| Name |
| |
| OML_interlace |
| |
| Name Strings |
| |
| GL_OML_interlace |
| |
| Contact |
| |
| Jon Leech, Silicon Graphics (ljp 'at' sgi.com) |
| |
| Status |
| |
| Complete. Approved by the Khronos SIG on July 19, 2001. |
| |
| Version |
| |
| Last Modified Date: 07/23/2001 |
| Author Revision: $Header: //depot/main/doc/registry/extensions/OML/interlace.spec#5 $ |
| |
| Number |
| |
| 239 |
| |
| Dependencies |
| |
| None. |
| |
| Overview |
| |
| This extension provides a way to interlace rows of pixels when |
| drawing, reading, or copying pixel rectangles or texture images. In |
| this context, interlacing means skiping over rows of pixels or |
| texels in the destination. This is useful for dealing with video |
| data since a single frame of video is typically composed from two |
| images or fields: one image specifying the data for even rows of the |
| frame and the other image specifying the data for odd rows of the |
| frame. |
| |
| The functionality provided by this extension is a combination |
| of the older SGIX_interlace and INGR_interlace_read extensions, |
| with changes applying interlacing to texture image queries. |
| |
| Issues |
| |
| * Should there be a single enumerant controlling both draw and read |
| operations? For the moment, we continue using separate enums, for |
| backwards compatibility with SGIX_interlace and INGR_interlace_read. |
| |
| * Can we use the same enum values as the older extensions? Possibly, |
| depending on the resolution of issues of exactly which operations |
| interlacing is applied to. For the moment we assume the same |
| values cannot be used. |
| |
| * Are there any GLX protocol issues relating to the actual vs. |
| specified size of the image being transferred? Probably not, since |
| unlike the effects of convolution, the image being transferred over |
| the wire is always the specified size; all that changes is where the |
| pixels are positioned in the frame buffer. |
| |
| * Discreet requested that INTERLACE_READ_OML apply to GetTexImage. The |
| extension does not support this because there's no easy way to |
| support it with any generality: with only the binary |
| INTERLACE_READ_OML setting available, the implementation could |
| return only the even rows, but would have no way of indicating that |
| only the odd rows should be returned. This is non-orthogonal |
| probably more frustrating than useful; a generic solution would |
| require creation of a GetTexSubImage call. |
| |
| * We may need to be more precise about exactly which operations |
| interlacing is and is not applied to. Currently it must be inferred |
| from other parts of the OpenGL Specification, and different |
| implementations are likely to disagree on this. Some language has |
| been added to section 6.1.4 to deal explicitly with GetTexImage, but |
| may be needed elsewhere as well. |
| |
| IP Status |
| |
| No known issues. |
| |
| New Procedures and Functions |
| |
| None. |
| |
| New Tokens |
| |
| Accepted by the <cap> parameter of Enable, Disable, and IsEnabled, |
| and by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv, |
| and GetDoublev: |
| |
| INTERLACE_OML 0x8980 |
| INTERLACE_READ_OML 0x8981 |
| |
| Additions to Chapter 2 of the OpenGL 1.2.1 Specification (OpenGL Operation) |
| |
| None. |
| |
| Additions to Chapter 3 of the OpenGL 1.2.1 Specification (Rasterization) |
| |
| - (3.6.4, p. 99) Insert the following subsection between "Pixel |
| Transfer Operations" and "Final Conversion" |
| |
| Interlacing |
| |
| This step applies only if INTERLACE_OML is enabled. All of the |
| groups which belong to a row m in the source image are treated as if |
| they belonged to the row 2 * m. If the source image has a height of |
| h rows, this effectively expands the height of the image to 2 * h - |
| 1 rows. After interlacing, only every other row of the image is |
| defined. If the interlaced pixel rectangle is rasterized to the |
| framebuffer, then only these rows are converted to fragments. If the |
| interlaced pixel rectangle is a texture image, then only these rows |
| are written to texure memory. |
| |
| In cases where errors can result from the specification of invalid |
| image dimensions, it is the resulting dimensions that are tested, |
| not the dimensions of the source image. (A specific example is |
| TexImage2D, which specifies constraints for image dimensions. Even |
| if TexImage2D is called with a null pixel pointer, the dimensions of |
| the resulting texture image are those that would result from the |
| effective expansion of the specified image due to interlacing.) |
| |
| Additions to Chapter 4 of the OpenGL 1.2.1 Specification (Per-Fragment |
| Operations and the Frame Buffer) |
| |
| - (4.3.2, p. 157) Modify the 5th paragraph of "Obtaining Pixels from |
| the Framebuffer" to read |
| |
| If INTERLACE_READ_OML is disabled, then ReadPixels obtains values |
| from the selected buffer for each pixel with lower left hand corner |
| at (x+i, y+j) for 0 <= i < width and 0 <= j < height; this pixel is |
| said to be the ith pixel in the jth row. |
| |
| If INTERLACE_READ_OML is enabled, then ReadPixels obtains values |
| from the selected buffer for each pixel with lower left hand corner |
| at (x+i, y+(j*2)) for 0 <= i < width and 0 <= j < height; this pixel |
| is said to be the ith pixel in the jth row. |
| |
| If any of these pixels lies outside of the window... |
| |
| |
| Additions to Chapter 5 of the OpenGL 1.2.1 Specification (Special Functions) |
| |
| None. |
| |
| Additions to Chapter 6 of the OpenGL 1.2.1 Specification (State and State Requests) |
| |
| - (6.1.4, p. 184) Insert in the second paragraph, following "... and |
| from the first image to the last for three-dimensional textures." |
| |
| The value of INTERLACE_READ_OML has no effect on the operation of |
| GetTexImage. |
| |
| Additions to the GLX 1.3 Specification |
| |
| None. |
| |
| Errors |
| |
| See above. |
| |
| New State |
| |
| Get Value Type Get Command Initial Value Attribute |
| --------- ---- ----------- ------------- --------- |
| INTERLACE_OML B IsEnabled False pixel/enable |
| INTERLACE_READ_OML B IsEnabled False pixel/enable |
| |
| New Implementation Dependent State |
| |
| None. |
| |
| Revision History |
| |
| * Revision 5, 07/24/2001 - Finalized Status for OpenML 1.0. |
| * Revision 4, 07/11/2001 - Assign enum values and extension number |
| for the registry. |
| * Revision 3 - formatting changes for OpenML Specification |
| * Revision 2 - expanded description of why GetTexImage doesn't support |
| interlaced readbacks. |
| * Revision 1 - derived from SGIX_interlace and INGR_interlace_read. |
