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I want to do a test for image brightness in the rendered image (bpy.data.images["Render Result"]). I would like to be able to see if the image is dark or light. An example output of the code could be The rendered image is: light

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You can access the pixel data directly through the Image.pixels property. It is a list of repeated RGBA values. To access a single pixel, use this:

img = bpy.data.images["myimage"]
img.pixels[(y * img.width + x) * 4 + channel] = value

where channel is 0 for red, 1 for green, 2 for blue and 3 for transparency.

If you wanted to compute, for example, the maximum blue amount of all pixels, Python magic can help you make it in one line:

bluemax = max(img.pixels[:][2::4])

Note that blue amount is number 2 out of 4 colors in total, therefore 2::4. In order to overcome a Blender API limitation, there are extra brackets [:] that convert image data to a classical list.

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    $\begingroup$ This just gives me 1.0 every time. $\endgroup$ – John Roper Nov 27 '16 at 19:19
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    $\begingroup$ The maximum value is often 1, and it means that at least one pixel of the whole is bright. If you want a more specific measure of brightness, be more specific in your question. $\endgroup$ – emu Nov 28 '16 at 14:03
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    $\begingroup$ So how to I calculate image brightness from this? $\endgroup$ – John Roper Nov 30 '16 at 1:09

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