TextureEvaluator.GetColor(), Ray Differential parameters?

bfrederick · October 28, 2021, 4:51pm

Hi ,

I’m trying to improve the results from the method:

TextureEvaluator.GetColor( Point3d uvw, Vector3d duvwdx, Vector3d duvwdy )

So I assumed using the duvwdx, duvwdy ray differential parameters might help.

However, I have no idea how to construct these vectors. I tried to follow the documentation and reference Pharr Humphreys, “Physically Based Rendering”, chapter 11. (Chapter 10.1.1 in the version I found. pg. 601.)

But… it’s a bit over my head. From what I can gather these two vectors duvwdx, duvwdy are x,y offsets (one pixel sample?) in the a tangent plane of the first parameter uvw?

Any thoughts @nathanletwory? I noticed you had a code comment about using these parameters in the overridden method of GetColor you posted here.

Any help would be much appreciated, I’m pretty new to this area of the API.

nathanletwory · October 28, 2021, 5:10pm

Btw, the PBR book is these days for free available on the internet at Physically Based Rendering: From Theory to Implementation .

In RhinoCycles I sample textures in this way:

github.com

mcneel/RhinoCycles/blob/b89ba6b40e4dd4c5f9329a0a284f1efd16d9f8f8/Converters/BitmapConverter.cs#L495

    
      
          				return GetEnumeratorImpl();
          			}
          
          
			IEnumerator<float> IEnumerable<float>.GetEnumerator()
          			{
          				return GetEnumeratorImpl();
          			}
          
          
			IEnumerator<float> GetEnumeratorImpl()
          			{
          				var halfpixelU = 0.5 / _width;
          				var halfpixelV = 0.5 / _height;
          				var duvw = new Vector3d(halfpixelU, halfpixelV, 0.0);
          
          
				if (_image_based)
          				{
          					duvw.X = duvw.Y = duvw.Z = 0.0;
          				}
          
          
				var pt = new Point3d();
          				var col4F = new Rhino.Display.Color4f();

So I get the half pixel values U and V based on the width and height of the texture respectively. This gives the vector I need.

When the texture is image based the vector is reset to be just zeroes.

The same vector is used for both the duvwdx and duvwdy cases.

bfrederick · October 28, 2021, 5:17pm

Amazing! Thanks for being so helpful and for the link!
Have a great day!