What do the parameters of stb_png_to_mem mean?

2

I am using the stb libraries to load textures on the fly for OpenGL. I'm trying to use the function from stb_image_write.h. I cannot find any documentation on how to properly use the function stbi_write_png_to_mem my code looks like the following

#ifndef STB_IMAGE_WRITE_IMPLEMENTATION
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include <stb/stb_image_write.h> 

int main(  )
{
    // data is filled in later in program
    int size , width , height ; 
    unsigned char* data = ( unsigned char* ) malloc( width * height ) ;

    ...
    ...
    ...


    // this is where the issue lies
    stbi_write_png_to_mem(  ) ; 

}

I've looked on github and through the source itself. I cannot find anything on google. I am not sure what the inputs to this function is needing as nothing is clear what anything means. the source to the function is

STBIWDEF unsigned char *stbi_write_png_to_mem(const unsigned char *pixels, int stride_bytes, int x, int y, int n, int *out_len)
{
   int force_filter = stbi_write_force_png_filter;
   int ctype[5] = { -1, 0, 4, 2, 6 };
   unsigned char sig[8] = { 137,80,78,71,13,10,26,10 };
   unsigned char *out,*o, *filt, *zlib;
   signed char *line_buffer;
   int j,zlen;

   if (stride_bytes == 0)
      stride_bytes = x * n;

   if (force_filter >= 5) {
      force_filter = -1;
   }

   filt = (unsigned char *) STBIW_MALLOC((x*n+1) * y); if (!filt) return 0;
   line_buffer = (signed char *) STBIW_MALLOC(x * n); if (!line_buffer) { STBIW_FREE(filt); return 0; }
   for (j=0; j < y; ++j) {
      int filter_type;
      if (force_filter > -1) {
         filter_type = force_filter;
         stbiw__encode_png_line((unsigned char*)(pixels), stride_bytes, x, y, j, n, force_filter, line_buffer);
      } else { // Estimate the best filter by running through all of them:
         int best_filter = 0, best_filter_val = 0x7fffffff, est, i;
         for (filter_type = 0; filter_type < 5; filter_type++) {
            stbiw__encode_png_line((unsigned char*)(pixels), stride_bytes, x, y, j, n, filter_type, line_buffer);

            // Estimate the entropy of the line using this filter; the less, the better.
            est = 0;
            for (i = 0; i < x*n; ++i) {
               est += abs((signed char) line_buffer[i]);
            }
            if (est < best_filter_val) {
               best_filter_val = est;
               best_filter = filter_type;
            }
         }
         if (filter_type != best_filter) {  // If the last iteration already got us the best filter, don't redo it
            stbiw__encode_png_line((unsigned char*)(pixels), stride_bytes, x, y, j, n, best_filter, line_buffer);
            filter_type = best_filter;
         }
      }
      // when we get here, filter_type contains the filter type, and line_buffer contains the data
      filt[j*(x*n+1)] = (unsigned char) filter_type;
      STBIW_MEMMOVE(filt+j*(x*n+1)+1, line_buffer, x*n);
   }
   STBIW_FREE(line_buffer);
   zlib = stbi_zlib_compress(filt, y*( x*n+1), &zlen, stbi_write_png_compression_level);
   STBIW_FREE(filt);
   if (!zlib) return 0;

   // each tag requires 12 bytes of overhead
   out = (unsigned char *) STBIW_MALLOC(8 + 12+13 + 12+zlen + 12);
   if (!out) return 0;
   *out_len = 8 + 12+13 + 12+zlen + 12;

   o=out;
   STBIW_MEMMOVE(o,sig,8); o+= 8;
   stbiw__wp32(o, 13); // header length
   stbiw__wptag(o, "IHDR");
   stbiw__wp32(o, x);
   stbiw__wp32(o, y);
   *o++ = 8;
   *o++ = STBIW_UCHAR(ctype[n]);
   *o++ = 0;
   *o++ = 0;
   *o++ = 0;
   stbiw__wpcrc(&o,13);

   stbiw__wp32(o, zlen);
   stbiw__wptag(o, "IDAT");
   STBIW_MEMMOVE(o, zlib, zlen);
   o += zlen;
   STBIW_FREE(zlib);
   stbiw__wpcrc(&o, zlen);

   stbiw__wp32(o,0);
   stbiw__wptag(o, "IEND");
   stbiw__wpcrc(&o,0);

   STBIW_ASSERT(o == out + *out_len);

   return out;
}

My goal is to essentially output my picture data to a png in memory, not a file, so I can use it elsewhere in my program formatted like a png would be.

c++
opengl
png
textures
stb-image
asked on Stack Overflow Jan 9, 2020 by killer • edited Jan 9, 2020 by BDL

1 Answer

2

Most of the parameters are the same ones that are also passed to stbi_write_png which has a documentation in the header file:

  • pixels is the uncompressed image data
  • stride_bytes is the number of bytes between two the start of two consecutive rows:

    "stride_in_bytes" is the distance in bytes from the first byte of a row of pixels to the first byte of the next row of pixels.

  • x (called w in stbi_write_png) is the width of the image
  • y (called h in stbi_write_png) is the height of the image
  • n (called comp in stbi_write_png) is the number of channels:

    Each pixel contains 'comp' channels of data stored interleaved with 8-bits per channel, in the following order: 1=Y, 2=YA, 3=RGB, 4=RGBA.

  • out_len is a output parameter which returns the number of bytes the compressed data has.
answered on Stack Overflow Jan 9, 2020 by BDL

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