[plt-scheme] alpha-colors in universe/world

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Wed Sep 30 20:41:12 EDT 2009

On Wed, Sep 30, 2009 at 5:35 PM, Jordan Johnson <jmj at fellowhuman.com> wrote:
> Hi all,
> One of my particularly enthusiastic students discovered the
> documentation for alpha-colors in the image.ss teachpack, and
> subsequently discovered that the image primitives (triangle, circle,
> ...) don't accept alpha-colors.
>  * Is there anything I can do to get his hands onto a set of primitives
>   that DO accept the alpha-colors and that are still compatible with
>   the HtDP approach?

He can write them! You'd compose a regular function, say the circle
function, with a conversion to alpha colors and the you'd multiply in
the alpha value you wanted and convert it back to a image.

Not exactly efficient, but perhaps more enlightening?

>  * Is this something that will likely change in the spiffy new version
>   that's coming out sometime in the next few months?

Yeah, sure. I can try to build that in this time (making it more
efficient). I've also considered a function that, given an image,
returns another image with a different transparency value (applied to
the whole image). I think that's possible in the low-level toolkit.
Thanks for the suggestion!

>  * If not either of those, can you suggest any answer I can give that's
>   better than "sorry, not possible"? :)
> This is a youngster I *really* would like to inspire to do neat things
> with DrScheme, since he's bright, highly motivated, and already pushing
> the limits of what we can do (or at least what limits I'm aware of) with
> the image libraries -- and politically, this would be a nice thing to
> get right, since a parent is a programmer/professor who is both highly
> involved with the school and highly skeptical of my use of DrScheme
> here.

Good luck with that. But, I have to say if it comes down to the
particular set of image functions that drscheme supports, then I'm not
sure I'd have a lot of respect for this parent/programmer/professor's
opinion. :(


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