/usr/share/celestia/shaders/night.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 the diffuse light from a single source
# 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 light color * object color
# c[32] contains the ambient light color * object color
# c[33] contains the haze color
# c[40] contains (0, 1, 0, specPower)
# Transform the vertex by the modelview matrix
DP4 o[HPOS].x, c[0], v[OPOS];
DP4 o[HPOS].y, c[1], v[OPOS];
DP4 o[HPOS].z, c[2], v[OPOS];
DP4 o[HPOS].w, c[3], v[OPOS];
# Compute the diffuse light component
DP3 R2, v[NRML], c[16];
# Clamp the diffuse component to zero
MAX R2, R2, c[40].xxxx;
# Use the fourth power of L dot N to create a sharper division between
# the lit and unlit sides of the planet.
ADD R2, c[40].y, -R2;
MUL R2, R2, R2;
MAD R2, R2, -R2, c[40].y;
# Output the texture
MOV o[TEX0], v[TEX0];
# Output the primary color
MOV R0, c[32];
MAD o[COL0], c[20], R2, R0;
END
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