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Optimizing alpha channels in directDraw


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#1 Despotismo   Members   -  Reputation: 122

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Posted 16 January 2000 - 08:47 AM

As long as the directDraw developing team decides not to support alpha-channels, we have to make the blending functions by ourselves. I''ve already have it working, but it''s panifully slow (it halves my fps). I''ve read that there is a way to use D3D to simulate 2D alpha blending. Any ideas?

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#2 Ro   Members   -  Reputation: 122

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Posted 16 January 2000 - 09:55 AM

You have to enable the alpha blending rendering state in the D3D device, and set the alpha blending functions. And you have to create a texture with an alpha channel. I''m still trying to figure out how to create that extra channel when I copy the bitmap to the DirectDraw surface. I assume I can''t just grab a DC from the surface and bitblt the HBITMAP to the device surface like the example IM programs do. Does anyone know if D3D allows us to just say what the alpha value is for the entire surface, or do we have to set the alpha value for each of the texels? I would rather have it just use a constant alpha value throughout the surface if possible.

#3 alexmoura   Members   -  Reputation: 450

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Posted 16 January 2000 - 10:07 AM

Know what ya mean... By the way how much alpha blitting are you going to do? If you don''t have huge surfaces wich need alpha blending, be sure to use bltfast or blt, and stick to using home-made routines only when absolutely necessary.
From what i understood from D3D, you could render a polygon with your texture onto any surface (with alpha), be it another texture or the backbuffer... dom''t know what it takes to make the projection look loke a flat image though.

#4 Despotismo   Members   -  Reputation: 122

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Posted 16 January 2000 - 11:47 AM

I am actually using home-made alpha blitters, but I need to make a considerably high quantity of alpha blends (a strategy´s game "fog of war" limits). With palettes It was quite easy and fast, but in nonpalettized modes, it´s painfully slow to use home made blitters. I´ve no idea about d3d, but the main idea would be to create a texture with the backbuffer, another one with the alpha surface, put them altogether and render them into the backbuffer?
Any code?

#5 Bracket   Members   -  Reputation: 240

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Posted 16 January 2000 - 02:58 PM

For a purely "2D approach" to alpha blending, check out this article on GameDev - its pretty fast: http://www.gamedev.net/reference/programming/graphics/article817.asp

If you want to invoke D3D (and you know you do.... all those 3D enhanced cards out there shouldn''t go to waste!), its actually pretty easy. Basically, make sure the pattern you want is loaded as a texture with its alpha channel (either created at run time or saved into the bitmap). You''ll have needed to acquire a Direct3D device, too. For each frame, put the bounds of the shape you want to fill into D3DTLVERTEX structures, which means you simply specify screen coordinates - no transformation or lighting overhead at all. Call BeginScene. Make sure you set the appropriate texture, one call to DrawPrimitive and then call EndScene. There are various render states you may want to play with, too, to ensure that you get exactly the effect you want. Its really nice and fast. Tobias Lensing''s page even supplies sample code for this (in the context of 2D):
http://members.xoom.com/dxfaq/

DirectX7 also includes library functions that pretty much do this for you!


#6 Despotismo   Members   -  Reputation: 122

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Posted 17 January 2000 - 11:26 AM

Thank you! This was exactly what I needed!




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