I'm using the AGG backend to output graphs as PNG files, but I have had
users complain that the resulting files are too large (the images are
being served via a web server).
The graphs are simple line graphs with less than 8 colours, so I'm
trying to change src/_backend_agg.cpp to output a 4 bit per pixel PNG
using PNG_COLOR_TYPE_PALETTE, rather than the default
PNG_COLOR_TYPE_RGB_ALPHA. The code currently looks like this (the
changes I have made have "PHB" in the comments):
src/_backend_agg.cpp, line 2077:
// PHB inserted code start
info_ptr->valid |= PNG_INFO_PLTE;
info_ptr->palette = (png_color*)malloc(256 * sizeof (png_color));
info_ptr->num_palette = 256;
// PHB inserted code end
if (setjmp(png_ptr->jmpbuf)) {
if (fpclose) fclose(fp);
png_destroy_write_struct(&png_ptr, &info_ptr);
delete [] row_pointers;
throw Py::RuntimeError("Error building image");
}
png_init_io(png_ptr, fp);
png_set_IHDR(png_ptr, info_ptr,
width, height, 4, // PHB Changed from 8 BPP
PNG_COLOR_TYPE_PALETTE, PNG_INTERLACE_NONE, // PHB
changed from PNG_COLOR_TYPE_RGB_ALPHA
PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
// this a a color image!
sig_bit.gray = 0;
sig_bit.red = 4; // PHB Changed from 8 to 4
sig_bit.green = 4; // PHB Changed from 8 to 4
sig_bit.blue = 4; // PHB Changed from 8 to 4
/* if the image has an alpha channel then */
sig_bit.alpha = 0; // PHB Changed from 8 to 0
This doesn't crash, but it outputs a stretched image - it looks like the
pixels that make up the image have zero pixels in between. I've had a
look through the libpng documentation here:
http://www.libpng.org/pub/png/libpng-1.2.5-manual.html#section-4.1 but
there isn't a lot of detail about how to set up palettes, etc. Does
anyone have any advice for me on how to get this working?
Cheers
Peter
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