Name NV_point_sprite Name Strings GL_NV_point_sprite Contact Matt Craighead, NVIDIA Corporation (mcraighead 'at' nvidia.com) Notice Copyright NVIDIA Corporation, 2001, 2002. IP Status No known IP issues. Status Shipping (version 1.1) Version NVIDIA Date: March 6, 2003 (version 1.3) Number 262 Dependencies Written based on the wording of the OpenGL 1.3 specification. Assumes support for the EXT_point_parameters extension. Overview Applications such as particle systems usually must use OpenGL quads rather than points to render their geometry, since they would like to use a custom-drawn texture for each particle, rather than the traditional OpenGL round antialiased points, and each fragment in a point has the same texture coordinates as every other fragment. Unfortunately, specifying the geometry for these quads can be quite expensive, since it quadruples the amount of geometry required, and it may also require the application to do extra processing to compute the location of each vertex. The goal of this extension is to allow such apps to use points rather than quads. When GL_POINT_SPRITE_NV is enabled, the state of point antialiasing is ignored. For each texture unit, the app can then specify whether to replace the existing texture coordinates with point sprite texture coordinates, which are interpolated across the point. Finally, the app can set a global parameter for the way to generate the R coordinate for point sprites; the R coordinate can either be zero, the input S coordinate, or the input R coordinate. This allows applications to use a 3D texture to represent a point sprite that goes through an animation, with filtering between frames, for example. Issues * Should this spec say that point sprites get converted into quads? RESOLVED: No, this would make the spec much uglier, because then we'd have to say that polygon smooth and stipple get turned off, etc. Better to provide a formula for computing the texture coordinates and leave them as points. * How are point sprite texture coordinates computed? RESOLVED: They move smoothly as the point moves around on the screen, even though the pixels touched by the point do not. The exact formula is given in the spec. Note that point sprites' T texture coordinate decreases, not increases, with Y; that is, point sprite textures go top-down, not bottom-up. * How do point sizes for point sprites work? RESOLVED: The original NV_point_sprite spec treated point sprites as being sized like aliased points, i.e., integral sizes only. This was a mistake, because it can lead to visible popping artifacts. In addition, it limits the size of points unnecessarily. This revised specification treats point sprite sizes more like antialiased point sizes, but with more leniency. Implementations may choose to not clamp the point size to the antialiased point size range. The set of point sprite sizes available must be a superset of the antialiased point sizes. However, whereas antialiased point sizes are all evenly spaced by the point size granularity, point sprites can have an arbitrary set of sizes. This lets implementations use, e.g., floating-point sizes. It is anticipated that this behavior change will not cause any problems for compatibility. In fact, it should be beneficial to quality. * Should there be a way to query the list of supported point sprite sizes? RESOLVED: No. If an implementation were to use, say, a single- precision IEEE float to represent point sizes, the list would be rather long. * Do mipmaps apply to point sprites? RESOLVED: Yes. They are similar to quads in this respect. * What of this extension's state is per-texture unit and what of this extension's state is state is global? RESOLVED: The GL_POINT_SPRITE_NV enable and POINT_SPRITE_R_MODE_NV state are global. The COORD_REPLACE_NV state is per-texture unit (state set by TexEnv is per-texture unit). * Should we create an entry point for the R mode? RESOLVED: No, we take advantage of the existing glPointParameter interface. Unfortunately, EXT_point_parameters does not define a PointParameteri entry point. This extension adds one. It could live without, but it's a little annoying to have to use a float interface to specify an enumerant. This is definitely not TexEnv state, because it is global, not per texture unit. * What should the suffix for PointParameteri[v] be? RESOLVED: NV. This is an NV extension, and therefore any new entry points must be NV also. This is a bit less aesthetically pleasing than matching the EXT suffixes of EXT_point_parameters, but it is the right thing to do. * Should there be a global on/off switch for point sprites, or should the per-unit enable imply that switch? RESOLVED: There is a global switch to turn it on and off. This is probably more convenient for both driver and app, and it simplifies the spec. * What should the TexEnv mode for point sprites be called? RESOLVED: After much deliberation, COORD_REPLACE_NV seems to be appropriate. * What is the motivation for each of the three point sprite R modes? The R mode is most convenient for applications that may already be drawing their own "point sprites" by rendering quads. These applications already need to put the R coordinate in R, and they do not need to change their code. The S mode is most convenient for applications that do not use vertex programs, because it allows them to use TexCoord1 rather than TexCoord3 to specify their third texture coordinate. This reduces the size of the vertex data. Applications that use vertex programs are largely unaffected by this, because they can map the input S texture coordinate into the output R coordinate if they so desire. The zero mode may allow some applications to more easily obtain the behavior they want out of the dot product functionality of the NV_texture_shader extension. It reduces these dot products from three-component dot products into two-component dot products. In some implementations, it may also have higher performance than the other modes. There is no mode corresponding to the T or Q coordinates because we cannot envision a scenario where such modes would be useful. * What is the interaction with multisample points, which are round? RESOLVED: Point sprites are rasterized as squares, even in multisample mode. Leaving them as round points would make the feature useless. * How does the point sprite extension interact with fragment program extensions (ARB_fragment_program, NV_fragment_program, etc)? RESOLVED: The primary issue is how the interpolanted texture coordinate set appears when fragment attribute variables (ARB terminology) or fragment program attribute registers (NV terminology) are accessed. When point sprite is enabled and the GL_COORD_REPLACE_NV state for a given texture unit is GL_TRUE, the texture coordinate set for that texture unit is (s,t,r,1) where the point sprite-overriden s, t, and r are described in the amended Section 3.3 below. The important point is that q is forced to 1. For fragment program extensions, q cooresponds to the w component of the respective fragment attribute. * What push/pop attribute bits control the state of this extension? RESOLVED: POINT_BIT for all the state. Also ENABLE_BIT for the POINT_SPRITE_NV enable. New Procedures and Functions void PointParameteriNV(enum pname, int param) void PointParameterivNV(enum pname, const int *params) New Tokens Accepted by the parameter of Enable, Disable, and IsEnabled, by the parameter of GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev, and by the parameter of TexEnvi, TexEnviv, TexEnvf, TexEnvfv, GetTexEnviv, and GetTexEnvfv: POINT_SPRITE_NV 0x8861 When the parameter of TexEnvf, TexEnvfv, TexEnvi, TexEnviv, GetTexEnvfv, or GetTexEnviv is POINT_SPRITE_NV, then the value of may be: COORD_REPLACE_NV 0x8862 When the and parameters of TexEnvf, TexEnvfv, TexEnvi, or TexEnviv are POINT_SPRITE_NV and COORD_REPLACE_NV respectively, then the value of or the value pointed to by may be: FALSE TRUE Accepted by the parameter of PointParameteriNV, PointParameterfEXT, PointParameterivNV, PointParameterfvEXT, GetBooleanv, GetIntegerv, GetFloatv, and GetDoublev: POINT_SPRITE_R_MODE_NV 0x8863 When the parameter of PointParameteriNV, PointParameterfEXT, PointParameterivNV, or PointParameterfvEXT is POINT_SPRITE_R_MODE_NV, then the value of or the value pointed to by may be: ZERO S R Additions to Chapter 2 of the OpenGL 1.3 Specification (OpenGL Operation) None. Additions to Chapter 3 of the OpenGL 1.3 Specification (Rasterization) Insert the following paragraphs after the second paragraph of section 3.3 (page 63): "Point sprites are enabled or disabled by calling Enable or Disable with the symbolic constant POINT_SPRITE_NV. The default state is for point sprites to be disabled. When point sprites are enabled, the state of the point antialiasing enable is ignored. The point sprite R coordinate mode is set with one of the commands void PointParameter{if}NV(enum pname, T param) void PointParameter{if}vNV(enum pname, const T *params) where pname is POINT_SPRITE_R_MODE_NV. The possible values for param are ZERO, S, and R. The default value is ZERO. The point sprite texture coordinate replacement mode is set with one of the commands void TexEnv{if}(enum target, enum pname, T param) void TexEnv{if}v(enum target, enum pname, const T *params) where target is POINT_SPRITE_NV and pname is COORD_REPLACE_NV. The possible values for param are FALSE and TRUE. The default value for each texture unit is for point sprite texture coordinate replacement to be disabled." Replace the first two sentences of the fourth paragraph of section 3.3 (page 63) with the following: "The effect of a point width other than 1.0 depends on the state of point antialiasing and point sprites. If antialiasing and point sprites are disabled, ..." Replace the first sentences of the sixth paragraph of section 3.3 (page 64) with the following: "If antialiasing is enabled and point sprites are disabled, ..." Insert the following paragraphs at the end of section 3.3 (page 66): "When point sprites are enabled, then point rasterization produces a fragment for each framebuffer pixel whose center lies inside a square centered at the point's (x_w, y_w), with side length equal to the current point size. All fragments produced in rasterizing a point sprite are assigned the same associated data, which are those of the vertex corresponding to the point, with texture coordinates s, t, and r replaced with s/q, t/q, and r/q, respectively. If q is less than or equal to zero, the results are undefined. However, for each texture unit where COORD_REPLACE_NV is TRUE, these texture coordinates are replaced with point sprite texture coordinates. The s coordinate varies from 0 to 1 across the point horizontally, while the t coordinate varies from 0 to 1 vertically. The r coordinate depends on the value of POINT_SPRITE_R_MODE_NV. If this is set to ZERO, then the r coordinate is set to zero. If it is set to S, then the r coordinate is set to the s texture coordinate before coordinate replacement takes place. If it is set to R, then the r coordinate is set to the r texture coordinate before coordinate replacement takes place. The following formula is used to evaluate the s and t coordinates: s = 1/2 + (x_f + 1/2 - x_w) / size t = 1/2 - (y_f + 1/2 - y_w) / size where size is the point's size, x_f and y_f are the (integral) window coordinates of the fragment, and x_w and y_w are the exact, unrounded window coordinates of the vertex for the point. The widths supported for point sprites must be a superset of those supported for antialiased points. There is no requirement that these widths must be equally spaced. If an unsupported width is requested, the nearest supported width is used instead." Replace the text of section 3.3.1 (page 66) with the following: "The state required to control point rasterization consists of the floating-point point width, a bit indicating whether or not antialiasing is enabled, a bit indicating whether or not point sprites are enabled, the current value of the point sprite R coordinate mode, and a bit for the point sprite texture coordinate replacement mode for each texture unit." Replace the text of section 3.3.2 (page 66) with the following: "If MULTISAMPLE is enabled, and the value of SAMPLE_BUFFERS is one, then points are rasterized using the following algorithm, regardless of whether point antialiasing (POINT_SMOOTH) is enabled or disabled. Point rasterization produces a fragment for each framebuffer pixel with one or more sample points that intersect a region centered at the point's (x_w, y_w). This region is a circle having diameter equal to the current point width if POINT_SPRITE_NV is disabled, or a square with side equal to the current point width if POINT_SPRITE_NV is enabled. Coverage bits that correspond to sample points that intersect the region are 1, other coverage bits are 0. All data associated with each sample for the fragment are the data associated with the point being rasterized, with the exception of texture coordinates when POINT_SPRITE_NV is enabled; these texture coordinates are computed as described in section 3.3. Point size range and number of gradations are equivalent to those supported for antialiased points when POINT_SPRITE_NV is disabled. The set of point sizes supported is equivalent to those for point sprites without multisample when POINT_SPRITE_NV is enabled." Additions to Chapter 4 of the OpenGL 1.3 Specification (Per-Fragment Operations and the Frame Buffer) None. Additions to Chapter 5 of the OpenGL 1.3 Specification (Special Functions) None. Additions to Chapter 6 of the OpenGL 1.3 Specification (State and State Requests) None. GLX Protocol Two new GL rendering commands are added. The following commands are sent to the server as part of a glXRender request: PointParameteriNV 2 12 rendering command length 2 4221 rendering command opcode 4 ENUM pname 0x8126 n==1 POINT_SIZE_MIN_ARB 0x8127 n==1 POINT_SIZE_MAX_ARB 0x8128 n==1 POINT_FADE_THRESHOLD_SIZE_ARB 0x8863 n==1 POINT_SPRITE_R_MODE_NV 4 INT32 param PointParameterivNV 2 8+4*n rendering command length 2 4222 rendering command opcode 4 ENUM pname 0x8126 n==1 POINT_SIZE_MIN_ARB 0x8127 n==1 POINT_SIZE_MAX_ARB 0x8128 n==1 POINT_FADE_THRESHOLD_SIZE_ARB 0x8129 n==3 DISTANCE_ATTENUATION_ARB 0x8863 n==1 POINT_SPRITE_R_MODE_NV 4*n LISTofINT32 params Errors None. New State (table 6.12, p. 220) Get Value Type Get Command Initial Value Description Sec Attribute --------- ---- ----------- ------------- ----------- ------ --------- POINT_SPRITE_NV B IsEnabled False point sprite enable 3.3 point/enable POINT_SPRITE_R_MODE_NV Z3 GetIntegerv ZERO R coordinate mode 3.3 point (table 6.17, p. 225) Get Value Type Get Command Initial Value Description Sec Attribute --------- ---- ----------- ------------- ----------- ------ --------- COORD_REPLACE_NV 2* x B GetTexEnviv False coordinate replacement 3.3 point enable NVIDIA Implementation Details This extension was first supported for GeForce4 Ti only in NVIDIA's Release 25 drivers. Future drivers will support this extension on all GeForce products. However, the extension is only hardware-accelerated on the GeForce3 and GeForce4 Ti platforms. In addition, there are restrictions on the cases that are accelerated on the GeForce3. In order to ensure that you get hardware acceleration on GeForce3, make sure that: 1. The point sprite R mode is set to GL_ZERO. (This is the default.) 2. Coordinate replacement is turned on for texture unit 3 and for no other texture units. This is non-obvious; using texture unit zero will _not_ be accelerated. Also, if coordinate replacement is off for _all_ texture units, that's also unaccelerated. So, in the typical usage case where you just want a single texture on some points, you should enable TEXTURE_2D on unit 3 but disable it on unit zero. The GeForce4 Ti platform supports point sprites as large as 8192, but the spacing between sizes becomes larger as the size increases. All other platforms do not support point sprite sizes above 64. ATI Implementation Details This extension is supported on the Radeon 8000 series and later platforms. In order to ensure that Radeon 8000 series will accelerate point sprite rendering using TCL hardware, make sure that the point sprite R mode is set to GL_ZERO. (This is the default.) Radeon 8000 series can render points as large as 2047. Revision History June 4, 2002 - Added implementation details section. Fixed a typo in the overview. Changed behavior of point sizes so that fractional sizes are allowed and so that implementations can support large point sprites or use floating-point point size representations. Significant rewrite of spec language to cover this new point size behavior. July 5, 2002 - Finished GLX protocol. March 6, 2003 - Added issue to clarify Q handling for fragment program extensions. Added issue to clarify push/pop attrib handling. Adjusted state tables so COORD_REPLACE_NV state appears in the texture environment and generation table.