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