Name EXT_depth_clamp Name Strings GL_EXT_depth_clamp Contact Gert Wollny (gert.wollny 'at' collabora.com) Notice Copyright (c) 2019 Collabora LTD Copyright (c) 2009-2013 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Status Complete. Version Version 1, 2019/01/24. Based on ARB_depth_clamp version 4, modified 2009/08/02. Number #309 Dependencies OpenGL ES 2.0 is required. Written based on the wording of the OpenGL ES 3.2 specification. Overview Conventional OpenGL clips geometric primitives to a clip volume with six faces, two of which are the near and far clip planes. Clipping to the near and far planes of the clip volume ensures that interpolated depth values (after the depth range transform) must be in the [0,1] range. In some rendering applications such as shadow volumes, it is useful to allow line and polygon primitives to be rasterized without clipping the primitive to the near or far clip volume planes (side clip volume planes clip normally). Without the near and far clip planes, rasterization (pixel coverage determination) in X and Y can proceed normally if we ignore the near and far clip planes. The one major issue is that fragments of a primitive may extend beyond the conventional window space depth range for depth values (typically the range [0,1]). Rather than discarding fragments that defy the window space depth range (effectively what near and far plane clipping accomplish), the depth values can be clamped to the current depth range. This extension provides exactly such functionality. This functionality is useful to obviate the need for near plane capping of stenciled shadow volumes. The functionality may also be useful for rendering geometry "beyond" the far plane if an alternative algorithm (rather than depth testing) for hidden surface removal is applied to such geometry (specifically, the painter's algorithm). Similar situations at the near clip plane can be avoided at the near clip plane where apparently solid objects can be "seen through" if they intersect the near clip plane. New Procedures and Functions None New Tokens Accepted by the parameter of Enable, Disable, and IsEnabled, and by the parameter of GetBooleanv, GetIntegerv, GetFloatv: DEPTH_CLAMP_EXT 0x864F Additions to Chapter 12 of the OpenGL ES 3.2 Specification (Fixed-Function Vertex Post-Processing) -- Section 12.5 "Primitive Clipping" Add to the end of the 1st paragraph: "Depth clamping is enabled with the generic Enable command and disabled with the Disable command. The value of the argument to either command is DEPTH_CLAMP_EXT. If depth clamping is enabled, the "-w_c <= z_c <= w_c" plane equation are ignored by view volume clipping (effectively, there is no near or far plane clipping)." Additions to Chapter 15 of the OpenGL ES 3.2 Specification (Writing Fragments and Samples to the Framebuffer) -- Section 15.1.3 "Depth buffer test" Add to the end of the 2nd paragraph: "If depth clamping (see section 12.15) is enabled, before the incoming fragment's z_w is compared z_w is clamped to the range [min(n,f),max(n,f)], where n and f are the current near and far depth range values (see section 12.6.1)." Additions to the AGL/GLX/WGL Specifications None GLX Protocol None Errors None New State Add to table 6.4, transformation state Get Value Type Get Command Initial Value Description Sec -------------- ---- ----------- ------------- -------------- ------ DEPTH_CLAMP_EXT B IsEnabled False Depth clamping 12.5 on/off New Implementation Dependent State None Issues See the issue list in GL_ARB_depth_clamp. Can fragments with w_c <=0 be generated when this extension is supported? RESOLUTION: No. The inequalities in OpenGL ES Specification 12.5 clarify that only primitives that lie in the region w_c >= 0 can be produced by clipping and the vertex normalization in 12.6 clarifies that values w_c = 0 are prohibited. Hence fragments with w_c <= 0 should also never be generated when this extension is supported. How does this extension differ from ARB_depth_clamp? Instead of DEPTH_CLAMP the parameter is called DEPTH_CLAMP_EXT. Push/pop attrib bits are not relevant for OpenGL ES. Revision History Version 1, 2019/01/25 (Gert Wollny) - rewrite ARB_depth_clamp against OpenGL ES 3.2 instead of OpenGL 3.1.