Merge pull request #825 from cx20/add_gltf_sample_model

Added glTF Sample model

index.js

@@ -366,7 +366,8 @@ function makeTutorialFeatureTestModelLinks() {
                      || modelName == 'NegativeScaleTest' 
                      || modelName == 'SuzanneMorphSparse' 
                      || modelName == 'TextureEncodingTest' 
-                     || modelName == 'TextureLinearInterpolationTest' ) {
+                     || modelName == 'TextureLinearInterpolationTest' 
+                     || modelName == 'VertexColorAlphaClipTest' ) {
                 td.appendChild(createlink(engines[i].name, 'tutorialModels', modelName, 'glTF', scale));
                 td.appendChild(document.createElement('br'));
                 td.appendChild(createlink(engines[i].name, 'tutorialModels', modelName, 'glTF-Binary', scale));

tutorialModels/VertexColorAlphaClipTest/README.body.md

@@ -0,0 +1,84 @@
+## Screenshot
+
+![screenshot](screenshot/screenshot-large.jpg)
+
+## Description
+
+This model tests what happens when vertex colors contain alpha values and a material uses `alphaCutoff`.  The above screenshot shows what this model looks like when the test passes.
+
+## Common Problems
+
+### Problem: Noisy alpha clipping
+
+![screenshot](screenshot/fail_alphaClipNoise.jpg)
+
+The two middle cubes (yellow and green) show a lot of fragment noise in the above screenshot. Here's a zoomed-in view to show detail on the green cube:
+
+![screenshot](screenshot/fail_noise_detail.png)
+
+The exact reason this noise is present can vary by rendering engine implementation, platform, and GPU in use.  It's possible that discarding masked fragments too early in a shader (prior to gradients and texture lookup usage) may be a contributing factor in some implementations.
+
+### Problem: No alpha mask
+
+![screenshot](screenshot/fail_noAlphaCutoff.jpg)
+
+The above screenshot shows a renderer that is not doing any alpha masking against vertex color alpha values.  In this case, a large red "X" appears over the left-most sample that is intended to be left blank.  This indicates a test failure.
+
+### Problem: Incorrect alpha comparison
+
+![screenshot](screenshot/fail_noEquality.jpg)
+
+In the above screenshot, the two middle cubes are missing, indicating that when the alpha value exactly matches the `alphaCutoff`, the result is clipped.
+
+The glTF specification [Alpha Coverage section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#alpha-coverage) has this to say:
+
+> When `alphaMode` is set to `MASK` the `alphaCutoff` property specifies the cutoff threshold. If the alpha value is greater than or equal to the `alphaCutoff` value then it is rendered as fully opaque, otherwise, it is rendered as fully transparent.
+
+The specification makes special mention of the equality case presented here by the two middle cubes in yellow and green.  They should be rendered fully opaque.  So a rendering similar to the above screenshot with the missing middle cubes is a test failure.
+
+## Label Text
+
+In case the text on the labels is hard to read or needs translation, it is reproduced below:
+
+### Far left sample
+
+```
+THIS SPACE INTENTIONALLY LEFT BLANK
+
+There should not be an X here.
+```
+
+```
+   Blank space
+  COLOR_0.a = 0.48
+alphaCutoff = 0.5
+   (less than)
+```
+
+### Second sample, yellow box
+
+```
+   Solid Box
+  COLOR_0.a = 0.5
+alphaCutoff = 0.5
+   (equal)
+```
+
+### Third sample, green box
+
+```
+   Solid Box
+  COLOR_0.a = 1.0
+alphaCutoff = 1.0
+   (equal)
+```
+
+### Far right sample, blue box
+
+```
+   Solid Box
+  COLOR_0.a = 0.51
+alphaCutoff = 0.5
+  (greater than)
+```
+