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Min / Max rangesNormally, you would look in Maxwells console panel to determine what the min/max ranges are for a certain grid, and enter those in the corresponding VEC_MOD_x, or FLOAT_x parameters.
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Finding out which grids
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exist in an OpenVDB file
In order to find out how many grids are actually present in a VDB file, what their names are, what type of values they hold (float or vectors) and what these values are, simply load the vdb file using the MaxwellVolumetric extension and hit render. Look in the Console panel for detailed info about the data in the vdb file.
Texturing OpenVDB volumetrics
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To experiment, you can use some of the VDB files found at the official download page of the OpenVDB website. In the following examples we have used the "explosion.vdb" file. |
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By changing the corresponding min/max values, you can remap the gradient on the volumetric cloud, to make parts of it appear more prominently, or even make the gradient start to repeat. For example, if you set the max value to 0.2 instead of 0.884 in this case, the entire gradient will be mapped to parts of the grid that fall within the range 0.0 - 0.2. In areas of the grid that exceed that range, the grid will be repeated (if texture tiling is turned on), this time between the range 0.2-0.4, then repeated again between 0.4 -0.6, and again between 0.6 - 0.8. So we would have approximately 4 repeats in this case. On the other hand, if you set a max value double of what is stored in the grid, the actual max value of the grid will be "reached" much sooner and only the left half of the gradient will be visible on the volumetric. Finally, if you have texture tiling turned off and you use a smaller max value than the grid holds, when in areas in of the grid which have higher max values then than what you've specified, the texture won't be repeated and instead it's the materials Reflectance 0 color that which will show. The following four renders demonstrate using different max values and tiling set to on and off:
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