feat: interpolation in shader UI controls invlerp/transfer function and use interpolation in multi-channel setup for volume rendering

[seankmartin]

Summary

Adds an interpolation parameter to the invlerp and transferFunction shader ui controls, which enables trilinear interpolation when sampling the data. The default remains as using the nearest neighbour.

New parameter

#uicontrol invlerp contrast
#uicontrol vec3 color color
void main() {
  if (VOLUME_RENDERING) {
    float contrast_value = contrast(true /* interpolation */);
    emitRGBA(vec4(color * contrast_value, contrast_value));
  }
  else {
    float contrast_value = contrast(false /* no interpolation */);
    emitRGB(color * contrast_value);
  }
}

[jbms]

I suppose it may commonly be desired to have interpolation enabled for volume rendering and disabled otherwise, which indeed makes it less useful to have as a property of the invlerp control unless we also add separate volume rendering and non-volume rendering options.

[seankmartin]

Thanks Jeremy, I think maybe best to go down a route similar to Option 1 for now then. Unsure what would be clearer for the user in the shader.

Could do something like:

#uicontrol invlerp contrast

contrast() /* as normal, has all the available parameters */
contrastInterpolated() /* with interpolation */

As opposed to an overload of the contrast function that takes in a bool as a parameter.

Edit: just realised we can't do that because we could get redefinition if the user has another invlerp called contrastInterpolated. Will set back to using overloads.

[seankmartin]

Hi @jbms @fcollman I've expanded this to include a test for the behaviour and also the docs to include the new parameter

[chrisj]

code looks good!