Add property attribute point clouds sample

[javagl]

A DRAFT for an example showing point clouds with property attributes and statistics.

[javagl]

Updated the sample to add a README, screenshot and sandcastle.

Preview: https://github.com/javagl/3d-tiles-samples/tree/add-property-attribute-point-clouds/glTF/EXT_structural_metadata/PropertyAttributesPointCloud

The line

int classification = int(fsInput.metadata.classification + 0.5);

is still to be discussed: It should not be necessary to round up there.

[lilleyse]

sRGB -> linear conversion needed here?

[lilleyse]

The line

int classification = int(fsInput.metadata.classification + 0.5);

is still to be discussed: It should not be necessary to round up there.

It seems like the attributes are being interpolated even though the mode is gl.POINTS. My theory is that the GPU is treating each point as a quad, duplicating the attributes at each corner, and running interpolation math. I wonder if the flat qualifier in WebGL 2 would fix this.

I put together some custom shaders to help debug this.

Version 1: vertex shader only approach - works!

  CLASSIFICATION_SHADER: new Cesium.CustomShader({
    varyings: {
      v_color: Cesium.VaryingType.VEC3,
    },
    vertexShaderText: `
    void vertexMain(VertexInput vsInput, inout czm_modelVertexOutput vsOutput) {
        int classif = int(vsInput.attributes.classification);
        
        vec3 color = vec3(1);
        if (classif == 0) {
            color = vec3(0,0.5,0);
        }
        else if (classif == 1) {
            color = vec3(0.5,0.5,0.5);
        }

        v_color = color;
    }`,
    fragmentShaderText: `
    void fragmentMain(FragmentInput fsInput, inout czm_modelMaterial material)
    {
        material.diffuse = v_color;
    }
    `,
  }),

Version 2: fragment shader approach with custom varying - fails!

  CLASSIFICATION_SHADER: new Cesium.CustomShader({
    varyings: {
      v_classif: Cesium.VaryingType.FLOAT,
    },
    vertexShaderText: `
    void vertexMain(VertexInput vsInput, inout czm_modelVertexOutput vsOutput) {
        v_classif = vsInput.attributes.classification;
    }`,
    fragmentShaderText: `
    void fragmentMain(FragmentInput fsInput, inout czm_modelMaterial material)
    {
        vec3 color = vec3(1);
        int classification = int(v_classif);
        if (classification == 0) {
            color = vec3(0,0.5,0);
        }
        else if (classification == 1) {
            color = vec3(0.5,0.5,0.5);
        }
        material.diffuse = color;
    }
    `,
  }),

[javagl]

I have updated the colors with sRGB (not sure whether it's worth updating the screenshot as well), and applied the workaround that @lilleyse suggested. The issue of imprecise int values in the fragment shader is now tracked at CesiumGS/cesium#10699 (also linked in the sandcastle), so I think this is ready for review.

[javagl]

I actually had noticed that in the comment, but didn't update it when copying the shader from the comment to the sandcastle. Updated.