vpSetClassifierTable(3)

💖 vpSetClassifierTable — bestie's guide to opacity transfer functions 💖

okay so real talk, this is vpSetClassifierTable(3) from the Library Functions Manual (that's the VolPack crew's manual, section 3, the library functions girlies). it's the function you call when you wanna tell VolPack how to turn voxel values into opacity. she's giving very important, lowkey the whole vibe of your volume rendering depends on her. let's get into it 🫶

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✨ NAME ✨

vpSetClassifierTable — specify an opacity transfer function 🌸

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✨ SYNOPSIS ✨

#include <volpack.h>

vpResult
vpSetClassifierTable(vpc, param_num, param_field, table, table_size)
    vpContext *vpc;
    int param_num, param_field;
    float *table;
    int table_size;

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✨ ARGUMENTS ✨

• vpc — your VolPack context, the one you got from vpCreateContext. she's the main character, everything runs through her 💅
• param_num — the parameter number, like, which one of the opacity transfer function's parameters you're setting up.
• param_field — the field number, aka which voxel field actually holds that parameter.
• table — the lookup table, bestie! it's literally the mapping from voxel field value ➡️ opacity. the glow-up pipeline ✨
• table_size — size of table, and yes, it's in bytes. don't get it twisted.

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✨ DESCRIPTION ✨

so vpSetClassifierTable is what you use to specify the lookup tables that define an opacity transfer function. and here's the tea: the opacity transfer function has to be in this exact form:

opacity = f0(param0) * f1(param1) * ... * fN(paramN)

where each of those factors is a function of one voxel field. (if you need something more general than that, keep reading, there's a whole section for you at the bottom, don't worry 🥰)

you call vpSetClassifierTable once for each factor, each time handing over a lookup table that defines that factor. once the function is fully specified, you compute opacities by calling one of the classification functions: vpClassifyScalars, vpClassifyVolume, or vpClassifyScanline. here's how that goes: to classify a voxel, those functions take the values in the voxel's fields and use them as indexes into the opacity transfer function tables, grab one value from each table, and multiply all those values together to get the opacity for that voxel. math queen behavior fr fr 🧮👑

the opacity should be a number in the range 0.0-1.0, so normally each table holds numbers in that range too. keep it in range, bestie, it's giving responsible.

the parameters of the opacity transfer function are numbered from zero, and the total number of parameters must be specified using vpSetVoxelSize before calling vpSetClassifierTable. do it in that order, i'm literally begging 🙈

and no, the number of parameters is independent of the number of voxel fields. so some fields might not be used as parameters at all, and one field could totally be used multiple times — though that's inefficient, since all the tables indexed by the same parameter could just be merged into one table. just merge them. be efficient. slay ✨

each table should be a 1D array of numbers with one entry for each possible value of the corresponding voxel field. the voxel field must contain an unsigned integer quantity. normally you'd declare it like this:

float table[MAX_FIELD_VALUE + 1];

where MAX_FIELD_VALUE is the maximum possible value in the voxel field, as specified with vpSetVoxelField. the table can also be dynamically allocated if that's your vibe.

⚠️ okay this part is important: VolPack will never free the table, not even if the VolPack context gets destroyed with vpDestroyContext. so you, the application, are responsible for freeing that memory when it's appropriate. no cap. don't leak, bestie, it's not cute 😭

you can initialize or modify the values in the table at any time before or after calling vpSetClassifierTable — but it has to be before you call one of the classification routines. also, peep vpRamp for a convenient way to initialize tables with piece-wise linear ramps. she's the shortcut 🎀

classification functions that aren't expressible in that form? totally handleable, here's the play:

1. first, write your own routine that computes the opacity for each voxel.
2. next, quantize the opacities using somewhere between 8 and 16 bits per voxel, and store those quantized values in one field of each voxel.
3. finally, define an opacity transfer function table that maps the quantized opacities into a floating point number in the range 0.0-1.0, and use that table to define a single-parameter opacity transfer function by calling vpSetClassifierTable.

boom. iconic. 💅

oh also — the minimum opacity threshold should be set at the same time as the opacity transfer function. see the VP_MIN_VOXEL_OPACITY argument to vpSetd. don't forget her 🫶

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✨ STATE VARIABLES ✨

you can retrieve the current classification function parameters with these state variable codes (check vpGeti(3) for the deets): VP_CLASSIFY_FIELD_COUNT, VP_CLASSIFY_FIELDS, VP_CLASSIFY_TABLES, VP_CLASSIFY_TABLE_SIZES, VP_MIN_VOXEL_OPACITY ✨

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✨ ERRORS ✨

the normal return value is VP_OK. if things go sideways, this is the error return value you might get:

• VPERROR_BAD_VALUE — either param_num or field_num is out of range. 💀 out of range is not the vibe.

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✨ SEE ALSO ✨

VolPack(3), vpCreateContext(3), vpClassifyScanline(3), vpClassifyScalars(3), vpClassifyVolume(3), vpRamp(3)

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and that's the whole page, bestie, you ate it 🍓💜

this lil masterpiece comes courtesy of the VolPack manual — it's the vpSetClassifierTable(3) page, section 3 of the Library Functions Manual. no date on this one, she's timeless like that 👑✨ go forth and classify, you absolute legend 🌈🦋