[SDL] Somethng wrong with my scaling function?

[Jonathan Dearborn]

Hey,

I think something is wrong with this line:
src_y = (y * src_w) * scale_y;
Here, you're scaling the source y-coord, but you should be more careful.  The part in parentheses is giving you an integer that represents the index of the beginning of a line.  Then you scale this index by a double.  That will end up giving you fractions of a line as the apparent beginning of the line.  You would probably be best off by scaling the y-coordinate, rounding or flooring it, then finding the start of the line.
Try replacing that line similar to this:
src_y = int(y * scale_y) * src_w;


Good luck,
Jonny D



> From: mir_ctx at imap.cc
> To: sdl at lists.libsdl.org
> Date: Fri, 14 Dec 2007 08:29:12 -0800
> Subject: [SDL] Somethng wrong with my scaling function?
> 
> Hi.
> 
> I'm not sure if this problem is due to misunderstanding of
> the way SDL works or just a stupid error on my part. For
> some reason, this vanilla "nearest neighbour" scaling function
> I've written correctly scales in the X direction but not in
> the Y direction (the image appears interlaced or totally 
> corrupted with fractional scaling values):
> 
> void
> scale(SDL_Surface *dst, const SDL_Surface *src, double scale_x, double
> scale_y)
> {
>   unsigned int dst_w = dst->w;
>   unsigned int dst_h = dst->h;
>   unsigned int src_w = src->w;
>   unsigned int x;
>   unsigned int y;
>   unsigned int src_y;
>   unsigned int src_x;
>   unsigned int dst_y;
>   uint32 *dst_pix = dst->pixels;
>   uint32 *src_pix = src->pixels;
> 
>   scale_x = 1 / scale_x;
>   scale_y = 1 / scale_y;
> 
>   for (y = 0; y < dst_h; ++y) {
>     src_y = (y * src_w) * scale_y;
>     dst_y = y * dst_w;
>     for (x = 0; x < dst_w; ++x) {
>       src_x = x * scale_x;
>       dst_pix[dst_y + x] = src_pix[src_y + src_x];
>     }
>   }
> }
> 
> 
> I have made sure that dst is large enough to hold the scaled
> pixels and that both surfaces are in 32 bit RGBA format.
> 
> Is there some glaring error here?
> 
> Any help appreciated.
> -- 
>   
>   mir_ctx at imap.cc
> 
> -- 
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