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richgel999's profile
Rich Geldreich
Rich Geldreich
Rich Geldreich
@richgel999

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Rich Geldreich

@richgel999

Entrepreneur at http://Binomial.info , Khronos member working on Basis. Ex-Valve/MS,Age1DE,3/Halo Wars/Portal2/DotA2/CS:GO.

Portland, OR
sites.google.com/site/richgel99/
Joined March 2011

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    1. Rich Geldreich‏ @richgel999 Nov 3

      BC7 has a weakness: There’s only a single alpha mode that supports 2 subsets (mode 7). And it assumes alpha is correlated with RGB. The other alpha modes are only 1 subset modes, so they can’t handle complex RGB blocks well.

      1 reply 3 retweets 11 likes
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    2. Rich Geldreich‏ @richgel999 Nov 3

      So if the block has uncorrelated alpha and is complex enough in RGB, there’s not a lot you can do. Either RGB suffers or alpha suffers.

      2 replies 1 retweet 1 like
      Show this thread
    3. Rich Geldreich‏ @richgel999 Nov 3

      Also: Modes 4 and 5 support component rotations, allowing one of the RGB components to be encoded seperately. This results in different looking artifacts vs. all other modes.

      1 reply 1 retweet 1 like
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    4. Rich Geldreich‏ @richgel999 Nov 3

      So as a developer using BC7, you need to be very careful what you decide to put into the alpha channel. Ideally it should be correlated (resemble strongly) RGB. It’s not really a fully separate channel like DXT5/BC3.

      2 replies 1 retweet 1 like
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    5. Rich Geldreich‏ @richgel999 Nov 3

      Our BC7 encoders will handle uncorrelated alpha better than the open source ones. We’re putting a lot of effort into this.

      2 replies 1 retweet 3 likes
      Show this thread
    6. bmcnett‏ @bmcnett Nov 3
      Replying to @richgel999

      it's not as bad as you describe, because each block can decide which channel is independent. with bc3 it's always the same channel that gets to "be alpha"

      1 reply 0 retweets 1 like
      Rich Geldreich‏ @richgel999 Nov 3
      Replying to @bmcnett

      First, this is exactly the feature that I'm seeing some AAA developers disable (mode 4/5 component rotation) because it leads to different artifacts vs. the other modes. Even with rotation support, existing BC7 encoders are weak with uncorrelated alpha: http://richg42.blogspot.com/2018/10/bc7-with-decorrelated-alpha.html …

      9:37 AM - 3 Nov 2018
      • 1 Retweet
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      • TriΔng3l🔺
      1 reply 1 retweet 1 like
        1. New conversation
        2. Rich Geldreich‏ @richgel999 Nov 3
          Replying to @richgel999 @bmcnett

          This problem is easily reproduced: just put in different non-trivial uncorrelated signals into RGB vs. A and watch the open source BC7 encoders trash alpha. I've been seeing this for months but I haven't had time to investigate it until now.

          2 replies 0 retweets 2 likes
        3. Rich Geldreich‏ @richgel999 Nov 3
          Replying to @richgel999 @bmcnett

          What's actually happening is the alpha signals developers are using with BC7 are either relatively simple or are highly correlated vs. RGB. If you stray too far then either RGB or alpha quality suffers. I found this while exhaustively testing and benchmarking existing encoders.

          1 reply 0 retweets 1 like
        4. bmcnett‏ @bmcnett Nov 3
          Replying to @richgel999

          games I'm familiar with moved on from the concept of rgb vs alpha in textures years ago, and treat them as a bag of channels.

          1 reply 0 retweets 2 likes
        5. Rich Geldreich‏ @richgel999 Nov 3
          Replying to @bmcnett

          Yea, I hear you, but BC7 and the encoders don't look at texture data that way. There are tons of assumptions made. BC7 unfortunately doesn't support 3-4 separate encoding planes. If you ignore this and just jam in whatever something's going to get trashed.

          1 reply 0 retweets 1 like
        6. Rich Geldreich‏ @richgel999 Nov 3
          Replying to @richgel999 @bmcnett

          If you do treat your textures as a bag of channels, you should try to intelligently reorder the channels. Try to ensure that RGB is correlated and that each block's RGB vectors lie along 1-2 lines, etc.

          1 reply 0 retweets 1 like
        7. Sebastian Aaltonen‏ @SebAaltonen Nov 3
          Replying to @richgel999 @bmcnett

          If you pack some fully independent data to alpha channel and need consistent high quality result for that data, you choose BC3. This is where I would still choose BC3 over BC7. Not a problem since all GPUs supporting BC7 support BC3 as well. No shader/codee changes needed.

          2 replies 0 retweets 8 likes
        8. Rich Geldreich‏ @richgel999 Nov 3
          Replying to @SebAaltonen @bmcnett

          Yes, exactly. Except now your RGB data only has a single subset (leading to blocky artifacts). But at least your alpha channel doesn't get mangled.

          1 reply 0 retweets 3 likes
        9. Rich Geldreich‏ @richgel999 Nov 3
          Replying to @richgel999 @SebAaltonen @bmcnett

          Other options: Configure your BC7 encoder to only use modes 4,5, which may give you higher quality (due to component rotation). Or, modify your BC7 encoder to favor modes 4,5 more often.

          1 reply 2 retweets 4 likes
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