| Name |
| |
| NV_conservative_raster |
| |
| Name Strings |
| |
| GL_NV_conservative_raster |
| |
| Contact |
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| Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com) |
| |
| Contributors |
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| Michael Chock, NVIDIA Corporation |
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| Status |
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| Shipping. |
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| Version |
| |
| Last Modified Date: March 27, 2015 |
| Revision: 3 |
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| Number |
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| OpenGL Extension #465 |
| OpenGL ES Extension #228 |
| |
| Dependencies |
| |
| This extension is written against the OpenGL 4.3 specification |
| (Compatibility Profile) but may be used with the Core profile or |
| OpenGL ES 2.0 or later. |
| |
| When this extension is used with the Core profile or an OpenGL ES |
| context, references to functionality specific to the Compatibility |
| Profile can be ignored. |
| |
| Overview |
| |
| This extension adds a "conservative" rasterization mode where any pixel |
| that is partially covered, even if no sample location is covered, is |
| treated as fully covered and a corresponding fragment will be shaded. |
| |
| A new control is also added to modify window coordinate snapping |
| precision. |
| |
| These controls can be used to implement "binning" to a low-resolution |
| render target, for example to determine which tiles of a sparse texture |
| need to be populated. An app can construct a framebuffer where there is |
| one pixel per tile in the sparse texture, and adjust the number of |
| subpixel bits such that snapping occurs to the same effective grid as when |
| rendering to the sparse texture. Then triangles should cover (at least) |
| the same pixels in the low-res framebuffer as they do tiles in the sparse |
| texture. |
| |
| |
| New Procedures and Functions |
| |
| void SubpixelPrecisionBiasNV(uint xbits, uint ybits); |
| |
| New Tokens |
| |
| Accepted by the <cap> parameter of Enable, Disable, IsEnabled: |
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| CONSERVATIVE_RASTERIZATION_NV 0x9346 |
| |
| Accepted by the <pname> parameter of GetBooleanv, GetDoublev, |
| GetIntegerv, and GetFloatv: |
| |
| SUBPIXEL_PRECISION_BIAS_X_BITS_NV 0x9347 |
| SUBPIXEL_PRECISION_BIAS_Y_BITS_NV 0x9348 |
| MAX_SUBPIXEL_PRECISION_BIAS_BITS_NV 0x9349 |
| |
| Additions to Chapter 13 of the OpenGL 4.3 (Compatibility Profile) Specification |
| (Fixed-Function Vertex Post-Processing) |
| |
| Modify subsection 13.6.1 "Controlling the Viewport", p. 469 |
| |
| Add after the formula for the vertex's window coordinates: |
| |
| The vertex's window x and y coordinates may be optionally converted |
| to fixed-point values with <N> fractional bits. If CONSERVATIVE_- |
| RASTERIZATION_NV is disabled (see section 14.6.X), then <N> is the |
| implementation-dependent value of SUBPIXEL_BITS. If CONSERVATIVE_- |
| RASTERIZATION_NV is enabled, <N> is computed as the sum of the value of |
| SUBPIXEL_BITS and programmable values of SUBPIXEL_PRECISION_BIAS_{X,Y}- |
| _BITS_NV. These values may be set with the command |
| |
| void SubpixelPrecisionBiasNV(uint xbits, uint ybits); |
| |
| When these values are non-zero, the invariance requirement of section 14.2 |
| may not apply because the subpixel precision may not be the same at all |
| window coordinates. The initial values of SUBPIXEL_PRECISION_BIAS_{X,Y}- |
| _BITS_NV are zero. If <xbits> or <ybits> are greater than the value of |
| MAX_SUBPIXEL_PRECISION_BIAS_BITS_NV, the error INVALID_VALUE is generated. |
| |
| |
| Additions to Chapter 14 of the OpenGL 4.3 (Compatibility Profile) Specification |
| (Fixed-Function Primitive Assembly and Rasterization) |
| |
| Add a new subsection at the end of 14.6, 14.6.X "Conservative Rasterization" |
| |
| Point, line, and polygon rasterization may optionally be made conservative |
| by calling Enable and Disable with a <pname> of CONSERVATIVE_- |
| RASTERIZATION_NV. When conservative rasterization is enabled, rather than |
| evaluating coverage at individual sample locations, a determination is made |
| of whether any portion of the pixel (including its edges and corners) is |
| covered by the primitive. If any portion of the pixel is covered, then a |
| fragment is generated with all coverage samples turned on. Conservative |
| rasterization may also generate fragments for pixels near the edges of |
| rasterized point or line primitives, even if those pixels are not covered |
| by the primitive. The set of such pixels is implementation-dependent, but |
| implementations are encouraged to evaluate coverage as precisely as |
| possible. |
| |
| If CONSERVATIVE_RASTERIZATION_NV is enabled, points are rasterized |
| according to multisample rasterization rules (section 14.4.3), except that |
| a fragment will be generated for a framebuffer pixel if the circle |
| (POINT_SPRITE disabled) or square (POINT_SPRITE enabled) covers any portion |
| of the pixel, including its edges or corners. When performing conservative |
| rasterization of points, the POINT_SMOOTH enable is ignored and treated as |
| disabled. |
| |
| If CONSERVATIVE_RASTERIZATION_NV is enabled, lines are rasterized according |
| to multisample rasterization rules (section 14.5.4), except that the |
| LINE_STIPPLE and LINE_SMOOTH enables are ignored and treated as disabled. |
| |
| If CONSERVATIVE_RASTERIZATION_NV is enabled, polygons are rasterized |
| according to multisample rasterization rules (section 14.6.6), except that |
| the POLYGON_SMOOTH enable is ignored and treated as disabled. Polygons with |
| an area of zero generate no fragments, even for pixels that contain a |
| vertex or edge of the zero-area polygon. |
| |
| Modify the new Subsection "Drawing Textures" from the NV_draw_texture |
| extension: |
| |
| In either case, the set of fragments generated is not affected by the |
| CULL_FACE, POLYGON_SMOOTH, POLYGON_OFFSET_FILL enables, or PolygonMode |
| state. The CONVSERVATIVE_RASTERIZATION_NV enable does apply, and fragments |
| will be generated for all pixels which have any portion covered by the |
| rectangle. All fragments generated for the rectangle will have a Z window |
| coordinate of <z>. |
| |
| |
| Interactions with OpenGL ES and Core Profiles |
| |
| If using OpenGL ES or a Core profile, references to LINE_SMOOTH, |
| LINE_STIPPLE, POINT_SMOOTH, and POLYGON_SMOOTH are ignored and treated as |
| disabled. POINT_SPRITE is ignored and treated as enabled. For OpenGL ES, |
| references to PolygonMode are ignored. |
| |
| |
| New Implementation Dependent State |
| |
| Minimum |
| Get Value Type Get Command Value Description Sec. |
| --------- ------- ----------- ------- ------------------------ ------ |
| MAX_SUBPIXEL_PRECISION_- Z+ GetIntegerv 1 Max number of extra bits 13.6.1 |
| BIAS_BITS_NV |
| |
| |
| New State |
| |
| Get Value Get Command Type Initial Value Description Sec. Attribute |
| --------- ----------- ---- ------------- ----------- ---- --------- |
| CONSERVATIVE_RASTERIZATION_NV IsEnabled B FALSE Enable conservative 14.6.X enable |
| rasterization rules |
| SUBPIXEL_PRECISION_BIAS_X_BITS_NV GetIntegerv Z+ 0 Additional window x 13.6.1 viewport |
| coordinate precision |
| SUBPIXEL_PRECISION_BIAS_Y_BITS_NV GetIntegerv Z+ 0 Additional window y 13.6.1 viewport |
| coordinate precision |
| |
| Additions to the AGL/GLX/WGL Specifications |
| |
| None. |
| |
| GLX Protocol |
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| None. |
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| Modifications to the OpenGL Shading Language Specification, Version 4.30 |
| |
| None. |
| |
| Errors |
| |
| INVALID_VALUE is generated by SubpixelPrecisionBiasNV if <xbits> or |
| <ybits> are greater than the value of MAX_SUBPIXEL_PRECISION_BIAS_BITS_NV. |
| |
| Issues |
| |
| (1) How is invariance affected by the precision bias? |
| |
| RESOLVED: Invariance may be broken for large enough values of the bias. It |
| is expected that an implementation has enough precision to support |
| SUBPIXEL_BITS for a MAX_VIEWPORT_DIMS size viewport, but if the combination |
| of viewport size and total subpixel precision exceed that then less |
| precision may be used for large x,y coordinates. |
| |
| (2) Do zero area primitives generate fragments in conservative raster? |
| |
| RESOLVED: No, although in some cases that may not be the desired behavior. |
| If a primitive is truly zero area (e.g. two vertices of a triangle have |
| identical positions), then drawing nothing is probably fine. If the |
| primitive happens to be zero area due to subpixel precision then generating |
| fragments may be desirable, but this spec does define that behavior. |
| |
| The primary reason to discard zero area primitives is that attribute |
| interpolation is not well-defined when the area is zero. |
| |
| (3) How does centroid interpolation work for a conservative primitive? |
| |
| RESOLVED: Since a fragment generated by a conservative primitive is |
| considered "fully covered", any location within the pixel may be used for |
| interpolation. This implies that the interpolation may occur outside of the |
| original primitive, causing attribute extrapolation. |
| |
| (4) How is depth coordinate evaluation handled for conservative |
| rasterization? |
| |
| RESOLVED: The "extrapolation" issue for attributes also applies to depth |
| evaluation. |
| |
| (5) How does NV_draw_texture interact with this extension? |
| |
| RESOLVED: The DrawTextureNV command rasterizes conservatively. |
| |
| Revision History |
| |
| Revision 3, 2015/03/27 |
| - Add ES interactions |
| |
| Revision 2, 2014/09/26 (Jon Leech) |
| - Add missing return type to function |
| |
| Revision 1 |
| - Internal revisions. |
