Re: [pygame] Using Numeric & Surfarray with alpha values

["Matthew"<mp31199@xxxxxxxxxxxxxxxxx>]

On Sat, 26 Nov 2005 18:28:16 -0500
 Mike Rotondo <mr001m@xxxxxxxxxxxxxxxxxx> wrote:
> We are writing a 2D particle engine and we would like to 
> render each particle with gradually decreasing alpha values. 
> However, our original image for the particles has per-pixel 
> alpha, so we can't use set-alpha. So, we are going to have to 
> modify each pixel's alpha value ith Numeric, it seems.
>
> We have a value for the time left for the particle to live, 
> named lifetime, and a value for how long it will live, named 
> decay. The value (lifetime/decay) is the percent of the 
> original alpha value we want to render each pixel with. So, 
> if a pixel in the original image has alpha 200, and a 
> particular particle has lifetime 50 and decay 100, we would 
> want to render that pixel in that particle with an alpha of 
> 200 * (50/100) = 100.
>
> We are trying to do this with numeric & surfarray, but are 
> having trouble with array types. The best we've been able to 
> do is to create a list comprehension which does the job:
>
> [[pixel and (pixel * particle.life / particle.decay) for pixl 
> in row] for row in array]
>
> This uses short circuit evaluation and the fact that booleans 
> return the last object they looked at to return either the 
> pixel's alpha value if it is 0, or the pixel's alpha value 
> times our percentage if it is not 0.
>
> array is pygame.surfarray.array_alpha(original_image)
>
> SO, my question is: What is the best way to translate the 
> above list comprehension into Numeric, so as to speed it up?
>
> Thanks,
> Mike Rotondo
I don't know but it sounds cool!
Sincerely, Matt