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Transparency
Most of the time when you display a bitmap, you'd like the have some part of it appear
transparent. How else are you going to be able to see Jupiter when you fly your Vegan
starship over it? It would look like a rectangle with a ship in it flying over Jupiter
without transparency effects. What you need to do is set a certain colour to be the
transparent colour, thus removing the unsightly black rectangle, and leaving only the
colourful ship behind. I tend to use black, so that's what we'll do here.
"CKey" is defined as a DirectDraw Color Key. The "CKey" contains a "low" value and a "high"
value. This enables us to define a range of colours as transparent (if we want). For our
purposes a single colour will do fine: Black. We set "CKey.low" and "CKey.high" to vbBlack.
All we have to do after that is set the "Sprite.SetColorKey" method using the color key we've
created. Also the flag "DDCKEY_SRCBLT" indicates that we want to apply this key to the source
bitmap when we blt it. This will make more sense in a second:
BackBuffer.Blt DestRect, Sprite, SrcRect, DDBLT_KEYSRC Or DDBLT_WAIT
Now when we blt the surface we have to specify the "DDBLT_KEYSRC" flag to ensure that the color
key of the source (the "Sprite" surface) is considered transparent during the blt. That's all
there is to it!
Have a look at my DirectX 7 sample project to see it in action.
