Dave Akers (dla@sgi.com)
Fri, 13 Aug 1999 14:41:12 -0700
Here's what happens in your example scenario:
The "-lut 50 55" creates a linear ramp table with 256 floating point
entries, and fills these 256 entries with values ranging from 0 to 1 (the
ramp is centered at 50, with width 55.) Since your data is in RGBA
format, the single-valued LUT data is duplicated within Volumizer to form
RGBA 4-tuples. Lookups to this table work independently in R,G,B, and A.
So let's say your value was (r,g,b,a). This is rescaled into indeces (r',
g', b', a') from 0..255, and these values are used to index into the
color lookup table. It's just a level of indirection in each channel for
calculating final values.
Here's a section of the glColorTableSGI man page that provides a more
general explanation:
> Each pixel group has color components (R, G, B, A) which are in the
> range
> [0.0, 1.0]. The color components are rescaled to the size of the
> color
> lookup table to form an index. Then a subset of the components
> based on
> the internal format of the table are replaced by the table entry
> selected
> by that index. If the color components and contents of the table
> are
> represented as follows:
>
> Representation Meaning
> _________________________________________________
> r Table index computed from R
> g Table index computed from G
> b Table index computed from B
> a Table index computed from A
> L[i] Luminance value at table index i
> I[i] Intensity value at table index i
> R[i] Red value at table index i
> G[i] Green value at table index i
> B[i] Blue value at table index i
> A[i] Alpha value at table index i
>
> Then the result of color table lookup is as follows:
>
> Resulting Texture Components
> Table Internal Format R G B A
> _____________________________________________________
> GL_ALPHA R G B A[a]
> GL_LUMINANCE L[r] L[g] L[b] At
> GL_LUMINANCE_ALPHA L[r] L[g] L[b] A[a]
> GL_INTENSITY I[r] I[g] I[b] I[a]
> GL_RGB R[r] G[g] B[b] A
> GL_RGBA R[r] G[g] B[b] A[a]
Let me know if you still have any questions..
-Dave
This archive was generated by hypermail 2.0b2 on Mon Nov 01 1999 - 14:21:40 PST