I'm making a script that determines the color of a face, based on the texture. I have an object that is UV unwrapped and has a texture assigned. What I would like to do is find the pixels that are in the UV space of a certain face.

How to find the pixels covering the UV space of the selected face?

I have something that works, but it is terribly slow. For every face I loop over all pixels of the texture image and determine if they are inside the UV space of the face. Not the best method I think :D

In the Python API documentation I found something about MeshTextureFace and this seems exactly what I need. But I have no idea how I could use this.

Any ideas?


1 Answer 1


By the sounds of it, you just need to optimize what you have.

Some suggestions.

  • Don't check every pixel, at the very least calculate a bounding box around the face's UV's and only check those pixels.
  • Rather then do a point-in-polygon test for each pixel. Use a scan-line method. See: Darel Rex Finley's Efficient Polygon Fill, Or Blenders version of it fill_poly_v2i_n

Note that being a scripting language Python isn't ideal for these tasks - but it can likely be sped up enough to be usable as long as you don't need real-time feedback.

Update, made a more efficient version of Darel Rex's algorithm, see link.
(avoids looping over all coordinates for each y-pixel).

  • $\begingroup$ Thanks Campbell, I will try to use the method of Darel Rex Finely. So there is no 'easy' way? No access to some already existing functionality in Blender I could use? $\endgroup$
    – jasperge
    Dec 6, 2013 at 23:14
  • $\begingroup$ @jasperge, no, generally per-pixel operations are not very good to do in Python, so we dont provide helper functions for these kinds of tasks. It shouldn't be too hard to port fill_poly_v2i_n to Python (think it reads a little easier then the original). $\endgroup$
    – ideasman42
    Dec 7, 2013 at 2:52
  • $\begingroup$ Sorry, for slow reaction. I finally got around to implement the fill_poly_v2i_n. Together with some other optimizations (still no bounding box), the processing time went from a few hours to a few minutes! $\endgroup$
    – jasperge
    Feb 19, 2014 at 12:36
  • $\begingroup$ @jasperge would be good if you could post the code here, or at least a link to it. $\endgroup$
    – ideasman42
    Feb 19, 2014 at 13:21
  • $\begingroup$ Indeed it would. But the problem is it's too much tied in with the rest of the code. I will see if I can get the core part out and post it here. $\endgroup$
    – jasperge
    Feb 19, 2014 at 13:30

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