opencv保存png alpha通道的办法

2011/10/14 10:43
阅读数 2.4K

We are currently facing the situation that we need to process PNG files that contain an alpha channel. Unfortunately OpenCV curently does not support so (but there’s a hack – but that didn’t help in my situation). The documentation for cvLoadImage contains a sad little note:

Note that in the current implementation the alpha channel, if any, is stripped from the output image, e.g. 4-channel RGBA image will be loaded as RGB.

Diving into the code I figured alpha channels have been disabled on purpose. Around line 220 in grfmt_png.cpp (the respective PNG encoder / decoder) it reads: png_set_strip_alpha( png_ptr ).

So the hope would be, if one disables this line libpng should be able to read in the alpha channel. Indeed it does, if you specify  a large enough buffer to write in (CV_8UC4 instead of CV_8UC3). So we need  a few more patches to make it work throughout:
1. In grfmt_ png.cpp, line 170 add  a line that allows to push through the type (alpha channel) detection: 
// Allow for PNG alpha channel here
if(color_type == PNG_COLOR_TYPE_RGB_ALPHA) { m_type = CV_8UC4;
}

2. In grfmt_png.cpp, around line 220 allow for an alpha channel by commenting the following statements:

//png_set_strip_alpha( png_ptr );
3. In loadsave.cpp , line 222 we finally tweak the parent routine to be more flexible towards the channels returned. Here we replace the “3″ by CV_MAT_CN(type):
type = CV_MAKETYPE(CV_MAT_DEPTH(type), CV_MAT_CN(type));
(Note all changes were performed against  OpenCV 2.0)

After a recompile you should be able to read alpha-channeled pngs as well!

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