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[Charybdis]

Hi there
I am currently trying to set the blend system in opengles to render sprites as a white silhouette. Basically, I want to simply draw the alpha channel of a sprite. So, a pixel should be white where the src alpha is 1 and background colour when src alpha is 0. That is, I want it to completely ignore src colour.
It's turning out to be more difficult than I thought.
The problem is that no matter what I set for the first argument of glBlendFunc, it will always MULTIPLY the src colour.
So I thought the answer must lie with glBlendEquation or glBlendFuncSeparate. I have read the description of how they work but I just dont get it.
Can anyone help? I hope I have made the problem clear. Perhaps I should mention that I am also using premultipled alpha textures although I think it's irrelevant for this problem.
Cheers

[rivon]

I don't know OGL but isn't it possible, when loading the sprites, to just rewrite the color part with white and leave only the alpha unchanged?

[Christian Knudsen]

I don't know much about OpenGLES, so this might only be possible in regular OpenGL. But you can have a texture that's white, then draw the sprite with the alpha values onto this texture as an FBO, but setting glColorMask to only write to the alpha buffer. That'll leave you with the white texture with the alpha values from the sprite.

[Charybdis]

I don't know OGL but isn't it possible, when loading the sprites, to just rewrite the color part with white and leave only the alpha unchanged?

That is certainly esy to do, but my reason for wanting to do this is to flash the agent sprites white when taking damage, oldskool style. I dont want to have a duplicate textures for all agents in memory just for that.
I'm sure it must be possible with blend modes.

[Charybdis]

I don't know much about OpenGLES, so this might only be possible in regular OpenGL. But you can have a texture that's white, then draw the sprite with the alpha values onto this texture as an FBO, but setting glColorMask to only write to the alpha buffer. That'll leave you with the white texture with the alpha values from the sprite.

Good suggestion, thanks. But it seems like overkill for what I want to do.
I can't believe its so difficult. All I want to do is draw an alpha mask by fiddling with blend modes/equations.

[wademcgillis]

OpenGLES X.Y

What are the values of X.Y that you are using?

Edit:
Or, to elaborate:

If 1.X, use fog. GL_FOG_START should be zero GL_FOG_END should be zero. TADA! Instant flash to a color.

Code:

Enable fog.
Set start and end distances to zero.
Set color to whatever you want.
Set mode to whatever.
DRAW THE THING.
Disable fog.

If 2.X, use fragment shaders to change the colors of your image.

Code:

Enable shader and pass the color you want to it
DRAW THE THING.
Disable shader

[Xienen]

Wade has good suggestions for both versions of OpenGLES, but I can tell you that if you'd like to go the glBlend route for either of them, you'll definitely want to use glBlendFuncSeparate.  These parameters might work for you, but you'll have to test it: GL_ONE, GL_DST_COLOR, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA

[Evan Balster]

I will also nudge you towards writing a simple shader.  That's the proper way to approach GL programming these days when you're doing anything nontrivial.  No blend mode will do what you want for black source and black destination, as they always multiply by source and destination color by definition.

[Xienen]

I will also nudge you towards writing a simple shader.  That's the proper way to approach GL programming these days when you're doing anything nontrivial.  No blend mode will do what you want for black source and black destination, as they always multiply by source and destination color by definition.

Ah, Evan is correct.  There's no good way to do that with blend modes.

[Charybdis]

use fog

Brilliant! Why the hell didnt I think of that? You, sir, are a bloody genius. Cheers!!!!    

[Charybdis]

Nooooooooo!!
Fog not playing nice with premultiplied alpha
Waaaaagh!!! 

[Charybdis]

Phew, alpha test to save the day. Lucky I have no soft edges.