Added "expanding" option for soft particles, which requires hardware depth test off. Improve soft particles documentation on the renderpath page. Closes #1623.

Docs/Reference.dox

@@ -1589,7 +1589,16 @@ The render path starts by allocating a readable depth-stencil texture the same s
 
 The ForwardDepth.xml render path does the same, but using a linear depth rendertarget instead of a hardware depth texture. The advantage is better compatibility (guaranteed to work without checking \ref Graphics::GetReadableDepthSupport "GetReadableDepthSupport()") but it has worse performance as it will perform an additional full scene rendering pass.
 
-Renderpaths whose name ends with "SP" implement soft particles by comparing the scene depth to particles' depth.
+\section RenderPaths_SoftParticles Soft particles rendering
+
+Soft particles rendering is a practical example of utilizing scene depth reading. Renderpaths whose name ends with "SP" enable soft particles by adding the SOFTPARTICLES shader compile define for the alpha pass. This is recognized by the LitParticle & UnlitParticle shaders.
+
+Soft particles can be implemented in two contrasting approaches: "shrinking" and "expanding". In the shrinking approach (default) depth test can be left on and the soft particle shader starts to reduce particle opacity when the particle geometry approaches solid geometry. In the expanding approach the particles should have depth test off, and the shader instead starts to reduce the particle opacity when the particle geometry overshoots the solid geometry.
+
+For the expanding mode, see the "Expand" family of particle techniques in Bin/CoreData/Techniques. The downside is that performance can be lower due to not being able to use the hardware depth test, and the same technique can not be used when readable depth is not available, as otherwise the particles would clip through any solid geometry.
+
+Note the SoftParticleFadeScale shader parameter which can be used to control the distance over which the fade
+will take effect. The renderpath sets the default value (1) but it can be adjusted by materials.
 
 \section RenderPaths_ForwardLighting Forward lighting special considerations
 

bin/CoreData/Shaders/GLSL/LitParticle.glsl

@@ -103,15 +103,28 @@ void PS()
     #endif
 
     // Soft particle fade
+    // In expand mode depth test should be off. In that case do manual alpha discard test first to reduce fill rate
     #ifdef SOFTPARTICLES
+        #ifdef EXPAND
+            if (diffColor.a < 0.01)
+                discard;
+        #endif
+
         float particleDepth = vWorldPos.w;
         #ifdef HWDEPTH
             float depth = ReconstructDepth(texture2DProj(sDepthBuffer, vScreenPos).r);
         #else
             float depth = DecodeDepth(texture2DProj(sDepthBuffer, vScreenPos).rgb);
         #endif
-        float diffZ = abs(depth - particleDepth) * (cFarClipPS - cNearClipPS);
-        float fade = clamp(1.0 - diffZ * cSoftParticleFadeScale, 0.0, 1.0);
+
+        #ifdef EXPAND
+            float diffZ = max(particleDepth - depth, 0.0) * (cFarClipPS - cNearClipPS);
+            float fade = clamp(diffZ * cSoftParticleFadeScale, 0.0, 1.0);
+        #else
+            float diffZ = abs(depth - particleDepth) * (cFarClipPS - cNearClipPS);
+            float fade = clamp(1.0 - diffZ * cSoftParticleFadeScale, 0.0, 1.0);
+        #endif
+
         diffColor.a = max(diffColor.a - fade, 0.0);
     #endif
 

bin/CoreData/Shaders/GLSL/UnlitParticle.glsl

@@ -57,15 +57,28 @@ void PS()
     #endif
 
     // Soft particle fade
+    // In expand mode depth test should be off. In that case do manual alpha discard test first to reduce fill rate
     #ifdef SOFTPARTICLES
+        #ifdef EXPAND
+            if (diffColor.a < 0.01)
+                discard;
+        #endif
+
         float particleDepth = vWorldPos.w;
         #ifdef HWDEPTH
             float depth = ReconstructDepth(texture2DProj(sDepthBuffer, vScreenPos).r);
         #else
             float depth = DecodeDepth(texture2DProj(sDepthBuffer, vScreenPos).rgb);
         #endif
-        float diffZ = abs(depth - particleDepth) * (cFarClipPS - cNearClipPS);
-        float fade = clamp(1.0 - diffZ * cSoftParticleFadeScale, 0.0, 1.0);
+
+        #ifdef EXPAND
+            float diffZ = max(particleDepth - depth, 0.0) * (cFarClipPS - cNearClipPS);
+            float fade = clamp(diffZ * cSoftParticleFadeScale, 0.0, 1.0);
+        #else
+            float diffZ = abs(depth - particleDepth) * (cFarClipPS - cNearClipPS);
+            float fade = clamp(1.0 - diffZ * cSoftParticleFadeScale, 0.0, 1.0);
+        #endif
+
         #ifndef ADDITIVE
             diffColor.a = max(diffColor.a - fade, 0.0);
         #else

bin/CoreData/Shaders/HLSL/LitParticle.hlsl

@@ -168,14 +168,27 @@ void PS(float2 iTexCoord : TEXCOORD0,
     #endif
 
     // Soft particle fade
+    // In expand mode depth test should be off. In that case do manual alpha discard test first to reduce fill rate
     #ifdef SOFTPARTICLES
+        #ifdef EXPAND
+            if (diffColor.a < 0.01)
+                discard;
+        #endif
+
         float particleDepth = iWorldPos.w;
         float depth = Sample2DProj(DepthBuffer, iScreenPos).r;
         #ifdef HWDEPTH
             depth = ReconstructDepth(depth);
         #endif
-        float diffZ = abs(depth - particleDepth) * (cFarClipPS - cNearClipPS);
-        float fade = saturate(1.0 - diffZ * cSoftParticleFadeScale);
+
+        #ifdef EXPAND
+            float diffZ = max(particleDepth - depth, 0.0) * (cFarClipPS - cNearClipPS);
+            float fade = saturate(diffZ * cSoftParticleFadeScale);
+        #else
+            float diffZ = abs(depth - particleDepth) * (cFarClipPS - cNearClipPS);
+            float fade = saturate(1.0 - diffZ * cSoftParticleFadeScale);
+        #endif
+
         diffColor.a = max(diffColor.a - fade, 0.0);
     #endif
 

bin/CoreData/Shaders/HLSL/UnlitParticle.hlsl

@@ -113,14 +113,27 @@ void PS(float2 iTexCoord : TEXCOORD0,
     #endif
     
     // Soft particle fade
+    // In expand mode depth test should be off. In that case do manual alpha discard test first to reduce fill rate
     #ifdef SOFTPARTICLES
+        #if defined(EXPAND) && !defined(ADDITIVE)
+            if (diffColor.a < 0.01)
+                discard;
+        #endif
+
         float particleDepth = iWorldPos.w;
         float depth = Sample2DProj(DepthBuffer, iScreenPos).r;
         #ifdef HWDEPTH
             depth = ReconstructDepth(depth);
         #endif
-        float diffZ = abs(depth - particleDepth) * (cFarClipPS - cNearClipPS);
-        float fade = saturate(1.0 - diffZ * cSoftParticleFadeScale);
+
+        #ifdef EXPAND
+            float diffZ = max(particleDepth - depth, 0.0) * (cFarClipPS - cNearClipPS);
+            float fade = saturate(diffZ * cSoftParticleFadeScale);
+        #else
+            float diffZ = abs(depth - particleDepth) * (cFarClipPS - cNearClipPS);
+            float fade = saturate(1.0 - diffZ * cSoftParticleFadeScale);
+        #endif
+
         #ifndef ADDITIVE
             diffColor.a = max(diffColor.a - fade, 0.0);
         #else

bin/CoreData/Techniques/DiffLitParticleAlphaExpand.xml

@@ -0,0 +1,5 @@
+<technique vs="LitParticle" ps="LitParticle" psdefines="DIFFMAP EXPAND" >
+    <pass name="alpha" depthwrite="false" depthtest="false" blend="alpha" />
+    <pass name="litalpha" depthwrite="false" depthtest="false" blend="addalpha" />
+    <pass name="shadow" vs="Shadow" ps="Shadow" />
+</technique>

bin/CoreData/Techniques/DiffUnlitParticleAddExpand.xml

@@ -0,0 +1,3 @@
+<technique vs="UnlitParticle" ps="UnlitParticle" vsdefines="VERTEXCOLOR" psdefines="DIFFMAP VERTEXCOLOR ADDITIVE EXPAND">
+    <pass name="alpha" depthwrite="false" depthtest="false" blend="add" />
+</technique>

bin/CoreData/Techniques/DiffUnlitParticleAlphaExpand.xml

@@ -0,0 +1,3 @@
+<technique vs="UnlitParticle" ps="UnlitParticle" vsdefines="VERTEXCOLOR" psdefines="DIFFMAP VERTEXCOLOR EXPAND">
+    <pass name="alpha" depthwrite="false" depthtest="false" blend="alpha" />
+</technique>