[plt-scheme] 2htdp/image

On Mon, Nov 16, 2009 at 9:28 PM, Todd O'Bryan <toddobryan at gmail.com> wrote:
>
> I think I'm confused. If all bitmaps are images, but equality is done
> cleverly, what will happen if you do
>
> (equal? (overlay BITMAP (line 20 30 "black"))
>        (add-line BITMAP 0 0 20 30 "black"))
>
> And if you have to handle bitmaps anyway, how is color-list->image or
> any of the crop functions any more problematic?
>
> In other words, suppose I make an image in DrScheme and save it as a
> PNG. When I read it back in, I've lost all the image-y information, so
> is it only equal to another bitmap that's pixel-identical, but not to
> an image that's pixel-identical (say the one I saved)?
>
> Sorry to be dense, but I'm just trying to understand how this is going
> to work when you start mixing user-created images and images loaded
> from outside the environment.
>
> Todd

Todd,

There are a lot of images that are pixel-wise equal in this system,
but not image-equal, even without resorting to bitmaps.  Two images
that are equal are guaranteed to continue to be equal after applying
(the same) scale and rotation operations to them; if you fix something
to a bitmap and then apply scaling or rotation, you will get a
different result, however.  So by differentiating bitmaps from other
images, we lose some aspects of "ideal" image equality, but gain
others.

--Carl