Mail Archives: djgpp/1997/01/25/16:00:06
frenchc writes:
>wonderful, but does anybody know how to keep sprites from clipping when
>you rotate them? I assume this is just a limitation (?feature) of the
>asm routine or algo used (I don't really know rotational algos except
You are right - its a bug/feature that results from the rotation
algorithm I'm using.
My technique is to map straight lines from the source bitmap onto other
straight lines across the destination bitmap, and then trace along these
lines copying each pixel (once you've set up the direction vectors, it's
a simple linear interpolation). Logically I should trace horizontal
lines through the source image, but this would result in some
destination pixels being missed due to rounding errors, and would play
havoc with SVGA bank switching efficiency. So instead I trace horizontal
lines through the destination bitmap, mapping them onto arbitrary lines
through the source. At the moment I just scan over an area of the
destination the same size as the source image, so any parts of the
source that get rotated outside this rectangle are missed.
The way to fix this would be to expand the destination region covered by
the scan, depending on the size of the source image. I _think_ this will
be fairly easy to do - I'll have a play with it and see if I can make it
work without too much overhead.
I've also heard mention of a rotation algorithm based on shearing the
image first horizontally and then vertically, which can rotate by up to
45 degrees (flips can then be used for the other 7 octants). I've never
tried implementing this: does anyone have any experience of it? Could it
be faster than my line-tracing method?
/*
* Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
* Ghoti: 'gh' as in 'enough', 'o' as in 'women', and 'ti' as in 'nation'.
*/
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