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// 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)
//
// more context at http://altdevblogaday.org/2011/04/06/a-smaller-jpg-encoder/.
//
// follow me on twitter @mmalex http://twitter.com/mmalex
//
// fixed a buffer overrun
u32 crc_table[256]={0};
void make_crc_table(void)
{
for (int n = 0; n < 256; n++) {
u32 c = n;
for (int k = 0; k < 8; k++) if (c & 1) c = 0xedb88320L ^ (c >> 1); else c = c >> 1;
crc_table[n] = c;
}
}
u32 update_crc(u32 c, unsigned char *buf, int len)
{
if (!crc_table[0]) make_crc_table();
for (int n = 0; n < len; n++) c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
return c;
}
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+w); // +w: ensure we can at least store when uncompressed line (w-53-X might be enough)
u8 *scanline=zbuf+53+z.avail_out;
z.next_out=zbuf+41;
for (int y=0;y<h;++y)
{
u8 *dst=scanline; *dst++ = 0;
u8 *src=((u8*)img)+y*p;
for (int x=0;x<p;++x) *dst++ = *src++; // this line was overrunning the buffer in the original version
z.avail_in=1+p; z.next_in=scanline;
int err = 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(update_crc(0xffffffffL,pnghdr+12,17)^0xffffffffL);
memcpy(zbuf,pnghdr,41); // header
memcpy(z.next_out+4,"\x49\x45\x4e\x44\xae\x42\x60\x82",8); // footer
*(u32*)z.next_out =htonl(update_crc(0xffffffffL, zbuf+41-4, len_out+4)^0xffffffffL);
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);
}
@mmalex

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mmalex commented Apr 7, 2011

Hi Andreas! well spotted on the memory bug; however it's +p you want, not +w....
maybe I'll fork my original, I have some other edits...

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AndreasOM commented Apr 7, 2011

Yep, youre right.
In "most" cases it fits anyway, but not in all...

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