Re: [openMSX-devel] Blending

[Wouter V. <wou...@pi...>]

On Tue, 29 Apr 2003, Maarten ter Huurne wrote:

> Hi,
>
> I want to improve the handling of SCREEN6 border and sprite pixels. On SDLLo,
> two adjecent narrow pixels are blended into a single wide pixel to cope with
> the low resolution. Wouter created a nice blending method that I want to
> re-use.
>
> Currently the blending method is duplicated in CharacterConverter and
> BitmapConverter. Since SpriteConverter and SDLRenderer need it as well, it is
> a good idea to isolate the blending method. The blending method depends on a
> blending mask which is calculated once at initialisation. Even though that
> state is very small, I decided to make a class especially for Blending. I did
> something similar before with DisplayMode and that worked out fine.

Sounds like a good idea. The algorithm is small but, as you describe
below, not trivial. So it certainly makes sense to put it in a seperate
class.

> It took me a while to figure out how the algorithm worked, but once understood
> I think it's nice and efficient. However, it cannot handle the case where the
> most significant bit of a pixel is used: the result of the addition "colour1
> + colour2" is clipped before it is divided by 2. When the pixel format is
> 24bpp or 15bpp with the highest bits unused, this is not a problem. However,
> on big endian RGBA could have the lowest bits unused and the highest used,
> and 16bpp has all bits used. So the current blend implementation can fail for
> some pixel formats.
>
> I tried to correct this problem without changing the algorithm too much. I
> think I succeeded, but this is complex code, so I would like a second
> opinion. Wouter (or anyone else interested), please review the code. The
> Blender class is attached.

Looks ok to me.


Wouter