feat(examples): add a zlib backed PNG encoder

Real PNG output through deflate, which is what makes the pkg-config
dependency load bearing rather than merely declared.

examples/raytracer/src/image/png.c

@@ -0,0 +1,95 @@
+#include "rt_config.h"
+#include "rt_image.h"
+
+#ifndef HAVE_ZLIB
+
+int image_write_png(const image *im, const char *path) {
+    (void)im;
+    (void)path;
+    return -1; /* built without zlib; callers fall back to PPM */
+}
+
+#else
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <zlib.h>
+
+static void put_be32(unsigned char *p, unsigned long v) {
+    p[0] = (unsigned char)((v >> 24) & 0xFF);
+    p[1] = (unsigned char)((v >> 16) & 0xFF);
+    p[2] = (unsigned char)((v >> 8) & 0xFF);
+    p[3] = (unsigned char)(v & 0xFF);
+}
+
+static int put_chunk(FILE *f, const char *type, const unsigned char *data, unsigned long len) {
+    unsigned char header[8];
+    put_be32(header, len);
+    memcpy(header + 4, type, 4);
+    if (fwrite(header, 1u, 8u, f) != 8u) return -1;
+    if (len && fwrite(data, 1u, len, f) != len) return -1;
+
+    unsigned long crc = crc32(0uL, (const Bytef *)type, 4u);
+    if (len) crc = crc32(crc, (const Bytef *)data, (uInt)len);
+
+    unsigned char tail[4];
+    put_be32(tail, crc);
+    return fwrite(tail, 1u, 4u, f) == 4u ? 0 : -1;
+}
+
+int image_write_png(const image *im, const char *path) {
+    if (!im || !path) return -1;
+
+    /* One filter byte (type 0, None) in front of every scanline. */
+    size_t stride = (size_t)im->width * 3u;
+    size_t raw_len = ((size_t)im->height) * (stride + 1u);
+    unsigned char *raw = (unsigned char *)malloc(raw_len);
+    if (!raw) return -1;
+
+    for (int y = 0; y < im->height; y++) {
+        unsigned char *dst = raw + (size_t)y * (stride + 1u);
+        dst[0] = 0;
+        memcpy(dst + 1, im->rgb + (size_t)y * stride, stride);
+    }
+
+    uLongf comp_len = compressBound((uLong)raw_len);
+    unsigned char *comp = (unsigned char *)malloc(comp_len);
+    if (!comp) {
+        free(raw);
+        return -1;
+    }
+    int zr = compress2(comp, &comp_len, raw, (uLong)raw_len, Z_BEST_COMPRESSION);
+    free(raw);
+    if (zr != Z_OK) {
+        free(comp);
+        return -1;
+    }
+
+    FILE *f = fopen(path, "wb");
+    if (!f) {
+        free(comp);
+        return -1;
+    }
+
+    static const unsigned char sig[8] = {137, 80, 78, 71, 13, 10, 26, 10};
+    int rc = fwrite(sig, 1u, 8u, f) == 8u ? 0 : -1;
+
+    unsigned char ihdr[13];
+    put_be32(ihdr, (unsigned long)im->width);
+    put_be32(ihdr + 4, (unsigned long)im->height);
+    ihdr[8] = 8;  /* bit depth */
+    ihdr[9] = 2;  /* colour type: truecolour */
+    ihdr[10] = 0; /* deflate */
+    ihdr[11] = 0; /* adaptive filtering */
+    ihdr[12] = 0; /* no interlace */
+
+    if (rc == 0) rc = put_chunk(f, "IHDR", ihdr, sizeof ihdr);
+    if (rc == 0) rc = put_chunk(f, "IDAT", comp, (unsigned long)comp_len);
+    if (rc == 0) rc = put_chunk(f, "IEND", NULL, 0uL);
+
+    free(comp);
+    return (fclose(f) == 0 && rc == 0) ? 0 : -1;
+}
+
+#endif