Name ARB_point_sprite Name Strings GL_ARB_point_sprite Contact Matt Craighead, NVIDIA Corporation (mcraighead 'at' nvidia.com) Mark Kilgard, NVIDIA Corporation (mjk 'at' nvidia.com) Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com) Notice Copyright (c) 2003-2013 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 IP Status No known IP issues. Status Approved by the ARB on July 24, 2003. Version Last Modified Date: July 22, 2003 Revision: 7 Number ARB Extension #35 Dependencies Written based on the wording of the OpenGL 1.4 specification. NV_point_sprite affects the definition of this extension. Overview Applications such as particle systems have tended to 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 expensive, since it quadruples the amount of geometry required, and may also require the application to do extra processing to compute the location of each vertex. The purpose of this extension is to allow such applications to use points rather than quads. When GL_POINT_SPRITE_ARB 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. 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 below. A point sprite can be thought of as a quad whose upper-left corner has (s,t) texture coordinates of (0,0) and whose lower-right corner has texture coordinates of (1,1), as illustrated in the following figure. In the figure "P" is the center of the point sprite, and "O" is the origin (0,0) of the window coordinate system. Note that the y window coordinate increases from bottom-to-top but the t texture coordinate of point sprites increases from top-to-bottom. ^ +y| (0,0) | +-----+ | | | | | P | | | | | +-----+ | (1,1) | +x O---------------> Applications using a single texture for both point sprites and other geometry need to account for the fixed coordinate mapping of point sprites. * Is the ARB specification different from the NV version? RESOLVED: Yes. The point sprite R mode has been removed. The wording has also been updated to reflect version 1.4 of the core OpenGL specification however. The enumerant values are unchanged. * How do point sizes for point sprites work? RESOLVED: This specification treats point sprite sizes 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. * 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_ARB enable is global. The COORD_REPLACE_ARB state is per-texture unit (state set by TexEnv is per-texture unit). * 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: COORD_REPLACE_ARB. * 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 interpolated 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_ARB state for a given texture unit is GL_TRUE, the texture coordinate set for that texture unit is (s,t,0,1) where the point sprite-overridden s and t are described in the amended Section 3.3 below. The important point is that r and q are forced to 0 and 1, respectively. For fragment program extensions, r and q correspond to the z and w components of the respective fragment attribute. * How does this extension interact with PolygonMode? RESOLVED: If a polygon is rendered in point mode and POINT_SPRITE_ARB is enabled, its vertices will be rendered as point sprites. * How does this extension interact with the point size attenuation functionality in ARB_point_parameters and OpenGL 1.4? RESOLVED: Point sprites sizes are attenuated just like the sizes of non-sprite points. * 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_ARB enable. * How are point sprites clipped? RESOLVED: Point sprites are transformed as points, and standard point clipping operations are performed. This can cause point sprites that move off the edge of the screen to disappear abruptly, in the same way that regular points do. As with any other primitive, standard per-fragment clipping operations (scissoring, window ownership test) still apply. New Procedures and Functions None 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_ARB 0x8861 When the parameter of TexEnvf, TexEnvfv, TexEnvi, TexEnviv, GetTexEnvfv, or GetTexEnviv is POINT_SPRITE_ARB, then the value of may be: COORD_REPLACE_ARB 0x8862 When the and parameters of TexEnvf, TexEnvfv, TexEnvi, or TexEnviv are POINT_SPRITE_ARB and COORD_REPLACE_ARB respectively, then the value of or the value pointed to by may be: FALSE TRUE Additions to Chapter 2 of the OpenGL 1.4 Specification (OpenGL Operation) None. Additions to Chapter 3 of the OpenGL 1.4 Specification (Rasterization) Insert the following paragraphs after the second paragraph of section 3.3 (page 66): "Point sprites are enabled or disabled by calling Enable or Disable with the symbolic constant POINT_SPRITE_ARB. 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 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_ARB and pname is COORD_REPLACE_ARB. 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 second paragraph of section 3.3.1 (page 67) 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 fourth paragraph of section 3.3.1 (page 68) with the following: "If antialiasing is enabled and point sprites are disabled, ..." Insert the following paragraphs at the end of section 3.3.1 (page 70): "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_ARB is TRUE, these texture coordinates are replaced with point sprite texture coordinates. The s coordinate varies from 0 to 1 across the point horizontally left-to-right, while the t coordinate varies from 0 to 1 vertically top-to-bottom. The r and q coordinates are replaced with the constants 0 and 1, respectively. 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 3.3.2 (page 70) with the following: "The state required to control point rasterization consists of the floating-point point width, three floating-point values specifying the minimum and maximum point size and the point fade threshold size, three floating-point values specifying the distance attenuation coefficients, a bit indicating whether or not antialiasing is enabled, a bit indicating whether or not point sprites are enabled, and a bit for the point sprite texture coordinate replacement mode for each texture unit." Replace the text of section 3.3.3 (page 70) 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_ARB is disabled, or a square with side equal to the current point width if POINT_SPRITE_ARB 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_ARB 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_ARB is disabled. The set of point sizes supported is equivalent to those for point sprites without multisample when POINT_SPRITE_ARB is enabled." Additions to Chapter 4 of the OpenGL 1.4 Specification (Per-Fragment Operations and the Frame Buffer) None. Additions to Chapter 5 of the OpenGL 1.4 Specification (Special Functions) None. Additions to Chapter 6 of the OpenGL 1.4 Specification (State and State Requests) None. Interactions with NV_point_sprite ARB_point_sprite is a functional subset of NV_point_sprite. The only functional difference between the extensions is that NV_point_sprite provides the POINT_SPRITE_R_MODE_NV control. This mode allows applications to specify how the r texture coordinates for point sprites are replaced. The r coordinate can be replaced with the corresponding s texture coordinate ("S" mode), left unchanged ("R" mode), or replaced with the constant zero ("ZERO" mode). ARB_point_sprite always replaces r texture coordiantes of point sprites with zero. Since ARB_point_sprite is functionally compatible with the default r mode from NV_point_sprite, the two extensions can coexist nicely. Enumerant values from NV_point_sprite are reused. If NV_point_sprite is supported, the language describing the replacement of r coordinates for point sprites (forced to zero) is replaced with the corresponding language from NV_point_sprite (controlled by POINT_SPRITE_R_MODE_NV). Errors None. New State (table 6.12, p. 220) Get Value Type Get Command Initial Value Description Sec Attribute --------- ---- ----------- ------------- ----------- ------ --------- POINT_SPRITE_ARB B IsEnabled False point sprite enable 3.3 point/enable (table 6.17, p. 225) Get Value Type Get Command Initial Value Description Sec Attribute --------- ---- ----------- ------------- ----------- ------ --------- COORD_REPLACE_ARB 2* x B GetTexEnviv False coordinate replacement 3.3 point enable Revision History Initially based on the NV_point_sprite specification but updated for OpenGL 1.4. Rev. Date Author Changes ---- -------- -------- -------------------------------------------- 7 07/22/03 pbrown Marked point parameter issue resolved. 6 07/18/03 pbrown Removed POINT_SPRITE_R_MODE_CONTROL. Expanded on spec issue documenting the inversion of the "t" texture coordinate relative to the "y" window coordinate. Added issues on interaction with PolygonMode, clipping, and point parameters. Documented interaction with NV_point_sprite. Removed now unneeded point parameter interaction section and GLX protocol.