Nouveau - Mar 2013 - [Bug 61854] New: [nv50]glClipPlane not clipping correctly for glsl

https://bugs.freedesktop.org/show_bug.cgi?id=61854

          Priority: medium
            Bug ID: 61854
          Assignee: nouveau at lists.freedesktop.org
           Summary: [nv50]glClipPlane not clipping correctly for glsl
          Severity: normal
    Classification: Unclassified
                OS: Linux (All)
          Reporter: bill at taniwha.org
          Hardware: x86-64 (AMD64)
            Status: NEW
           Version: git
         Component: Drivers/DRI/nouveau
           Product: Mesa

Created attachment 75953
  --> https://bugs.freedesktop.org/attachment.cgi?id=75953&action=edit
blender file demonstrating FF vs GLSL behavior of glClipPlane

hw: NVIDIA Corporation G80 [GeForce 8800 GTS] (rev a2)


The problem was found using the alt-b clipping border option in blender.

In blender's default scene, press alt-b with the mouse in the 3d view and
"select" part of the cube. It gets clipped properly. However, turn on
matcap
(press N to get the properties panel, scroll down and open the display
sub-panel, and turn on "Matcap") and the clipping of the cube will
depend on
the viewer's position and angle with respect to the clipping planes.
However,
the highlighted wire-frame of the cube is always clipped correctly.

I was told by Ton (blender leader) that blender uses glClipPlane for the
clipping border, and that enabling matcap simply causes blender to use glsl for
the solid rendering.

Although I've included the url to the bug report in blender's bug
tracker, I've
also attached the same blend file to this report. For the blend file, turning
off matcap will cause the cube to clip correctly, independently of the
viewer's
position and angle. Zooming in a little (mouse wheel) with matcap enabled will
cause the cube's surface to become partially visible, but it will still be
incorrect.

Blender bug tracker url:
http://projects.blender.org/tracker/?func=detail&atid=231&aid=34492&group_id=9

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--- Comment #1 from bill at taniwha.org ---
Ok, thank's to calim's help, it turns out that blender wasn't
writing to
gl_ClipVertex. However, this is because __GLSL_CG_DATA_TYPES is not defined.
Should nouveau define it? Or is there another define that can be used in
blender's shader?

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--- Comment #2 from Christoph Bumiller <e0425955 at student.tuwien.ac.at>
---
__GLSL_CG_DATA_TYPES is an NV blob-specific feature related to Cg. How does
that have anything to do with gl_ClipVertex ? And no, there's no way
we're
going to define it with mesa.

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Christoph Bumiller <e0425955 at student.tuwien.ac.at> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |NOTOURBUG

--- Comment #3 from Christoph Bumiller <e0425955 at student.tuwien.ac.at>
---
blender's shader says:
"gl_ClipVertex works only on NVIDIA graphic cards so we have to check with
__GLSL_CG_DATA_TYPES"

The GL 2.0 spec says:
"If gl ClipVertex is not written by the vertex shader, its value is
unde?ned,
which implies that the results of clipping to any client-de?ned clip planes are
also undefined."

I don't know with which driver they tested this (maybe on a gallium driver
before gl_ClipVertex was handled, which came rather late ?), but they can't
expect undefined behaviour to work everywhere.

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