Bug 61854

Summary:	[nv50]glClipPlane not clipping correctly for glsl
Product:	Mesa	Reporter:	bill
Component:	Drivers/DRI/nouveau	Assignee:	Nouveau Project <nouveau>
Status:	RESOLVED NOTOURBUG	QA Contact:
Severity:	normal
Priority:	medium
Version:	git
Hardware:	x86-64 (AMD64)
OS:	Linux (All)
Whiteboard:
i915 platform:		i915 features:
Attachments:	blender file demonstrating FF vs GLSL behavior of glClipPlane

Description bill 2013-03-05 12:54:53 UTC

Created attachment 75953 [details]
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

Comment 1 bill 2013-03-07 13:01:27 UTC

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?

Comment 2 Christoph Bumiller 2013-03-07 18:35:51 UTC

__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.

Comment 3 Christoph Bumiller 2013-03-07 19:43:17 UTC

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 undeﬁned,
which implies that the results of clipping to any client-deﬁned 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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