Bitmap/Read a PPM file: Difference between revisions

=={{header|REXX}}==

The input file   '''Lenna50.ppm'''   is a '''PPM''' format of

<br>the input file &nbsp;&nbsp; '''Lenna50.jpg''' &nbsp;&nbsp; used elsewhere on Rosetta Code.

This REXX program handles alternative delimiters as well as comments within the PPM header.

<lang rexx>/*REXX program reads a PPM formatted image file, and creates a gray─scale image of it. */

parse arg iFN oFN /*obtain optional argument from the CL.*/

if iFN=='' | iFN=="," then iFN= 'Lenna50' /*Not specified? Then use the default.*/

if oFN=='' | oFN=="," then oFN= 'greyscale' /* " " " " " " */

iFID= iFN'.ppm'; oFID= oFN'.ppm' /*complete the input and output FIDs.*/

call charin iFID, 1, 0 /*set the position of the input file. */

y=charin(iFID, , copies(9, digits() ) ) /*read the entire input file ───► X */

parse var y id 3 c 4 3 width height # pixels /*extract header info from the PPM hdr.*/

LF= 'a'x /*define a comment separator (in hdr).*/ /* ◄─── LF delimiters & comments*/

if c==LF then do; commentEND=pos(LF, y, 4) /*point to the last char in the comment*/ /* ◄─── LF delimiters & comments*/

parse var y =(commentEND) +1 width height # pixels /* ◄─── LF delimiters & comments*/

end /* ◄─── LF delimiters & comments*/

/* [↓] has an alternative delimiter? */ /* ◄─── LF delimiters & comments*/

z=pos(LF, height); if z\==0 then parse var height height =(z) +1 # pixels /* ◄─── LF delimiters & comments*/

z=pos(LF, # ); if z\==0 then parse var # # =(z) +1 pixels /* ◄─── LF delimiters & comments*/

chunk=4000 /*chunk size to be written at one time.*/

LenPixels= length(pixels)

do j=0 for 256; _=d2c(j); @._=j; @@.j=_ /*build two tables for fast conversions*/

end /*j*/

call charout oFID, , 1 /*set the position of the output file. */

call charout oFID, id || width height #' ' /*write the header followed by a blank.*/

!=1

do until !>=LenPixels; $= /*$: partial output string so far.*/

do !=! by 3 for chunk /*chunk: # pixels converted at 1 time.*/

parse var pixels =(!) r +1 g +1 b +1 /*obtain the next RGB of a PPM pixel.*/

if r=='' then leave /*has the end─of─string been reached? */

_=(.2126*@.r + .7152*@.g + .0722*@.b )%1 /*an integer RGB greyscale of a pixel. */

$=$ || @@._ || @@._ || @@._ /*lump (grey) R G B pixels together. */

end /*!*/ /* [↑] D2C converts decimal ───► char*/

call charout oFID, $ /*write the next bunch of pixels. */

end /*until*/

call charout oFID /*close the output file just to be safe*/

say 'File ' oFID " was created." /*stick a fork in it, we're all done. */</lang>

{{out|output}}

<pre>

File greyscale.ppm was created.

</pre>