Fix the alpha gamma value when rendering legacy diffuse textures and materials

With the new PBR renderer, the faces using alpha blend with a simple diffuse
texture or a Blinn-Phong material are badly rendered (way too opaque), which
breaks rendering for an enormous amount of "legacy" (pre-PBR) objects.

It has been proposed (1) to introduce a new "alpha gamma" parameter for faces,
that would need to be set on legacy contents in order to fix the latter, which
would also force to introduce a no-mod permission exception for this parameter
alone, so that people owning a legacy no-mod object could still manually edit
it to fix its rendering.
This proposal is ill-conceived: how can anyone, in their right mind, hope that
*every* owner of *every* "legacy" object using alpha blending in SL will
bother to edit the said object (supposing they even know they can do it and
how to do it) ???

The *proper* solution is to fix the renderer itself, and automatically apply a
gamma correction factor to faces *not bearing* a glTF material.

This commit implements this fix (which is now part of the Cool VL Viewer and
proves that it works very nicely). It implements the same kind of rendering
fix (at the shader level) as (2), but instead automatically detects faces
rendering with a PBR material, and applies the correction to all faces *not*
using such a material.

In the event you want to let contents creators use linear alpha channels in
objects mixing legacy BP materials and PBR materials (even though I fail to
see how a creator would want to do that at all), I suggest you implement an
additional flag for faces, that could be set to antagonize this auto-fix
(this would let full leeway about the alpha channel scale for newly created
contents while not requiring to implement some weird no-mod permission
exception since this flag would be set by the creator themselves).

In the mean time, this fix solves one of the remaining major issues seen
with the PBR renderer.

(1) https://feedback.secondlife.com/bug-reports/p/pbr-client-opacity-issue-on-textures-with-alpha-channel-windows
(2) secondlife#2668

Author: vldevel

indra/llrender/llshadermgr.cpp

@@ -1313,6 +1313,7 @@ void LLShaderMgr::initAttribsAndUniforms()
 
     mReservedUniforms.push_back("minimum_alpha");
     mReservedUniforms.push_back("emissive_brightness");
+    mReservedUniforms.push_back("alpha_gamma");
 
     // Deferred
     mReservedUniforms.push_back("shadow_matrix");

indra/llrender/llshadermgr.h

@@ -156,6 +156,7 @@ class LLShaderMgr
 
         MINIMUM_ALPHA,                      //  "minimum_alpha"
         EMISSIVE_BRIGHTNESS,                //  "emissive_brightness"
+        ALPHA_GAMMA,                        //  "alpha_gamma"
 
         DEFERRED_SHADOW_MATRIX,             //  "shadow_matrix"
         DEFERRED_ENV_MAT,                   //  "env_mat"

indra/newview/app_settings/settings.xml

@@ -7478,6 +7478,17 @@
     <key>Value</key>
     <integer>0</integer>
   </map>
+  <key>RenderLegacyAlphaGamma</key>
+  <map>
+    <key>Comment</key>
+    <string>Gamma correction factor (clamped between 1.0 and 2.2) used to render non-PBR faces with alpha blend. 1.0 to disable.</string>
+    <key>Persist</key>
+    <integer>1</integer>
+    <key>Type</key>
+    <string>F32</string>
+    <key>Value</key>
+    <real>1.8</real>
+  </map>
   <key>RenderLocalLightCount</key>
   <map>
     <key>Comment</key>

indra/newview/app_settings/shaders/class2/deferred/alphaF.glsl

@@ -37,6 +37,7 @@ uniform mat3 env_mat;
 uniform vec3 sun_dir;
 uniform vec3 moon_dir;
 uniform int classic_mode;
+uniform float alpha_gamma;
 
 #ifdef USE_DIFFUSE_TEX
 uniform sampler2D diffuseMap;
@@ -215,7 +216,7 @@ void main()
     }
 
     color.rgb = diffuse_srgb.rgb;
-    color.a = final_alpha;
+    color.a = pow(final_alpha, alpha_gamma);
 
 #else // FOR_IMPOSTOR
 
@@ -263,7 +264,7 @@ void main()
 
     vec4 color = vec4(0.0);
 
-    color.a   = final_alpha;
+    color.a = pow(final_alpha, alpha_gamma);
 
     color.rgb = irradiance;
     if (classic_mode > 0)

indra/newview/app_settings/shaders/class3/deferred/materialF.glsl

@@ -37,6 +37,7 @@
 uniform float emissive_brightness;  // fullbright flag, 1.0 == fullbright, 0.0 otherwise
 uniform int sun_up_factor;
 uniform int classic_mode;
+uniform float alpha_gamma;
 
 vec4 applySkyAndWaterFog(vec3 pos, vec3 additive, vec3 atten, vec4 color);
 vec3 scaleSoftClipFragLinear(vec3 l);
@@ -420,7 +421,7 @@ void main()
 
     glare *= 1.0-emissive;
     glare = min(glare, 1.0);
-    float al = max(diffcol.a, glare) * vertex_color.a;
+    float al = pow(max(diffcol.a, glare) * vertex_color.a, alpha_gamma);
     float final_scale = 1;
     if (classic_mode > 0)
         final_scale = 1.1;

indra/newview/lldrawpoolalpha.cpp

@@ -595,6 +595,11 @@ void LLDrawPoolAlpha::renderAlpha(U32 mask, bool depth_only, bool rigged)
         above_water = !above_water;
     }
 
+     // Gamma correction factor for alpha faces without PBR material, in PBR
+    // rendering mode. HB
+    static LLCachedControl<F32> legacy_gamma(gSavedSettings,
+                                             "RenderLegacyAlphaGamma");
+    F32 alpha_gamma = llclamp((F32)legacy_gamma, 1.f, 2.2f);
 
     for (LLCullResult::sg_iterator i = begin; i != end; ++i)
     {
@@ -760,6 +765,8 @@ void LLDrawPoolAlpha::renderAlpha(U32 mask, bool depth_only, bool rigged)
                         current_shader->uniform4f(LLShaderMgr::SPECULAR_COLOR, spec_color.mV[VRED], spec_color.mV[VGREEN], spec_color.mV[VBLUE], spec_color.mV[VALPHA]);
                         current_shader->uniform1f(LLShaderMgr::ENVIRONMENT_INTENSITY, env_intensity);
                         current_shader->uniform1f(LLShaderMgr::EMISSIVE_BRIGHTNESS, brightness);
+                        // Fix for alpha gamma on non-PBR (legacy) faces. HB
+                        current_shader->uniform1f(LLShaderMgr::ALPHA_GAMMA, params.mHasPBR ? 1.f : alpha_gamma);
                     }
                 }
 

indra/newview/llspatialpartition.h

@@ -136,6 +136,8 @@ class LLDrawInfo final : public LLRefCount
     U8   mShiny = 0;
     bool mFullbright = false;
     bool mHasGlow = false;
+	// Set to 'true' when a PBR material is actually used to render this face. HB
+    bool mHasPBR = false;
 
     struct CompareTexture
     {

indra/newview/llvovolume.cpp

@@ -5372,10 +5372,9 @@ void LLVolumeGeometryManager::registerFace(LLSpatialGroup* group, LLFace* facep,
 
     U8 index = facep->getTextureIndex();
 
+    bool has_pbr = false;
     LLMaterial* mat = nullptr;
-
     LLUUID mat_id;
-
     auto* gltf_mat = (LLFetchedGLTFMaterial*)te->getGLTFRenderMaterial();
     llassert(gltf_mat == nullptr || dynamic_cast<LLFetchedGLTFMaterial*>(te->getGLTFRenderMaterial()) != nullptr);
     if (gltf_mat != nullptr)
@@ -5384,6 +5383,7 @@ void LLVolumeGeometryManager::registerFace(LLSpatialGroup* group, LLFace* facep,
         if (!facep->hasMedia() || (tex && tex->getType() != LLViewerTexture::MEDIA_TEXTURE))
         { // no media texture, face texture will be unused
             tex = nullptr;
+            has_pbr = true;
         }
     }
     else
@@ -5447,6 +5447,7 @@ void LLVolumeGeometryManager::registerFace(LLSpatialGroup* group, LLFace* facep,
         info->mCount + facep->getIndicesCount() <= (U32) gGLManager.mGLMaxIndexRange &&
 #endif
         info->mMaterialID == mat_id &&
+        info->mHasPBR == has_pbr &&
         info->mFullbright == fullbright &&
         info->mBump == bump &&
         (!mat || (info->mShiny == shiny)) && // need to break batches when a material is shared, but legacy settings are different
@@ -5501,6 +5502,7 @@ void LLVolumeGeometryManager::registerFace(LLSpatialGroup* group, LLFace* facep,
         draw_info->mShaderMask = shader_mask;
         draw_info->mAvatar = facep->mAvatar;
         draw_info->mSkinInfo = facep->mSkinInfo;
+        draw_info->mHasPBR = has_pbr;
 
         if (gltf_mat)
         {