vk: tools: add imagecompare tool to compare screenshots

Produces diff image file and total difference.

ref/vk/tools/Makefile

@@ -0,0 +1,2 @@
+imagecompare: imagecompare.c stb_image.h stb_image_write.h Makefile
+	${CC} -Wall -Werror -pedantic -lm -o imagecompare imagecompare.c

ref/vk/tools/imagecompare.c

@@ -0,0 +1,96 @@
+#include <stdio.h>
+
+#define STB_IMAGE_IMPLEMENTATION
+#include "stb_image.h"
+
+#define STB_IMAGE_WRITE_IMPLEMENTATION
+#include "stb_image_write.h"
+
+typedef struct {
+	int w, h;
+	int comp;
+	unsigned char* data;
+} image_t;
+
+static int imageLoad(image_t* img, const char *f) {
+	img->data = stbi_load(f, &img->w, &img->h, &img->comp, 0);
+	if (!img->data)
+		fprintf(stderr, "Unable to load image \"%s\"\n", f);
+	return !!(img->data);
+}
+
+static int imageSave(image_t* img, const char *f) {
+	return stbi_write_png(f, img->w, img->h, img->comp, img->data, 0);
+}
+
+/* LOL no need to clear anything
+static void imageFree(image_t *img) {
+	if (img->data)
+		stbi_image_free(img->data);
+}
+*/
+
+int main(int argc, char *argv[]) {
+	if (argc < 4) {
+		fprintf(stderr, "Usage: %s infile1 infile2 out_diff.png\n", argv[0]);
+		return 1;
+	}
+
+	image_t a, b;
+
+	if (!imageLoad(&a, argv[1]))
+		return 1;
+
+	if (!imageLoad(&b, argv[2]))
+		return 1;
+
+	if (a.w != b.w || a.h != b.h || a.comp != b.comp) {
+		fprintf(stderr, "Images metadata doesn't match: %s:%dx%d(%d) %s:%dx%d(%d)\n",
+			argv[1], a.w, a.h, a.comp,
+			argv[2], b.w, b.h, b.comp);
+		return 1;
+	}
+
+	image_t diff = {
+		.w = a.w,
+		.h = a.h,
+		.comp = a.comp,
+		.data = malloc(a.w * a.h * a.comp),
+	};
+
+	const unsigned char* ap = a.data, *bp = b.data;
+	unsigned char* dp = diff.data;
+
+	uint32_t diff_sum = 0;
+	for (int y = 0; y < a.h; ++y) {
+		for (int x = 0; x < a.w; ++x) {
+			const uint8_t ar = *(ap++);
+			const uint8_t ag = *(ap++);
+			const uint8_t ab = *(ap++);
+
+			const uint8_t br = *(bp++);
+			const uint8_t bg = *(bp++);
+			const uint8_t bb = *(bp++);
+
+			const uint8_t dr = abs((int)ar-br);
+			const uint8_t dg = abs((int)ag-bg);
+			const uint8_t db = abs((int)ab-bb);
+
+			*(dp++) = dr;
+			*(dp++) = dg;
+			*(dp++) = db;
+
+			const int diff = dr + dg + db;
+			diff_sum += diff;
+		}
+	}
+
+	const uint32_t total = a.w * a.h * a.comp * 256;
+
+	fprintf(stderr, "Total difference \"%s\" vs \"%s\": %d (%.03f%%)\n", argv[1], argv[2], diff_sum, (diff_sum * 100. / total));
+
+	if (!imageSave(&diff, argv[3]))
+		return 1;
+
+	return diff_sum == 0;
+}