vpShadeTable(3)

✨ vpShadeTable ✨

okay so bestie, this is a page from the Library Functions Manual — it's all about vpShadeTable(3), the VolPack function that fills in your shading lookup table. grab a strawberry drink 🍓 and let's get into it.

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💖 NAME 💖

vpShadeTable — compute the contents of the shading lookup table

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🌸 SYNOPSIS 🌸

#include <volpack.h>

vpResult
vpShadeTable(vpc)
    vpContext *vpc;

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🎀 ARGUMENTS 🎀

• vpc — the VolPack context, and like… you get it from vpCreateContext. that's the whole vibe. 🫶

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✨ DESCRIPTION ✨

so vpShadeTable computes the contents of the shading lookup table that you previously specified with vpLookupShader. for every single entry in that table, it evaluates the Phong shading equation using:

• the current lighting properties (the ones you set with vpSetLight),
• the current material properties (set with vpSetMaterial),
• and the current viewing parameters (set with the view transformation commands).

if you want the actual tea on how the shading calculation goes down, go read vpSetMaterial(3) — it has the full description. 👑

You should call this function before rendering a volume if any of the lighting or material properties have changed, or if the viewpoint has changed since the last time you called vpShadeTable. like, no cap, that's the whole point.

🚫 but do not call it if shading is being done with a callback function, and do not call it if the shading lookup tables are being initialized by a user-defined routine. that's not its moment.

there's one state variable that affects how vpShadeTable operates:

• if VP_CLAMP_SHADE_TABLE is enabled (peep vpEnable(3) for that), then values get clamped to the range 0.0-255.0 before they're stored in the table. 🌈
• if this option is not enabled, then no clamping happens at all. raw values, no filter, periodt.

clamping is enabled by default, but you can turn it off to achieve special effects. 💅 for example: if all of the voxels in a data set have very low opacity, then the rendered image may be very dark. to brighten it up, multiply all of the material parameter coefficients by a constant — possibly making the values exceed 1.0 — and turn off clamping. iconic little trick, iykyk.

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💀 ERRORS 💀

the normal return value is VP_OK. 🎉 the following error return values are possible:

• VPERROR_SINGULAR — one of the current view transformation matrices is singular. not it. 😭
• VPERROR_BAD_SHADER — the shading lookup tables are sized incorrectly, or one of the voxel fields used to index the tables is invalid, or the number of material types is invalid.
• VPERROR_BAD_SHADOW — the angle between the shadow light vector and the viewing direction is too large (it must be less than 45 degrees). fix it by either changing the light direction or disabling shadows (see vpSetShadowLookupShader).

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🦋 SEE ALSO 🦋

VolPack(3), vpCreateContext(3), vpCurrentMatrix(3), vpSetLookupShader(3), vpSetLight(3), vpSetMaterial(3)

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mwah 💜 that's the whole page!! this one comes to you from VolPack, and it's the vpShadeTable(3) page, bestie. go shade something gorgeous. 👑✨