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GR_CLIP_COORDS Inverted Axis

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First post, by lizard78

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Hi there,
Not sure if bug reports are welcome here, if not please feel free to disregard. I noticed a problem with dgvoodoo which I believe has to do with the use of homogeneous coordinates in Glide 3.x. I grabbed the latest version from github. Basically the Y axis gets flipped, not really sure why this happens but it definitely doesn't happen on a real Voodoo3. Example vertex configuration:
grCoordinateSpace(GR_CLIP_COORDS);
grVertexLayout(GR_PARAM_XY, 0, GR_PARAM_ENABLE);
grVertexLayout(GR_PARAM_Z, 8, GR_PARAM_ENABLE);
grVertexLayout(GR_PARAM_W, 12, GR_PARAM_ENABLE);
grVertexLayout(GR_PARAM_RGB, 16, GR_PARAM_ENABLE);
grVertexLayout(GR_PARAM_A, 28, GR_PARAM_ENABLE);

Window is set to GR_ORIGIN_LOWER_LEFT. I can provide more details if needed.

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Reply 1 of 4, by Dege

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Hi!

How could I test/reproduce it?
I modified and tried a sample app from the Glide3 SDK using clip space coords to do that with GR_ORIGIN_LOWER_LEFT, but the rendering didn't get inverted vertically.
It only should if the height parameter from grViewPort is negative.

But if the result is different from a real Voodoo then it's apparently a problem.

Reply 2 of 4, by lizard78

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Hi Dege,
I put together a quick demo scene for you and tested it (working) on a Voodoo 2. Attached it to this post. Hopefully you are able to log the Glide calls and notice something interesting, because it would be difficult for me to extract the code needed for a minimal repro. I'm still not 100% sure what causes this - all I know is inverting the Y axis for the clip coords seems to fix it (but gives disastrous results on the real voodoo 😁).

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  • ex2.png
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  • Filename
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