From 9187edcd45e97efd5b4d699c071bc1530f298d54 Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Sat, 8 May 2021 09:00:26 -0400
Subject: graphics: model: Add basic alpha mode support for glTF models.

---
 chickadee/graphics/model.scm | 11 +++++++----
 chickadee/graphics/pbr.scm   | 14 +++++++++-----
 data/shaders/pbr-frag.glsl   | 22 ++++++++++++++++++++++
 3 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/chickadee/graphics/model.scm b/chickadee/graphics/model.scm
index caec045..5228db8 100644
--- a/chickadee/graphics/model.scm
+++ b/chickadee/graphics/model.scm
@@ -905,11 +905,14 @@
             (let ((v (or (array-ref/optional obj "emissiveFactor")
                          #(1.0 1.0 1.0))))
               (vec3 (vector-ref v 0) (vector-ref v 1) (vector-ref v 2))))
+           ;; TODO: Sort primitives such that all OPAQUE and MASK
+           ;; objects are drawn first, then draw primitives with
+           ;; BLEND.
            (alpha-mode (match (or (string-ref/optional obj "alphaMode")
-                                  "BLEND")
-                         ("OPAQUE" 'opaque)
-                         ("MASK" 'mask)
-                         ("BLEND" 'blend)))
+                                  "OPAQUE")
+                         ("OPAQUE" 0)
+                         ("MASK" 1)
+                         ("BLEND" 2)))
            (alpha-cutoff (or (number-ref/optional obj "alphaCutoff") 0.5))
            (double-sided? (boolean-ref/optional obj "doubleSided"))
            (extensions (object-ref/optional obj "extensions"))
diff --git a/chickadee/graphics/pbr.scm b/chickadee/graphics/pbr.scm
index 4b5b192..1f9853f 100644
--- a/chickadee/graphics/pbr.scm
+++ b/chickadee/graphics/pbr.scm
@@ -24,6 +24,7 @@
 (define-module (chickadee graphics pbr)
   #:use-module (chickadee config)
   #:use-module (chickadee math vector)
+  #:use-module (chickadee graphics blend)
   #:use-module (chickadee graphics buffer)
   #:use-module (chickadee graphics color)
   #:use-module (chickadee graphics engine)
@@ -77,7 +78,7 @@
   (local-field emissive-texture pbr-material-emissive-texture)
   (bool emissive-texture-enabled pbr-material-emissive-texture-enabled)
   (int emissive-texcoord pbr-material-emissive-texcoord)
-  (local-field alpha-mode pbr-material-alpha-mode)
+  (int alpha-mode pbr-material-alpha-mode)
   (float alpha-cutoff pbr-material-alpha-cutoff)
   (local-field double-sided? pbr-material-double-sided?))
 
@@ -101,7 +102,7 @@
                      #:emissive-texture null-texture
                      #:emissive-texture-enabled #f
                      #:emissive-texcoord 0
-                     #:alpha-mode 'opaque
+                     #:alpha-mode 0
                      #:alpha-cutoff 1.0
                      #:double-sided? #f))
 
@@ -113,9 +114,12 @@
   (let* ((shader (graphics-variable-ref pbr-shader))
          (vattrs (vertex-array-attributes vertex-array))
          (sattrs (shader-attributes shader)))
-    (with-graphics-state ((g:cull-face-mode (if (pbr-material-double-sided? material)
-                                                no-cull-face-mode
-                                                back-cull-face-mode))
+    (with-graphics-state ((g:blend-mode (if (= (pbr-material-alpha-mode material) 2)
+                                            blend:alpha
+                                            blend:replace))
+                          (g:cull-face-mode (if (pbr-material-double-sided? material)
+                                                   no-cull-face-mode
+                                                   back-cull-face-mode))
                           (g:texture-0 (pbr-material-base-color-texture material))
                           (g:texture-1 (pbr-material-normal-texture material))
                           (g:texture-2 (pbr-material-occlusion-texture material))
diff --git a/data/shaders/pbr-frag.glsl b/data/shaders/pbr-frag.glsl
index ce252ab..5b21fe1 100644
--- a/data/shaders/pbr-frag.glsl
+++ b/data/shaders/pbr-frag.glsl
@@ -15,6 +15,7 @@ struct Material {
   vec3 emissiveFactor;
   bool emissiveTextureEnabled;
   int emissiveTexcoord;
+  int alphaMode;
   float alphaCutoff;
 };
 
@@ -49,6 +50,25 @@ vec4 sampleTexture(sampler2D tex, bool enabled, int texcoord, vec3 factor, vec4
   }
 }
 
+vec4 applyAlpha(vec4 color) {
+  // Apply alpha mode.
+  if(material.alphaMode == 0) {
+    return vec4(color.xyz, 1.0);
+  } else if(material.alphaMode == 1) {
+    if(color.a >= material.alphaCutoff) {
+      return vec4(color.xyz, 1.0);
+    } else {
+      discard;
+    }
+  } else if(material.alphaMode == 2) {
+    if(color.a <= 0.005) {
+      discard;
+    } else {
+      return color;
+    }
+  }
+}
+
 void main(void) {
   vec4 finalColor = sampleTexture(baseColorTexture,
                                   material.baseColorTextureEnabled,
@@ -77,6 +97,8 @@ void main(void) {
                               material.emissiveTexcoord,
                               material.emissiveFactor,
                               vec4(0.0, 0.0, 0.0, 0.0));
+  finalColor = applyAlpha(finalColor);
+
 #ifdef GLSL330
   fragColor = finalColor;
 #else
-- 
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