| Name |
| |
| ARB_polygon_offset_clamp |
| |
| Name Strings |
| |
| GL_ARB_polygon_offset_clamp |
| |
| Contact |
| |
| Piers Daniell (pdaniell 'at' nvidia.com) |
| |
| Contributors |
| |
| Eric Lengyel, Terathon Software |
| Tobias Hector, Imagination Technologies |
| |
| Notice |
| |
| Copyright (c) 2017 The Khronos Group Inc. Copyright terms at |
| http://www.khronos.org/registry/speccopyright.html |
| |
| Specification Update Policy |
| |
| Khronos-approved extension specifications are updated in response to |
| issues and bugs prioritized by the Khronos OpenGL Working Group. For |
| extensions which have been promoted to a core Specification, fixes will |
| first appear in the latest version of that core Specification, and will |
| eventually be backported to the extension document. This policy is |
| described in more detail at |
| https://www.khronos.org/registry/OpenGL/docs/update_policy.php |
| |
| Status |
| |
| Draft |
| |
| Version |
| |
| Last Modified Date: April 3, 2017 |
| Revision: 1 |
| |
| Number |
| |
| ARB Extension #193 |
| |
| Dependencies |
| |
| OpenGL 3.3 (either core or compatibility profiles). |
| |
| This extension is written against the OpenGL 4.5 (Compatibility Profile) |
| Specification (October 24, 2016). |
| |
| Overview |
| |
| This extension adds a new parameter to the polygon offset function |
| that clamps the calculated offset to a minimum or maximum value. |
| The clamping functionality is useful when polygons are nearly |
| parallel to the view direction because their high slopes can result |
| in arbitrarily large polygon offsets. In the particular case of |
| shadow mapping, the lack of clamping can produce the appearance of |
| unwanted holes when the shadow casting polygons are offset beyond |
| the shadow receiving polygons, and this problem can be alleviated by |
| enforcing a maximum offset value. |
| |
| IP Status |
| |
| No known IP claims. |
| |
| New Procedures and Functions |
| |
| void PolygonOffsetClamp(float factor, float units, float clamp); |
| |
| New Tokens |
| |
| Accepted by the <pname> parameters of GetBooleanv, GetIntegerv, |
| GetInteger64v, GetFloatv, and GetDoublev: |
| |
| POLYGON_OFFSET_CLAMP 0x8E1B |
| |
| Additions to Chapter 14 of the OpenGL 4.5 (Compatibility Profile) Specification |
| (Fixed-Function Primitive Assembly and Rasterization) |
| |
| -- Modify Open GL section 14.6.5 "Depth Offset" |
| |
| Replace the 1st paragraph... |
| |
| "The depth values of all fragments generated by the rasterization of |
| a polygon may be offset by a single value that is computed for that |
| polygon. The function that determines this value is specified with |
| the commands |
| |
| void PolygonOffsetClamp(float factor, float units, float clamp); |
| void PolygonOffset(float factor, float units); |
| |
| <factor> scales the maximum depth slope of the polygon, and <units> |
| scales an implementation-dependent constant that relates to the |
| usable resolution of the depth buffer. The resulting values are |
| summed to produce the polygon offset value, which may then be |
| clamped to a minimum or maximum value specified by <clamp>. The |
| values <factor>, <units>, and <clamp> may each be positive, |
| negative, or zero. Calling the command PolygonOffset is equivalent |
| to calling the command PolygonOffsetClamp with <clamp> equal to |
| zero." |
| |
| Replace the 6th paragraph... |
| |
| "The offset value o for a polygon is |
| _ |
| | m x <factor> + r x <units>, if <clamp> = 0 or NaN; |
| | |
| o = < min(m x <factor> + r x <units>, <clamp>), if <clamp> > 0; |
| | |
| |_ max(m x <factor> + r x <units>, <clamp>), if <clamp> < 0. |
| |
| m is computed as described above. If the depth buffer uses a fixed- |
| point representation, m is a function of depth values in the range |
| [0, 1], and o is applied to depth values in the same range." |
| |
| Additions to the AGL/EGL/GLX/WGL Specifications |
| |
| None |
| |
| GLX Protocol |
| |
| A new GL rendering command is added. The following command is sent |
| to the server as part of a glXRender request: |
| |
| PolygonOffsetClamp |
| 2 16 rendering command length |
| 2 4225 rendering command opcode |
| 4 FLOAT32 factor |
| 4 FLOAT32 units |
| 4 FLOAT32 clamp |
| |
| Errors |
| |
| None |
| |
| New State (OpenGL) |
| |
| Get Value Type Get Command Initial Value Description Sec Attrib |
| ------------------------ ---- ----------- ------------- -------------------- ----- ------- |
| POLYGON_OFFSET_CLAMP R GetFloatv 0 Polygon offset clamp 14.6.5 polygon |
| |
| New Implementation Dependent State |
| |
| None |
| |
| Issues |
| |
| 1) Should the PolygonOffsetClamp command specify only the <clamp> |
| parameter, or should it specify all three of the parameters |
| <factor>, <units>, and <clamp>? |
| |
| Defining a new command that specifies new state in addition to |
| state that can be specified with an existing command has a |
| precedent in the BlendFuncSeparate command. The argument can be |
| made that an application would usually want to set the <factor> |
| and <units> values at the same time it sets the <clamp> value, |
| and making one GL call is better than making two GL calls. |
| Furthermore, requiring that a call to PolygonOffset sets |
| POLYGON_OFFSET_CLAMP to zero insulates applications unaware |
| of the new state from failures to restore it to its initial |
| value in separate libraries, and this cannot be done if an |
| orthogonal command specifying only the <clamp> value were to be |
| defined. |
| |
| RESOLVED: This extension defines a new command that specifies |
| the <factor>, <units>, and <clamp> parameters. |
| |
| 2) What happens if <clamp> is infinity or NaN? |
| |
| As per Section 2.1, the result of providing an infinity or NaN |
| is unspecified. However, if <clamp> is positive or negative |
| infinity, then Equation (3.13), in the literal mathematical |
| sense, is effectively reduced to the case in which no clamping |
| occurs, and this should be the defined behavior. |
| |
| If <clamp> is a floating-point NaN, then we could leave the |
| result undefined, but that could lead to application code |
| working correctly with one implementation and then inexplicably |
| failing with another. It would be better to define the behavior |
| such that no clamping occurs. If this is not the behavior |
| exhibited by the hardware, then the implementation can turn all |
| infinites and NaNs into zero using the following code: |
| |
| int32_t clampBits = *(int32_t *) &clamp; |
| clampBits &= (((clampBits >> 23) & 0xFF) - 0xFF) >> 31; |
| |
| This ANDs with all one bits if and only if the floating-point |
| exponent is less than 255. Otherwise, it ANDs with all zero |
| bits. (This assumes a well-defined right shift of negative |
| integers.) |
| |
| RESOLVED: If <clamp> is infinity or NaN, then no clamping is |
| applied to the polygon offset. |
| |
| 3) What happens if <clamp> is a denormalized floating-point value? |
| |
| As per Section 2.1, the result of providing a denormalized value |
| must yield predictable results. However, some implementations |
| may treat denormalized values as equal to zero, and other |
| implementations may treat them as greater than or less than |
| zero. To ensure uniform behavior across all implementations, we |
| can require that denormalized values not be equal to zero. This |
| may necessitate that implementations convert denormalized values |
| to the smallest representable normalized value with the same |
| sign. |
| |
| RESOLVED: Denormalized values are not considered equal to zero |
| in Equation (3.13). |
| |
| Revision History |
| |
| Rev. Date Author Changes |
| ---- ---------- --------- ----------------------------------------------- |
| 1 2017-04-03 pdaniell Initial draft for OpenGL 4.6 forked from |
| GL_EXT_polygon_offset_clamp rev. 4 |
