Re: [sdl.web@gmail.com: Re: problem with transparent PNG image display]

[Richard Stallman <rms@gnu.org>]

[I sent this message a week ago but did not get a response.
Could we get the discussion moving again?]

    When no background color is saved in the image, we render using the
    background color of the face the image is first drawn with, so the
    problem is not immediately apparent with those images, but if you draw
    the same image object onto a different colored background, you will see
    the same problem occur.

The cache could record what background color was used for rendering.
Then, on displaying the image again, if the desired background is
different, that could cause a cache-mismatch, which would cause
re-rendering.

For efficiency, we should do that only for formats that allow
transparency.

Is it possible to tell easily whether a png image has any
transparency?  Then we could optimize the images that don't
have any transparency by not recording a background for it.

    I don't know which platforms will let us cache image objects with
    transparency, but the solution would be to do that, and draw the
    background separately each time the image is drawn.

There must be something I don't get.  What would stop us from caching
the actual image data, transparent or not?

Is the idea that we want to save the time to re-render the image?