bug fix png encoder

pngencode.cpp

// by alex evans, 2011.  released into the public domain.
// based on a first ever reading of the png spec, it occurs to me that a minimal png encoder should be quite simple.
// this is a first stab - may be buggy! the only external dependency is zlib and some basic typedefs (u32, u8)
//
// VERSION 0.02! now using zlib's crc rather than my own, and avoiding a memcpy and memory scribbler in the old one
// by passing the zero byte at the start of the scanline to zlib first, then the original scanline in place. WIN!
//
// more context at http://altdevblogaday.org/2011/04/06/a-smaller-jpg-encoder/.
//
// follow me on twitter @mmalex http://twitter.com/mmalex
//
u8* CompressPNG(void *img, int w, int h, int numchans, u32 &len_out) {
	int p=w*numchans;
	z_stream z={0}; 
	deflateInit(&z,-1);	
	u8 *zbuf=(u8*)malloc(53+(z.avail_out=deflateBound(&z,(1+p)*h))+1);	
	if (!zbuf) return 0;
	z.next_out=zbuf+41;	
	for (int y=0;y<h;++y) {
		Bytef zero=0; z.avail_in=1;z.next_in=&zero; deflate(&z,Z_NO_FLUSH);
		z.avail_in=p;z.next_in=((Bytef*)img)+y*p; deflate(&z,(y==h-1) ? Z_FINISH : Z_NO_FLUSH);
	}
	len_out=z.next_out-zbuf-41;
	u8 pnghdr[41]={0x89,0x50,0x4e,0x47,0x0d,0x0a,0x1a,0x0a,0x00,0x00,0x00,0x0d,0x49,0x48,0x44,0x52,
				   0,0,w>>8,w,0,0,h>>8,h,8,"\0\0\04\02\06"[numchans],0,0,0,0,0,0,0,
				   len_out>>24,len_out>>16,len_out>>8,len_out,0x49,0x44,0x41,0x54};
	*(u32*)(pnghdr+29)=htonl(crc32(0,pnghdr+12,17));	
	memcpy(zbuf,pnghdr,41); 
	memcpy(z.next_out+4,"\x49\x45\x4e\x44\xae\x42\x60\x82",8); // footer	
	*(u32*)z.next_out =htonl(crc32(0, zbuf+41-4, len_out+4));
	deflateEnd(&z); len_out+=53; return zbuf;
}
void PNGTest()
{
	u32 img[200][320];
	for (int y=0;y<200;++y) for (int x=0;x<320;++x) img[y][x]=x+(y<<8)+0xff000000;
	u32 blen; u8*zbuf = CompressPNG(img,16,16,4,blen);
	FILE *f=fopen("test.png","wb");
	fwrite(zbuf,1,blen,f);
	fclose(f);
	free(zbuf);
}

the generated png is invalid though:

/tmp $ pngcheck -vv test.png
File: test.png (488 bytes)
  chunk IHDR at offset 0x0000c, length 13
    16 x 16 image, 32-bit RGB+alpha, non-interlaced
  chunk IDAT at offset 0x00025, length 435
    zlib: deflated, 32K window, default compression
    row filters (0 none, 1 sub, 2 up, 3 avg, 4 paeth):
      0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 (16 out of 16)
  invalid chunk name "B`" (ffffffae 42 60 ffffff82)
ERRORS DETECTED in test.png

here is a patch to my program that fixes that:

--- pngencode.c	2024-01-24 14:52:34.490371971 +0100
+++ pngencode-fixed.c	2024-01-24 14:52:54.517190368 +0100
@@ -19,7 +19,7 @@
         int p = w * numchans;
         z_stream z = {0};
         deflateInit(&z, -1);
-        uint8_t* zbuf = malloc(53 + (z.avail_out = deflateBound(&z, (1+p)*h)) + 1);
+        uint8_t* zbuf = malloc(57 + (z.avail_out = deflateBound(&z, (1+p)*h)) + 1);
         if (!zbuf) return 0;
         z.next_out = zbuf + 41;
 
@@ -39,10 +39,10 @@
                 *len_out>>24, *len_out>>16, *len_out>>8, *len_out, 0x49, 0x44, 0x41, 0x54};
         *(unsigned int*)(pnghdr + 29) = htonl(crc32(0, pnghdr + 12, 17));
         memcpy(zbuf, pnghdr, 41);
-        memcpy(z.next_out + 4, "\x49\x45\x4e\x44\xae\x42\x60\x82", 8); /* footer */
+        memcpy(z.next_out + 4, "\0\0\0\0\x49\x45\x4e\x44\xae\x42\x60\x82", 12); /* footer */
         *(unsigned int*)z.next_out = htonl(crc32(0, zbuf+41-4, *len_out+4));
         deflateEnd(&z);
-        *len_out += 53;
+        *len_out += 57;
         return zbuf;
 }
 

here is the image it produces:

compile with gcc -lz pngencode.c