# Objective
Add depth prepass and support for opaque, alpha mask, and alpha blend modes for the 3D PBR target.
## Solution
NOTE: This is based on top of #2861 frustum culling. Just lining it up to keep @cart loaded with the review train. 🚂
There are a lot of important details here. Big thanks to @cwfitzgerald of wgpu, naga, and rend3 fame for explaining how to do it properly!
* An `AlphaMode` component is added that defines whether a material should be considered opaque, an alpha mask (with a cutoff value that defaults to 0.5, the same as glTF), or transparent and should be alpha blended
* Two depth prepasses are added:
* Opaque does a plain vertex stage
* Alpha mask does the vertex stage but also a fragment stage that samples the colour for the fragment and discards if its alpha value is below the cutoff value
* Both are sorted front to back, not that it matters for these passes. (Maybe there should be a way to skip sorting?)
* Three main passes are added:
* Opaque and alpha mask passes use a depth comparison function of Equal such that only the geometry that was closest is processed further, due to early-z testing
* The transparent pass uses the Greater depth comparison function so that only transparent objects that are closer than anything opaque are rendered
* The opaque fragment shading is as before except that alpha is explicitly set to 1.0
* Alpha mask fragment shading sets the alpha value to 1.0 if it is equal to or above the cutoff, as defined by glTF
* Opaque and alpha mask are sorted front to back (again not that it matters as we will skip anything that is not equal... maybe sorting is no longer needed here?)
* Transparent is sorted back to front. Transparent fragment shading uses the alpha blending over operator
Co-authored-by: Carter Anderson <mcanders1@gmail.com>
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