Talk:Alpha compositing

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[edit] Why alpha?

In the German wp a user asked why the alpha channel was called alpha channel. Does anybody has got an idea? --PhilippWeissenbacher 17:51, May 14, 2005 (UTC)

My guess: RGBA color space Mion 16:28, 21 January 2006 (UTC)

I found a reference and added the information to RGBA color space. --130.234.180.167 23:02, 3 July 2006 (UTC)


[edit] Merger

[edit] For

  • Merge it. They're the same thing. Ayavaron 21:12, 10 September 2006 (UTC)
  • Merge it. The information in blending is basically the A Over B formula when B is opaque. Alancnet 10:26, 14 September 2006 (UTC)
  • Merge it. Neither article tells the whole story now. I do, however, think [Alpha Blending] is the better title -- many other uses of "composite" are applied to reversable or decomposable preocesses. I think of blending as producing an inseparable product. Rndmcnlly 23:42, 15 September 2006 (UTC)
  • Merge. In particular, the alpha blending article raises the important point that a single alpha channel is not the only option for this technique. The compositing article should mention this, and discuss the limitations of conventional RGBA compositing, such as inaccuracy in depictions of transparency in composites, especially with refractive or colored transparent elements.

[edit] Against

[edit] Required Details

This article is missing a lot of important information. Before I propose replacement, I'll throw out what I'd like to see included:

  • a separate article, Coverage (Computer Science), talking about representation of partially transparent images
  • a photorealistic example (showing the result of combining at least 3 layers (A over (B over C)))
  • refocus article on the general case of blending pixels, both with arbitrary coverage
  • clarification between discussion of blending whole images and the blending of two specific pixels
  • discussion of different formulations of the compositing operators (mention whether they are associative or not)
  • a small table of all possible operations (T. Porter and T. Duff, "Compositing Digital Images", SIGGRAPH 84, 253-259)
  • a Separate section about implementation that discusses using associativity to allow distribution/parallelization, 0-1-only alpha bitmaps, integer-only representations, lookup table usage, setting up glBlendFunc

-- Rndmcnlly 01:00, 16 September 2006 (UTC)

[edit] Possible error

According to Porter & Duff, the over operator solves to cO = cA*1+cB*(1-aA), which is equivalent to CO = (CA*aA+CB*aB*(1-aA))/aO, which is the second formula mentioned, not the first. The first formula, B' = F*a + B*(1-a), was mentioned in passing as being similar to the over function, but not equal. 64.229.30.184 05:54, 23 May 2007 (UTC)


sep


Hmm. I'm another reader, who finds the first equation in flat contradiction of the others (the later "premultiplied" rule and the "associative" rule seem to be equivalent, but the first equation looks just plain wrong). Also it's not made clear the relation between the "over" operation in the first section and the "Alpha Blending" section below. 212.44.20.129 (talk) 11:26, 20 May 2008 (UTC)

[edit] Confusion

RGB triple should instead be RGBA 4-tuple?


I find this line to be confusing: "Thus, assuming that the pixel color is expressed using RGB triples, a pixel value of (0.0, 0.5, 0.0, 0.5) implies a pixel which is fully green and has 50% coverage."

Seems like it should be (0.0, 1.0, 0.0, 0.5) and called a RGBA 4-tuple. I'm afraid to change it, because maybe I just don't get it!

24.106.89.6 12:43, 5 September 2007 (UTC)

I've added this: (Explanation: The RGB values are the first three values, (0, 0.5, 0) and the alpha value is the fourth, 0.5. If the color were fully green, its RGB would be (0, 1, 0). Since this pixel is using a premultiplied alpha, all of the RGB values in the ordered triplet (0, 1, 0) are multiplied by 0.5 and then the alpha is added to the end to yield (0, 0.5, 0, 0.5). ) --Rajah (talk) 15:45, 5 May 2008 (UTC)