Tiny PNG output

nanopng.cpp

// c++ -o nanopng nanopng.cpp && ./nanopng /tmp/foo.png
#include <cstdio>

// write an uncompressed PNG file from a uint8 RGB buffer
struct Png {
    FILE*f; unsigned int tab[256], crc; ~Png() { fclose(f); }
    Png(const char* fn, int w, int h, const unsigned char* c) {
        crc=0x575e51f5;unsigned char d[]={137,80,78,71,13,
        10,26,10,0,0,0,13,73,72,68,82,73,68,65,84,120,1,0,
        0,0,73,69,78,68,174,66,96,130};/*chunk headers*/
        for (int i=0;i<256;i++)/*precompute crc tables*/
            for (unsigned int j=8,&v=tab[i]=i;j--;)
                v=(v>>1)^(0xedb88320u&(~(v&1)+1));
        fwrite(d,1,16,f=fopen(fn,"w")); /*write IHDR+IDAT*/
        *this>>w>>h<<8<<2<<0<<0<<0;int a=1,b=0;/*adler-32*/
        *this>>~crc>>(h*(w*3+6)+6);int len=w*3+1;/*nbytes*/
        fwrite(d+16,1,6,f);crc=0x13e5812d;
        for (;h--;) {/*DEFLATE raw block*/
            *this<<!h<<len<<(len>>8)<<~len<<(~len>>8);
            /*filter=0*/*this <<0;/*adler(0)*/b=(a+b)%65521;
            for (int x=w,v;x--;){ /*write & checksum*/
                v=*c++;*this<<v;a=(a+v)%65521;b=(a+b)%65521;
                v=*c++;*this<<v;a=(a+v)%65521;b=(a+b)%65521;
                v=*c++;*this<<v;a=(a+v)%65521;b=(a+b)%65521;
            }
        }
        *this>>((b<<16)|a);*this>>~crc;fwrite(d+22,1,12,f);
    }
    Png& operator<<(int b){crc=(crc>>8)^tab[(crc^fputc(b,f))&255];return *this;}
    Png& operator>>(unsigned int v) {return *this<<(v>>24)<<(v>>16)<<(v>>8)<<v;}
};

// example usage
int main(int argc, const char** argv) {
    int w = 640, h = 480;
    unsigned char* image = new unsigned char[3 * w * h];
    for (int y = 0, idx = 0; y < h; y++)
        for (int x = 0; x < w; x++, idx += 3)
            image[idx + 0] = (255 * x) / w,
            image[idx + 1] = (255 * y) / h,
            image[idx + 2] = 0;
    Png(argc < 2 ? "/tmp/test.png" : argv[1], w, h, image);
    delete [] image;
}