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Premultiplied Alpha: Creating/Loading Textures In Practice

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I've read the resources and everything I can find on this site and others about using premultiplied alpha textures. However, I can't find a good resource that says how to create/set the texture up! Do I need to create the textures with premultiplied alpha (something that appears to be either non-intuitive or difficult, and Photoshop is not available). Should I be doing the premultiply in code when I load the texture? Should I be rendering with a shader instead? Does DirectX handle it when I load the texture with the DXT2/4 values? (I understand those are disappearing in DX10, so that is probably not what I'm looking for; also because TomF says not to) I understand the rendering pipeline and how to set up the device/render states for premultiplied alpha, I just need to know how to get the texture to the proper state for it. My specifics: Using C# with Managed DirectX 9. Our current textures are TGA/PNG, generally with alpha channels. Trying to use premultiplied alpha textures for our particle systems (for fire/smoke/etc.,.) Again, I need a non-Photoshop solution. Resources I've already looked at: http://home.comcast.net/~tom_forsyth/blog.wiki.html#%5B%5BPremultiplied%20alpha%5D%5D http://www.td-grafik.de/ext/xfrog/alpha/index.html http://www.gamedev.net/community/forums/topic.asp?topic_id=428576

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First thing to realize is that you need 5 channels of information to fully utilize the potential of the premultiplied alpha format:
R, G, B = Color channels
A = Normal alpha blend channel, dest = lerp(alpha, source, dest)
Add = Additive, dest = dest + source * Add

Combining them equals the following equation (alpha blend first, then add):
dest = source * alpha + dest * invAlpha + source * add
or (add first, then alpha blend)
dest = (dest + source * add) * invAlpha + source * alpha

dest = source * (alpha + add) + dest * invAlpha
dest = dest * invAlpha + source (add * invAlpha + alpha)

For the first equation it's easy to see that (alpha + add) could be greater than one, wich could cause problems.
For the second version there's no way that the (add * invAlpha + alpha) could become greater than one, so I'd choose that version.

As for the textures, I would make a small tool that takes a standard alpha blend texture (RGBA) and an alpha only texture (optional) that controls the additive component as input and creates a new RGBA as output, i.e:

CoolFireTexture_rgba.tga <= Color and "normal" alpha blend channel.
CoolFireTexture_add.tga <= Greyscale containing additiveness.

for every pixel
temp = CoolFireTexture_add.a * (1 - CoolFireTexture_rgba.a) + CoolFireTexture_rgba.a;
output_rgba.r = CoolFireTexture_rgba.r * temp
output_rgba.g = CoolFireTexture_rgba.g * temp
output_rgba.b = CoolFireTexture_rgba.b * temp
output_rgba.a = CoolFireTexture_rgba.a

Just my 2c,
Disclaimer: I'm very tired and could be very wrong... :)

Edit: Spelling..

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Premultiplied alpha is just precomputing part of the alpha blending equation.

The equation starts out as C0 = C1 * A + C2 * (1 - A)

Since C2 (the background) is always changing and the A depends on what you're blending with, you can't really precompute that part of the equation.

However, since C1 and A are part of the same image, you can do that computation anytime beforehand and keep the value around and then the equation becomes C0 = C1p + C2 * (1 - A) and you save a tiny bit of computation.

It doesn't really matter when or where you premultiply the color values by the alpha - any time before rendering is "pre". It could be part of your loading code, or part of a preprocessing utility, or anywhere else you want.

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Thanks for the responses! I had a lot of success with this today. I'll try to remember to post some screenshots and code segments tomorrow to round out the thread.

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