254
 
'''

R[edit]

Library: pixmap
# Conversion from Grey to RGB uses the following code
setAs("pixmapGrey", "pixmapRGB",
function(from, to){
z = new(to, as(from, "pixmap"))
[email protected] = [email protected]
[email protected] = [email protected]
[email protected] = [email protected]
[email protected] = c("red", "green", "blue")
z
})
 
# Conversion from RGB to grey uses built-in coefficients of 0.3, 0.59, 0.11. To see this, type
getMethods(addChannels)
 
# We can override this behaviour with
setMethod("addChannels", "pixmapRGB",
function(object, coef=NULL){
if(is.null(coef)) coef = c(0.2126, 0.7152, 0.0722)
z = new("pixmapGrey", object)
[email protected] = coef[1] * [email protected] + coef[2] * [email protected] +
coef[3] * [email protected]
[email protected] = "grey"
z
})
 
# Colour image
plot(p1 <- pixmapRGB(c(c(1,0,0,0,0,1), c(0,1,0,0,1,0), c(0,0,1,1,0,0)), nrow=6, ncol=6))
 
#Convert to grey
plot(p2 <- as(p1, "pixmapGrey"))
 
# Convert back to "colour"
plot(p3 <- as(p2, "pixmapRGB"))

Racket[edit]

This image shows the output: http://imgur.com/e3Wi8RJ

I gave up on uploading to Rosetta Code.

 
#lang racket
(require racket/draw)
 
(define (gray->color gray-bm)
(define gray-dc (new bitmap-dc% [bitmap gray-bm]))
(define-values (w h) (send gray-dc get-size))
(define width (exact-floor w))
(define height (exact-floor h))
(define color-bm (make-bitmap width height))
(define color-dc (new bitmap-dc% [bitmap color-bm]))
(define pixels (make-bytes (* 4 width height)))
(send gray-dc get-argb-pixels 0 0 width height pixels)
(send color-dc set-argb-pixels 0 0 width height pixels)
color-bm)
 
(define (color->gray color-bm)
(define color-dc (new bitmap-dc% [bitmap color-bm]))
(define-values (w h) (send color-dc get-size))
(define width (exact-floor w))
(define height (exact-floor h))
(define gray-bm (make-bitmap width height))
(define gray-dc (new bitmap-dc% [bitmap gray-bm]))
(define pixels (make-bytes (* 4 width height)))
(send color-dc get-argb-pixels 0 0 width height pixels)
(for ([i (in-range 0 (* 4 width height) 4)])
(define α (bytes-ref pixels i))
(define r (bytes-ref pixels (+ i 1)))
(define g (bytes-ref pixels (+ i 2)))
(define b (bytes-ref pixels (+ i 3)))
(define l (exact-floor (+ (* 0.2126 r) (* 0.7152 g) (* 0.0722 b))))
(bytes-set! pixels (+ i 1) l)
(bytes-set! pixels (+ i 2) l)
(bytes-set! pixels (+ i 3) l))
(send gray-dc set-argb-pixels 0 0 width height pixels)
gray-bm)
 
(require images/icons/symbol)
(define rosetta (text-icon "Rosetta Code" #:color "red" #:height 80))
rosetta
(color->gray rosetta)
(gray->color (color->gray rosetta))
 

REXX[edit]

Note:   REXX uses characters instead of binary for storing numbers, so there is no rounding   (using characters to
          store numbers is almost the same as using decimal floating point).

/*REXX program converts a RGB (red─green─blue) image into a  grayscale/greyscale image. */
blue= '00 00 ff'x /*define the blue color (hexadecimal).*/
@.= blue /*set the entire image to blue color.*/
width= 60 /* width of the image (in pixels). */
height= 100 /*height " " " " " */
 
do col=1 for width
do row=1 for height /* [↓] C2D convert char ───> decimal*/
r= left(@.col.row, 1)  ; r=c2d(r) /*extract the component red & convert.*/
g=substr(@.col.row, 2, 1) ; g=c2d(g) /* " " " green " " */
b= right(@.col.row, 1)  ; b=c2d(b) /* " " " blue " " */
_= d2c( (.2126*r + .7152*g + .0722*b) % 1) /*convert RGB number ───► grayscale. */
@.col.row=copies(_, 3) /*redefine old RGB ───► grayscale. */
end /*row*/ /* [↑] D2C convert decimal ───► char*/
end /*col*/ /* [↑] x%1 is the same as TRUNC(x) */
/*stick a fork in it, we're all done. */

Other alternatives to express the   blue   color are:

  blue= "00 00 ff"x                              /*define the blue color  (hexadecimal).*/
blue= '00 00 FF'x /*define the blue color (hexadecimal).*/
blue= '0000ff'x /*define the blue color (hexadecimal).*/
 
blue= '00000000 00000000 11111111'b /*define the blue color (binary). */
blue= '000000000000000011111111'b /*define the blue color (binary) */
 
blue= 'zzy' /*define the blue color (character). */
/*not recommended because of rendering.*/
/*where Z is the character '00'x */
/*where Y is the character 'ff'x */
 
/*Both Z & Y are normally not viewable*/
/*on most terminals (appear as blanks).*/


Ruby[edit]

Extending Basic_bitmap_storage#Ruby

class RGBColour
def to_grayscale
luminosity = Integer(0.2126*@red + 0.7152*@green + 0.0722*@blue)
self.class.new(luminosity, luminosity, luminosity)
end
end
 
class Pixmap
def to_grayscale
gray = self.class.new(@width, @height)
@width.times do |x|
@height.times do |y|
gray[x,y] = self[x,y].to_grayscale
end
end
gray
end
end

Scala[edit]

Uses the Scala Basic Bitmap Storage class.

object BitmapOps {
def luminosity(c:Color)=(0.2126*c.getRed + 0.7152*c.getGreen + 0.0722*c.getBlue+0.5).toInt
 
def grayscale(bm:RgbBitmap)={
val image=new RgbBitmap(bm.width, bm.height)
for(x <- 0 until bm.width; y <- 0 until bm.height; l=luminosity(bm.getPixel(x,y)))
image.setPixel(x, y, new Color(l,l,l))
image
}
}

Sidef[edit]

Translation of: Perl
require('Image::Imlib2')
 
func tograyscale(img) {
var (width, height) = (img.width, img.height)
var gimg = %s'Image::Imlib2'.new(width, height)
for y,x in (^height ~X ^width) {
var (r, g, b) = img.query_pixel(x, y)
var gray = int(0.2126*r + 0.7152*g + 0.0722*b)
gimg.set_color(gray, gray, gray, 255)
gimg.draw_point(x, y)
}
return gimg
}
 
var (input='input.png', output='output.png') = ARGV...
var image = %s'Image::Imlib2'.load(input)
var gscale = tograyscale(image)
gscale.set_quality(80)
gscale.save(output)

Tcl[edit]

Library: Tk
package require Tk
 
proc grayscale image {
set w [image width $image]
set h [image height $image]
for {set x 0} {$x<$w} {incr x} {
for {set y 0} {$y<$h} {incr y} {
lassign [$image get $x $y] r g b
set l [expr {int(0.2126*$r + 0.7152*$g + 0.0722*$b)}]
$image put [format "#%02x%02x%02x" $l $l $l] -to $x $y
}
}
}

Photo images are always 8-bits-per-channel RGBA.

Vedit macro language[edit]

Conversion to a grayscale image.

//  Convert RGB image to grayscale (8 bit/pixel)
// #10 = buffer that contains image data
// On return:
// #20 = buffer for the new grayscale image
 
:RGB_TO_GRAYSCALE:
File_Open("|(VEDIT_TEMP)\gray.data", OVERWRITE+NOEVENT+NOMSG)
#20 = Buf_Num
BOF
Del_Char(ALL)
Buf_Switch(#10)
Repeat(File_Size/3) {
#9 = Cur_Char() * 2126
#9 += Cur_Char(1) * 7152
#9 += Cur_Char(2) * 722
Char(3)
Buf_Switch(#20)
Ins_Char(#9 / 10000)
Buf_Switch(#10)
}
Return

Conversion to a color image.

//  Convert grayscale image (8 bits/pixel) into RGB (24 bits/pixel)
// #20 = buffer that contains image data
// On return:
// #10 = buffer for the new RGB image
 
:GRAYSCALE_TO_RGB:
File_Open("|(VEDIT_TEMP)\RGB.data", OVERWRITE+NOEVENT+NOMSG)
#10 = Buf_Num
BOF
Del_Char(ALL)
Buf_Switch(#20) // input image (grayscale)
BOF
Repeat(File_Size) {
#9 = Cur_Char()
Char
Buf_Switch(#10) // output image (RGB)
Ins_Char(#9, COUNT, 3)
Buf_Switch(#20)
}
Return

Visual Basic .NET[edit]

Convert a Bitmap to Grayscale.

 
Imports System.Drawing.Imaging
 
Public Function Grayscale(ByVal Map As Bitmap) As Bitmap
 
Dim oData() As Integer = GetData(Map)
Dim oReturn As New Bitmap(Map.Width, Map.Height, Map.PixelFormat)
Dim a As Integer = 0
Dim r As Integer = 0
Dim g As Integer = 0
Dim b As Integer = 0
Dim l As Integer = 0
 
For i As Integer = 0 To oData.GetUpperBound(0)
a = (oData(i) >> 24)
r = (oData(i) >> 16) And 255
g = (oData(i) >> 8) And 255
b = oData(i) And 255
 
l = CInt(r * 0.2126F + g * 0.7152F + b * 0.0722F)
 
oData(i) = (a << 24) Or (l << 16) Or (l << 8) Or l
Next
 
SetData(oReturn, oData)
 
Return oReturn
 
End Function
 
Private Function GetData(ByVal Map As Bitmap) As Integer()
 
Dim oBMPData As BitmapData = Nothing
Dim oData() As Integer = Nothing
 
oBMPData = Map.LockBits(New Rectangle(0, 0, Map.Width, Map.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb)
 
Array.Resize(oData, Map.Width * Map.Height)
 
Runtime.InteropServices.Marshal.Copy(oBMPData.Scan0, oData, 0, oData.Length)
 
Map.UnlockBits(oBMPData)
 
Return oData
 
End Function
 
Private Sub SetData(ByVal Map As Bitmap, ByVal Data As Integer())
 
Dim oBMPData As BitmapData = Nothing
 
oBMPData = Map.LockBits(New Rectangle(0, 0, Map.Width, Map.Height), ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb)
 
Runtime.InteropServices.Marshal.Copy(Data, 0, oBMPData.Scan0, Data.Length)
 
Map.UnlockBits(oBMPData)
 
End Sub

zkl[edit]

Does an in-place conversion from a color PPM image to a gray scale PPM image (ie rgb is down sampled but remains rgb vs one byte color). If you wish to write a bit map (or some other format), check out Bitmap/PPM conversion through a pipe#zkl

Uses the PPM class from http://rosettacode.org/wiki/Bitmap/Bresenham%27s_line_algorithm#zkl

Translation of: BASIC256
fcn toGrayScale(img){  // in-place conversion
foreach x,y in (img.w,img.h){
r,g,b:=img[x,y].toBigEndian(3);
lum:=(0.2126*r + 0.7152*g + 0.0722*b).toInt();
img[x,y]=((lum*256) + lum)*256 + lum;
}
}
img:=PPM.readPPMFile("lena.ppm");
toGrayScale(img);
img.write(File("foo.ppm","wb"));
Output:

http://www.zenkinetic.com/Images/RosettaCode/lenaGray.jpg