Re: [pygame] Blitting to alpha channel or using another image's alpha channel

[Lenard Lindstrom <len-l@xxxxxxxxx>]

Weeble wrote:
> Can somebody suggest how to do this? I want to render parts of an
> image based on a dynamically generated mask. Right now I'm doing this
> by drawing the mask in black and white on another surface, then
> iterating over every pixel and changing the alpha value of the source
> image to match the red value of the corresponding pixel in the mask
> surface, then finally blitting this onto the target surface. This
> seems a bit convoluted and I get the feeling there's probably a more
> efficient way to do it. Is there a way either to 1. blit an 8-bit
> greyscale image directly into the alpha channel of an image, or 2.
> directly blit image A onto image B using image C as the alpha channel?
>   
You can use the BLEND_RGBA_MIN flag with a surface blit. Make sure the 
mask is all black and that all the target surface pixels have an alpha 
of 255. Before that was available (Python 1.8.1?) there was the 
surfarray module. surfarray.pixels_alpha lets one get at the alpha plane 
of a surface and manipulate it with array operations. See the shadowed 
effects in the Pygame cookbook for an example:

http://www.pygame.org/wiki/ShadowEffects?parent=CookBook

But you need either NumPy or Numeric for this to work.

--
Lenard Lindstrom
<len-l@xxxxxxxxx>