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69 lines
3.2 KiB
69 lines
3.2 KiB
Randi Rost
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Hewlett-Packard
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970-229-2447
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[email protected]
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-----------------------------------------------------------------------------
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OpenGL RGBA Image Rendering Pipeline (destination is an RGBA visual)
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1. Image data exist in main memory (as either RGBA or color index values)
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2. Extract subimage, performing byte swapping if necessary
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3. If pixels are RGBA pixels, convert all components to floats (logically,
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but not always necessary in practice)
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4. If pixels are RGBA pixels, convert all component groups to full RGBA
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(again, logically, but not necessary in practice)
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5a. If pixels are RGBA pixels, perform ax+b operation on each component
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5b. If pixels are color index pixels, perform index shift and offset on
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each index value
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6a. If pixels are RGBA values, go through lookup table (OpenGL RGBA to RGBA
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lookup, controlled by GL_MAP_COLOR)
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6b. If pixels are color index values, go through lookup table (OpenGL color
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index to RGBA lookup, controlled by GL_MAP_COLOR)
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7. LUT (COLOR_TABLE_SGI from the SGI_color_table extension)
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8. Convolve (including post-convolution scale and bias)
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9. LUT (POST_CONVOLUTION_COLOR_TABLE_SGI, from SGI_color_table extension)
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10. Color matrix (including post-color matrix scale and bias)
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11. LUT (POST_COLOR_MATRIX_COLOR_TABLE_SGI, from SGI_color_table extension)
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12. Histogram, min/max
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13. Zoom
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14. Write fragments to the display (includes normal per-fragment operations:
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texture, fog, blend, etc.)
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Notes:
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Steps #1-6 and #13-14 are in core OpenGL. The rest come from the imaging
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extensions. Steps #7, 9, 11 come from the SGI_color_table extension.
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Step #8 comes from the EXT_convolution extension. Step #10 comes from
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the SGI_color_matrix extension. Step #12 comes from the EXT_histogram
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extension.
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You may notice there is nothing to support image rotation. I would propose
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that we add an affine transformation stage after step #9 in order to support
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image scale, rotate, and translate with proper resampling controls. To follow
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the example set for the convolve and color matrix extensions, we may want
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to follow this with a LUT that would be the means for accomplishing
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window-level mapping.
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The zoom function defined by core OpenGL seems insufficient and occurs at the
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wrong point in the pipeline.
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How are the typical imaging scale/rotate/resample operations performed in
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the SGI model? Steps 1-9 are performed on an image that is being loaded into
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texture memory. A rectangle is drawn with the geometry, texture coordinates,
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and texturing controls to provide the proper scaling/rotation/resampling
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behavior. This approach seems to have several drawbacks:
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1) The programming model is not simple
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2) On many architectures, traffic on the graphics bus may be
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doubled (write to texture memory, followed by a read from
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texture memory and write into the framebuffer)
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3) Requires texture mapping hardware to have reasonable performance
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The attached extension specification proposes adding an image transformation
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step that occurs immediately after step #9. This image transformation step
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supports image scale/rotate/translate operations, followed by proper
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resampling, followed by another color lookup table that can be used for
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window level mapping.
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