Converts 24 bit PPM file into a 32 bit raw data blob that can be pasted to /dev/fb0 as is.

ppmtorgba.c

#include <stdlib.h>
#include <assert.h>
#include <stdio.h>
#include <inttypes.h>


int main(int argc, char* argv[])
{
	FILE* f = stdin;
	if (argc!=1 && argc!=2)
	{
		fprintf(stderr,"Usage: %s file.ppm > out.rgba\n", argv[0]);
		exit(1);
	}
	if (argc==2)
	{
		f = fopen(argv[1],"rb");
		assert(f);
	}
	char newl=0;
	int imw=0, imh=0;
	const int numscanned = fscanf(f, "P6 %d %d 255%c", &imw, &imh, &newl);
	if (numscanned != 3)
	{
		fprintf(stderr,"Input is not a valid 24-bit raw PPM image.\n");
		exit(2);
	}
	assert(imw);
	assert(imh);
	uint8_t* im_src = (uint8_t*) malloc(imw*imh*3);
	uint8_t* im_dst = (uint8_t*) malloc(imw*imh*4);
	assert(im_src && im_dst);

	const size_t numr = fread(im_src, imw*imh, 3, f);
	assert(numr == 3);
	fclose(f);

	uint8_t* reader = im_src;
	uint8_t* writer = im_dst;
	for (int i=0; i<imw*imh; ++i)
	{
		*writer++ = reader[2];	// R
		*writer++ = reader[1];	// G
		*writer++ = reader[0];	// B
		*writer++ = 255;	// A
		reader += 3;
	}

	fwrite(im_dst, imw*imh, 4, stdout);
	return 0;
}