/usr/share/celestia/shaders/rings.vp is in celestia-common 1.6.1+dfsg-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# Compute ring illumination. Assumes rings made of spherical particles,
# and no occlusion or shadowing between ring particles. We also compute the
# texture coordinates for the projected shadow of a planet.
# c[0]..c[3] contains the concatenation of the modelview and projection matrices.
# c[4]..c[7] contains the inverse transpose of the modelview
# c[15] contains the eye position in object space
# c[16] contains the light direction in object space
# c[17] contains H, the normalized sum of the eye and light direction
# c[20] contains the object color * light color
# c[32] contains the ambient light color
# c[33] contains the haze color
# c[34] contains the specular color * light color
# c[41] and c[42] contain the shadow projection matrix
# c[40] contains (0, 1, 0, specPower)
# c[90] contains (0, 0.5, 1, 0)
# v[OPOS] contains the per-vertex position
# v[NRML] contains the per-vertex normal
# v[TEX0] contains the per-vertex texture coordinate 0
# o[HPOS] output register for homogeneous position
# o[TEX0] output register for texture coordinate 0
# o[COL0] output register for primary color
# R0...R11 temporary registers
# Transform the vertex by the modelview matrix
DP4 R1.x, c[0], v[OPOS];
DP4 R1.y, c[1], v[OPOS];
DP4 R1.z, c[2], v[OPOS];
DP4 R1.w, c[3], v[OPOS];
# Get the vector from the eye to the vertex
ADD R4, c[15], -v[OPOS];
# Normalize it
DP3 R0.w, R4, R4;
RSQ R0.w, R0.w;
MUL R4.xyz, R4, R0.w;
# Compute the illumination
DP3 R2.x, R4, c[16];
ADD R2.x, R2.x, c[90].z;
MUL R2.x, R2.x, c[90].y;
# Output the primary color
MOV R0, c[32];
MAD o[COL0], c[20], R2.xxxx, R0;
# Output the texture
MOV o[TEX0], v[TEX0];
# MOV o[TEX1], v[TEX1];
# The second texture is the shadow; we need to compute the
# it from the vertex coordinate.
DP4 o[TEX1].x, c[41], v[OPOS];
DP4 o[TEX1].y, c[42], v[OPOS];
# Output the vertex
MOV o[HPOS], R1;
END
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