Mail Archives: djgpp/1997/02/27/16:34:35
>questions about Allegro BITMAPS. First of all, with non screen
>memory BITMAPS, are the lines just pointers to an allocated
>peice of memory, or is each line allocated individually to
>different spots?
They are just offsets into a single block of memory. The only times when
the lines may not be contiguous in memory is if it is an SVGA screen
bitmap or a sub-bitmap of something else...
> Secondly, In the demo program I get 25 fps SVGA 640x480 page
>flipping and and double buffered in the same mode I get 32 fps.
>Does anyone know why I would get higher frame rates here double
>buffered?
My guess would be that you have a fairly fast CPU and a very slow
graphics card (are you using VESA 1.x mode, by any chance?). In such a
situation, the most expensive task is transfering data to the video
memory. With double buffered drawing, each pixel is written to video
memory exactly once, wheras with page flipping some areas get
overwritten (first they are cleared to black, then a sprite is drawn on
top of them). In my experience page flipping is usually faster, but if
video memory is a lot slower than main memory, double buffering will win
out...
> And does anyone have any ideas or hints on how to optimize
>the 640x480 SVGA mode? Thanks for your help.
The main 'trick' with any optimisation task is not to do any work that
isnt' required. Don't draw any more than you absolutely have to, and try
to make use of image coherency from one frame to the next to minimise
the amount of screen space that needs updating.
Getting more technical, use linear framebuffers if at all possible. Try
to draw large things rather than small (a 32 pixel wide horizontal line
is a lot faster than 32 putpixels). Blitting and drawing sprites is
faster if the destination X coordinate is a multiple of four. And if you
are using a banked video mode, things will go much faster if you draw
things in bank-order, ie. sort all your objects by Y position before the
draw. If you have 200 sprites at random positions on the screen, the
chances are that it will take nearly 200 bank switches to draw them all,
but if you draw all the ones at the top of the screen first, Allegro
will be able to output several without having to switch banks, which
saves a lot of time...
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* Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
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