ShaderGenerator: Simplify shader if ColorWriteAttrib is C_off

A lot more could be done here (though the compiler will probably already do a lot of DCE since we're not writing to the output color) but the important bit for now is that we don't perform shadow mapping, since we don't want to read from the shadow buffer while we're rendering to it in the shadow pass

panda/src/pgraphnodes/shaderGenerator.cxx

@@ -40,6 +40,7 @@
 #include "directionalLight.h"
 #include "renderModeAttrib.h"
 #include "rescaleNormalAttrib.h"
+#include "colorWriteAttrib.h"
 #include "pointLight.h"
 #include "sphereLight.h"
 #include "spotlight.h"
@@ -443,6 +444,22 @@ analyze_renderstate(ShaderKey &key, const RenderState *rs) {
     have_alpha_blend = true;
   }
 
+  // Are we writing to the color channel at all?
+  bool have_color_write = false;
+  const ColorWriteAttrib *color_write;
+  rs->get_attrib_def(color_write);
+  if (color_write->get_channels() != ColorWriteAttrib::C_off) {
+    have_color_write = true;
+  } else {
+    key._flags |= ShaderKey::F_disable_color_write;
+
+    // Subsume alpha test so we can eliminate the color output
+    if (have_alpha_test) {
+      key._flags |= (alpha_test->get_mode() & 0x7) << ShaderKey::F_ALPHA_TEST_SHIFT;
+      key._alpha_test_ref = alpha_test->get_reference_alpha();
+    }
+  }
+
   if (have_alpha_blend || have_alpha_test) {
     key._flags |= ShaderKey::F_calc_primary_alpha;
   }
@@ -456,7 +473,7 @@ analyze_renderstate(ShaderKey &key, const RenderState *rs) {
     if (have_alpha_blend) {
       key._flags |= ShaderKey::F_disable_alpha_write;
     }
-    else {
+    else if (have_color_write) {
       key._flags |= ShaderKey::F_out_alpha_glow;
       if (have_alpha_test) {
         // Subsume the alpha test in our shader.
@@ -537,7 +554,7 @@ analyze_renderstate(ShaderKey &key, const RenderState *rs) {
         }
       }
 
-      if (shader_attrib->auto_shadow_on()) {
+      if (have_color_write && shader_attrib->auto_shadow_on()) {
         if (llnode->is_shadow_caster()) {
           light_flags |= ShaderKey::LF_has_shadows;
 
@@ -790,7 +807,7 @@ analyze_renderstate(ShaderKey &key, const RenderState *rs) {
     }
   }
 
-  if (shader_attrib->auto_ramp_on()) {
+  if (have_color_write && shader_attrib->auto_ramp_on()) {
     const LightRampAttrib *light_ramp;
     if (rs->get_attrib(light_ramp)) {
       key._light_ramp_mode = light_ramp->get_mode();
@@ -811,7 +828,7 @@ analyze_renderstate(ShaderKey &key, const RenderState *rs) {
 
   // Check for fog.
   const FogAttrib *fog;
-  if (rs->get_attrib(fog) && !fog->is_off()) {
+  if (have_color_write && rs->get_attrib(fog) && !fog->is_off()) {
     key._flags |= ((int)fog->get_fog()->get_mode() + 1) << ShaderKey::F_FOG_MODE_SHIFT;
   }
 
@@ -1392,7 +1409,9 @@ synthesize_shader(const RenderState *rs, const GeomVertexAnimationSpec &anim) {
   if (key._flags & (ShaderKey::F_out_aux_normal | ShaderKey::F_out_aux_glow)) {
     text << "\t out float4 o_aux : COLOR1,\n";
   }
-  text << "\t out float4 o_color : COLOR0,\n";
+  if ((key._flags & ShaderKey::F_disable_color_write) == 0) {
+    text << "\t out float4 o_color : COLOR0,\n";
+  }
 
   if (need_color) {
     if (key._flags & ShaderKey::F_vertex_color) {
@@ -2052,7 +2071,9 @@ synthesize_shader(const RenderState *rs, const GeomVertexAnimationSpec &anim) {
 
   // The multiply is a workaround for a radeon driver bug.  It's annoying as
   // heck, since it produces an extra instruction.
-  text << "\t o_color = result * 1.000001;\n";
+  if ((key._flags & ShaderKey::F_disable_color_write) == 0) {
+    text << "\t o_color = result * 1.000001;\n";
+  }
   if ((key._flags & ShaderKey::F_ALPHA_TEST_MASK) != 0) {
     text << "\t // Shader subsumes normal alpha test.\n";
   }
@@ -2070,7 +2091,7 @@ synthesize_shader(const RenderState *rs, const GeomVertexAnimationSpec &anim) {
   PT(Shader) shader = Shader::make(text.str(), Shader::SL_Cg);
   nassertr(shader != nullptr, nullptr);
 
-  shader->_subsumes_alpha_test = true;
+  shader->_subsumes_alpha_test = (key._flags & ShaderKey::F_ALPHA_TEST_MASK) != 0;
 
   reset_register_allocator();
 

panda/src/pgraphnodes/shaderGenerator.h

@@ -122,6 +122,7 @@ protected:
       F_ALPHA_TEST_MASK       = 0x07000000,
 
       F_use_shadow_filter     = 0x08000000,
+      F_disable_color_write   = 0x10000000,
     };
 
     enum TextureFlags {