Flate png filters

[wrkhenddher]

#113

Flate PNG reverse filters: Up, Average and Paeth

[wrkhenddher]

Generating xxxx.png.log

Modify pngrutil.c from https://github.com/glennrp/libpng

(e.g. printf scanlines before and after if filtering was used)

void /* PRIVATE */
png_read_filter_row(png_structrp pp, png_row_infop row_info, png_bytep row,
    png_const_bytep prev_row, int filter)
{
   /* OPTIMIZATION: DO NOT MODIFY THIS FUNCTION, instead #define
    * PNG_FILTER_OPTIMIZATIONS to a function that overrides the generic
    * implementations.  See png_init_filter_functions above.
    */

    printf("width = %d\n", row_info->width);
    printf("bit_depth = %d\n", row_info->bit_depth);
    printf("channels = %d\n", row_info->channels);
    printf("color_type = %d\n", row_info->color_type);
    printf("pixel_depth = %d\n", row_info->pixel_depth);
    printf("rowbytes = %zu\n", row_info->rowbytes);
    printf("filter = %d\n", filter);
    printf("data = [ ");
    for (int i = 0; i < row_info->rowbytes; i++) {
        printf("0x%.2x,", row[i] & 0xff);
    }
    printf(" ]\n");
               
   if (filter > PNG_FILTER_VALUE_NONE && filter < PNG_FILTER_VALUE_LAST)
   {
      if (pp->read_filter[0] == NULL)
         png_init_filter_functions(pp);

      pp->read_filter[filter-1](row_info, row, prev_row);
   }
   else
       png_debug1(1, "no filter %d", filter);
    
    printf("expected = [ ");
    for (int i = 0; i < row_info->rowbytes; i++) {
        printf("0x%.2x,", row[i] & 0xff);
    }
    printf(" ]\n");
}

https://github.com/glennrp/libpng/blob/fc32eb1662d29a4467bd4fd7d3484d607507420f/pngrutil.c#L4110-L4125

Recompile and run.

For Mac OSX, https://github.com/IGRSoft/libpng-xcode-project is available if you prefer Xcode.

Visualizing Rasters (loaded from xxx.png.log files)

$ cd <..pdfrw>/tests
$ jupyter notebook Render\ Bitmap.ipynb 

The notebook will start on your browser.

[wrkhenddher]

Closes #113