screenshot.cpp

#include <assert.h>
#include "screenshot.h"

static void fwriteUint16LE(FILE *fp, uint16_t n) {
	fputc(n & 0xFF, fp);
	fputc(n >> 8, fp);
}

static void fwriteUint32LE(FILE *fp, uint32_t n) {
	fwriteUint16LE(fp, n & 0xFFFF);
	fwriteUint16LE(fp, n >> 16);
}

static const uint16_t TAG_BM = 0x4D42;

void saveBMP(FILE *fp, const uint8_t *bits, const uint8_t *pal, int w, int h) {
	const int alignWidth = (w + 3) & ~3;
	const int imageSize = alignWidth * h;

	// Write file header
	fwriteUint16LE(fp, TAG_BM);
	fwriteUint32LE(fp, 14 + 40 + 4 * 256 + imageSize);
	fwriteUint16LE(fp, 0); // reserved1
	fwriteUint16LE(fp, 0); // reserved2
	fwriteUint32LE(fp, 14 + 40 + 4 * 256);

	// Write info header
	fwriteUint32LE(fp, 40);
	fwriteUint32LE(fp, w);
	fwriteUint32LE(fp, h);
	fwriteUint16LE(fp, 1); // planes
	fwriteUint16LE(fp, 8); // bit_count
	fwriteUint32LE(fp, 0); // compression
	fwriteUint32LE(fp, imageSize); // size_image
	fwriteUint32LE(fp, 0); // x_pels_per_meter
	fwriteUint32LE(fp, 0); // y_pels_per_meter
	fwriteUint32LE(fp, 0); // num_colors_used
	fwriteUint32LE(fp, 0); // num_colors_important

	// Write palette data
	for (int i = 0; i < 256; ++i) {
		fputc(pal[2], fp);
		fputc(pal[1], fp);
		fputc(pal[0], fp);
		fputc(0, fp);
		pal += 3;
	}

	// Write bitmap data
	const int pitch = w;
	bits += h * pitch;
	for (int i = 0; i < h; ++i) {
		bits -= pitch;
		fwrite(bits, w, 1, fp);
		int pad = alignWidth - w;
		while (pad--) {
			fputc(0, fp);
		}
	}
}