Bitmap/Read a PPM file
You are encouraged to solve this task according to the task description, using any language you may know.
Using the data storage type defined on this page for raster images, read an image from a PPM file (binary P6 prefered). (Read the definition of PPM file on Wikipedia.)
Task: Use write ppm file solution and grayscale image solution with this one in order to convert a color image to grayscale one.
Ada
<ada> with Ada.Characters.Latin_1; use Ada.Characters.Latin_1; with Ada.Integer_Text_IO; use Ada.Integer_Text_IO; with Ada.Streams.Stream_IO; use Ada.Streams.Stream_IO;
function Get_PPM (File : File_Type) return Image is
use Ada.Characters.Latin_1; use Ada.Integer_Text_IO;
function Get_Line return String is -- Skips comments Byte : Character; Buffer : String (1..80); begin loop for I in Buffer'Range loop Character'Read (Stream (File), Byte); if Byte = LF then exit when Buffer (1) = '#'; return Buffer (1..I - 1); end if; Buffer (I) := Byte; end loop; if Buffer (1) /= '#' then raise Data_Error; end if; end loop; end Get_Line;
Height : Integer; Width : Integer;
begin
if Get_Line /= "P6" then raise Data_Error; end if; declare Line : String := Get_Line; Start : Integer := Line'First; Last : Positive; begin Get (Line, Width, Last); Start := Start + Last; Get (Line (Start..Line'Last), Height, Last); Start := Start + Last; if Start <= Line'Last then raise Data_Error; end if; if Width < 1 or else Height < 1 then raise Data_Error; end if; end; if Get_Line /= "255" then raise Data_Error; end if; declare Result : Image (1..Height, 1..Width); Buffer : String (1..Width * 3); Index : Positive; begin for I in Result'Range (1) loop String'Read (Stream (File), Buffer); Index := Buffer'First; for J in Result'Range (2) loop Result (I, J) := ( R => Luminance (Character'Pos (Buffer (Index))), G => Luminance (Character'Pos (Buffer (Index + 1))), B => Luminance (Character'Pos (Buffer (Index + 2))) ); Index := Index + 3; end loop; end loop; return Result; end;
end Get_PPM; </ada> The implementation propagates Data_Error when the file format is incorrect. End_Error is propagated when the file end is prematurely met. The following example illustrates conversion of a color file to grayscale. <ada> declare
F1, F2 : File_Type;
begin
Open (F1, In_File, "city.ppm"); Create (F2, Out_File, "city_grayscale.ppm"); Put_PPM (F2, Color (Grayscale (Get_PPM (F1)))); Close (F1); Close (F2);
end; </ada>
C
It is up to the caller to open the file and pass the handler to the function. So this code can be used in Read image file through a pipe without modification. It only understand the P6 file format.
<c>#include <stdio.h>
- define PPMREADBUFLEN 256
image get_ppm(FILE *pf) {
char buf[PPMREADBUFLEN], *t; image img; unsigned int w, h, d; int r; if (pf == NULL) return NULL; t = fgets(buf, PPMREADBUFLEN, pf); if ( (t == NULL) || ( strncmp(buf, "P6\n", 3) != 0 ) ) return NULL; do { /* Px formats can have # comments after first line */ t = fgets(buf, PPMREADBUFLEN, pf); if ( t == NULL ) return NULL; } while ( strncmp(buf, "#", 1) == 0 ); r = sscanf(buf, "%u %u", &w, &h); if ( r < 2 ) return NULL; r = fscanf(pf, "%u\n", &d); if ( (r < 1) || ( d != 255 ) ) return NULL; img = alloc_img(w, h); if ( img != NULL ) { size_t rd = fread(img->buf, sizeof(pixel), w*h, pf); if ( rd < w*h ) { free_img(img); return NULL; } return img; }
}</c>
The following acts as a filter to convert a PPM file read from standard input into a PPM gray image, and it outputs the converted image to standard output (see Grayscale image, Write ppm file, and Raster graphics operations in general):
<c>#include <stdio.h> /* #include "imglib.h" */
int main() {
image source; grayimage idest; source = get_ppm(stdin); idest = tograyscale(source); free_img(source); source = tocolor(idest); output_ppm(stdout, source); free_img(source); free_img((image)idest); return 0;
}</c>
Forth
: read-ppm { fid -- bmp } pad dup 80 fid read-line throw 0= abort" Partial line" s" P6" compare abort" Only P6 supported." pad dup 80 fid read-line throw 0= abort" Partial line" 0. 2swap >number 1 /string \ skip space 0. 2swap >number 2drop drop nip ( w h ) bitmap { bmp } pad dup 80 fid read-line throw 0= abort" Partial line" s" 255" compare abort" Only 8-bits per color channel supported" 0 pad ! bmp bdim 0 do dup 0 do pad 3 fid read-file throw 3 - abort" Not enough pixel data in file" pad @ i j bmp b! loop loop drop bmp ;
\ testing round-trip 4 3 bitmap value test red test bfill green 1 2 test b! s" red.ppm" w/o create-file throw test over write-ppm close-file throw s" red.ppm" r/o open-file throw dup read-ppm value test2 close-file throw : bsize ( bmp -- len ) bdim * pixels bdata ; test dup bsize test2 dup bsize compare . \ 0 if identical
OCaml
<ocaml>let read_ppm ~filename =
let ic = open_in filename in let line = input_line ic in if line <> "P6" then invalid_arg "not a P6 ppm file"; let line = input_line ic in let line = try if line.[0] = '#' (* skip comments *) then input_line ic else line with _ -> line in let width, height = Scanf.sscanf line "%d %d" (fun w h -> (w, h)) in let line = input_line ic in if line <> "255" then invalid_arg "not a 8 bit depth image"; 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 for y = 0 to pred height do for x = 0 to pred width do r_channel.{x,y} <- (input_byte ic); g_channel.{x,y} <- (input_byte ic); b_channel.{x,y} <- (input_byte ic); done; done; close_in ic; (all_channels, r_channel, g_channel, b_channel)</ocaml>
and converting a given color file to grayscale: <ocaml>let () =
let img = read_ppm ~filename:"logo.ppm" in let img = to_color(to_grayscale ~img) in output_ppm ~oc:stdout ~img;
- </ocaml>
sending the result to stdout
allows to see the result without creating a temporary file sending it through a pipe to the display utility of ImageMagick:
ocaml script.ml | display -
Vedit macro language
// Load a PPM file // @10 = filename // On return: // #10 points to buffer containing pixel data, // #11 = width, #12 = height. :LOAD_PPM: File_Open(@10) BOF Search("|X", ADVANCE) // skip "P6" #11 = Num_Eval(ADVANCE) // #11 = width Match("|X", ADVANCE) // skip separator #12 = Num_Eval(ADVANCE) // #12 = height Match("|X", ADVANCE) Search("|X", ADVANCE) // skip maxval (assume 255) Del_Block(0,CP) // remove the header Return
Example of usage. In addition to LOAD_PPM routine above, you need routine RGB_TO_GRAYSCALE from Grayscale image and routine SAVE_PPM from Write ppm file.
// Load RGB image Reg_Set(10, "|(USER_MACRO)\example.ppm") Call("LOAD_PPM") // Convert to grayscale #10 = Buf_Num Call("RGB_TO_GRAYSCALE") Buf_Switch(#10) Buf_Quit(OK) // Convert to RGB Call("GRAYSCALE_TO_RGB") // Save the image Reg_Set(10, "|(USER_MACRO)\example_gray.ppm") Call("SAVE_PPM") // Cleanup and exit Buf_Switch(#20) Buf_Quit(OK) return