Re: [AD] al_set_target_bitmap question

[allefant <allefant@xxxxxxxxxx>]

On Jan 9, 2008 10:36 AM, Peter Hull <peterhull90@xxxxxxxxxx> wrote:
> I was imagining there might be problems if you loaded an image from a
> file and then tried to draw on it. If your main display was OpenGL you
> would have to upload the bitmap to the graphics card, draw using
> pbuffers or whatever, then get it back into main memory.
>
> Initially I thought that it was going to work like Java or GDI+, where
> you start with the memory bitmap, then create a display from it, draw,
> then flush/dispose of the display. Having looked at the current
> implementation, it seems that loading an image automatically asks the
> current display to allocate a bitmap, then copies the image to it.
>
> I know this was discussed at length before, I suppose I just haven't
> got my head around it yet.
>

Well, currently, there are two types of bitmaps: Memory bitmaps and
display bitmaps, the latter being roughly the same as video bitmaps in
A4. When you load an image, you get one of the two, by default a
display bitmap. This means users by default are free from having to
first load a memory bitmap for everything like in A4, just to then
create video bitmaps or AllegroGL textures out of it before drawing.

When drawing to bitmaps, whether it is a memory bitmap or not decides
if the display is involved (like e.g. a driver draws to it using GL
commands) or we use memory implementations to draw to it. And there
was no need for something like the GDI "graphics contexts", as we have
almost no state in Allegro. (We tried to create an alternate API with
an ALLEGRO_GRAPHICS structure in addition to ALLEGRO_DISPLAY and
ALLEGRO-BITMAP at one point, should still be on the wiki somewhere,
but it didn't seem useful.) The only state we have so far as far as I
can see is:

per-thread: display, blending
per-display: target
per-bitmap: clipping

I.e. basically the same as in A4, except for the display and target.
Likely most A5 apps will have a single display (window) only, so you
never care about the possibility to choose a different one. But if you
have multiple windows, it is very easy to do it. And the drawing
target means you don't specify a bitmap parameter to each drawing
operation, but the current target is used.