Revised gradient.frag fragment shader:

 * use texture and textureOffset instead of clumsy getElement3DNormalized
 * introducing Sobel filtered gradients

core/glsl/tools/gradient.frag

@@ -30,38 +30,36 @@
 #include "tools/texture3d.frag"
 
 /**
- * Compute the gradient using forward differences on the texture's alpha channel.
+ * Compute the gradient using forward differences on the texture's red channel.
  * \param   tex         3D texture to calculate gradients for
  * \param   texCoords   Lookup position in texture coordinates
  */
 vec3 computeGradientForwardDifferences(in sampler3D tex, in TextureParameters3D texParams, in vec3 texCoords) {
-    vec3 offset = texParams._sizeRCP;
-    float v = getElement3DNormalized(tex, texParams, texCoords).r;
-    float dx = getElement3DNormalized(tex, texParams, texCoords + vec3(offset.x, 0.0, 0.0)).r;
-    float dy = getElement3DNormalized(tex, texParams, texCoords + vec3(0, offset.y, 0)).r;
-    float dz = getElement3DNormalized(tex, texParams, texCoords + vec3(0, 0, offset.z)).r;
+    float v = texture(tex, texCoords).r;
+    float dx = textureOffset(tex, texCoords, ivec3(1, 0, 0)).r;
+    float dy = textureOffset(tex, texCoords, ivec3(0, 1, 0)).r;
+    float dz = textureOffset(tex, texCoords, ivec3(0, 0, 1)).r;
     return vec3(v - dx, v - dy, v - dz) * texParams._voxelSize;
 }
 
 /**
- * Compute the gradient using central differences on the texture's alpha channel.
+ * Compute the gradient using central differences on the texture's red channel.
  * \param   tex         3D texture to calculate gradients for
  * \param   texCoords   Lookup position in texture coordinates
  */
 vec3 computeGradientCentralDifferences(in sampler3D tex, in TextureParameters3D texParams, in vec3 texCoords) {
-    vec3 offset = texParams._sizeRCP;
-    float dx = getElement3DNormalized(tex, texParams, texCoords + vec3(offset.x, 0.0, 0.0)).r;
-    float dy = getElement3DNormalized(tex, texParams, texCoords + vec3(0, offset.y, 0)).r;
-    float dz = getElement3DNormalized(tex, texParams, texCoords + vec3(0, 0, offset.z)).r;
-    float mdx = getElement3DNormalized(tex, texParams, texCoords + vec3(-offset.x, 0, 0)).r;
-    float mdy = getElement3DNormalized(tex, texParams, texCoords + vec3(0, -offset.y, 0)).r;
-    float mdz = getElement3DNormalized(tex, texParams, texCoords + vec3(0, 0, -offset.z)).r;
-    return vec3(mdx - dx, mdy - dy, mdz - dz) * texParams._voxelSize * vec3(0.5);
+    float dx = textureOffset(tex, texCoords, ivec3(1, 0, 0)).r;
+    float dy = textureOffset(tex, texCoords, ivec3(0, 1, 0)).r;
+    float dz = textureOffset(tex, texCoords, ivec3(0, 0, 1)).r;
+    float mdx = textureOffset(tex, texCoords, ivec3(-1, 0, 0)).r;
+    float mdy = textureOffset(tex, texCoords, ivec3(0, -1, 0)).r;
+    float mdz = textureOffset(tex, texCoords, ivec3(0, 0, -1)).r;
+    return vec3(mdx - dx, mdy - dy, mdz - dz) * texParams._voxelSize * 0.5;
 }
 
 
 /**
- * Compute the gradient using filtered central differences on the texture's alpha channel.
+ * Compute the gradient using filtered central differences on the texture's red channel.
  * \param   tex         3D texture to calculate gradients for
  * \param   texCoords   Lookup position in texture coordinates
  */
@@ -80,3 +78,108 @@ vec3 computeGradientFilteredCentralDifferences(in sampler3D tex, in TextureParam
     vec3 mix1 = mix(mix(g5, g6, 0.5), mix(g7, g8, 0.5), 0.5);
     return mix(g0, mix(mix0, mix1, 0.5), 0.75);
 }
+
+
+/**
+ * Compute the gradient using the sobel filter on the texture's red channel.
+ * \param   tex         3D texture to calculate gradients for
+ * \param   texCoords   Lookup position in texture coordinates
+ */
+vec3 computeGradientSobel(in sampler3D tex, in vec3 texCoords) {
+    const float sobelScale = 1.0 / 44.0;
+    vec3 sobel = vec3(0.0);
+    float value = 0.0;
+   
+    value = textureOffset(tex, texCoords, ivec3(-1, -1, -1)).r;
+    sobel.x -= value; 
+    sobel.y -= value; 
+    sobel.z -= value; 
+    value = textureOffset(tex, texCoords, ivec3( 0, -1, -1)).r;
+    sobel.y -= 3.0 * value; 
+    sobel.z -= 3.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1, -1, -1)).r;
+    sobel.x += value; 
+    sobel.y -= value; 
+    sobel.z -= value; 
+   
+    value = textureOffset(tex, texCoords, ivec3(-1,  0, -1)).r;
+    sobel.x -= 3.0 * value; 
+    sobel.z -= 3.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 0,  0, -1)).r;
+    sobel.z -= 6.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1,  0, -1)).r;
+    sobel.x += 3.0 * value; 
+    sobel.z -= 3.0 * value; 
+   
+    value = textureOffset(tex, texCoords, ivec3(-1,  1, -1)).r;
+    sobel.x -= value; 
+    sobel.y += value; 
+    sobel.z -= value; 
+    value = textureOffset(tex, texCoords, ivec3( 0,  1, -1)).r;
+    sobel.y +=  3.0 * value; 
+    sobel.z -=  3.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1,  1, -1)).r;
+    sobel.x += value; 
+    sobel.y += value; 
+    sobel.z -= value; 
+
+   
+    value = textureOffset(tex, texCoords, ivec3(-1, -1,  0)).r;
+    sobel.x -= 3.0 * value; 
+    sobel.y -= 3.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 0, -1,  0)).r;
+    sobel.y -= 6.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1, -1,  0)).r;
+    sobel.x += 3.0 * value; 
+    sobel.y -= 3.0 * value; 
+   
+    value = textureOffset(tex, texCoords, ivec3(-1,  0,  0)).r;
+    sobel.x -= 6.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1,  0,  0)).r;
+    sobel.x += 6.0 * value; 
+   
+    value = textureOffset(tex, texCoords, ivec3(-1,  1,  0)).r;
+    sobel.x -=  3.0 * value; 
+    sobel.y +=  3.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 0,  1,  0)).r;
+    sobel.y +=  6.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1,  1,  0)).r;
+    sobel.x +=  3.0 * value; 
+    sobel.y +=  3.0 * value; 
+   
+   
+    value = textureOffset(tex, texCoords, ivec3(-1, -1,  1)).r;
+    sobel.x -= value; 
+    sobel.y -= value; 
+    sobel.z += value; 
+    value = textureOffset(tex, texCoords, ivec3( 0, -1,  1)).r;
+    sobel.y -= 3.0 * value; 
+    sobel.z += 3.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1, -1,  1)).r;
+    sobel.x += value; 
+    sobel.y -= value; 
+    sobel.z += value; 
+   
+    value = textureOffset(tex, texCoords, ivec3(-1,  0,  1)).r;
+    sobel.x -= 3.0 * value; 
+    sobel.z += 3.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 0,  0,  1)).r;
+    sobel.z += 6.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1,  0,  1)).r;
+    sobel.x += 3.0 * value; 
+    sobel.z += 3.0 * value; 
+   
+    value = textureOffset(tex, texCoords, ivec3(-1,  1,  1)).r;
+    sobel.x -= value; 
+    sobel.y += value; 
+    sobel.z += value; 
+    value = textureOffset(tex, texCoords, ivec3( 0,  1,  1)).r;
+    sobel.y += 3.0 * value; 
+    sobel.z += 3.0 * value; 
+    value = textureOffset(tex, texCoords, ivec3( 1,  1,  1)).r;
+    sobel.x += value; 
+    sobel.y += value; 
+    sobel.z += value; 
+   
+    return -sobelScale * sobel;
+}
\ No newline at end of file

core/pipeline/processordecoratorshading.cpp

@@ -33,16 +33,17 @@
 #include "core/properties/propertycollection.h"
 
 namespace campvis {
-    static const GenericOption<ProcessorDecoratorShading::GradientMethod> gradientOptions[3] = {
+    static const GenericOption<ProcessorDecoratorShading::GradientMethod> gradientOptions[4] = {
         GenericOption<ProcessorDecoratorShading::GradientMethod>("Forward", "Forward Differences", ProcessorDecoratorShading::ForwardDifferences),
         GenericOption<ProcessorDecoratorShading::GradientMethod>("Central", "Central Differences", ProcessorDecoratorShading::CentralDifferences),
+        GenericOption<ProcessorDecoratorShading::GradientMethod>("Sobel", "Sobel Filter", ProcessorDecoratorShading::SobelFilter),
         GenericOption<ProcessorDecoratorShading::GradientMethod>("FilteredCentral", "Filtered Central Differences", ProcessorDecoratorShading::FilteredCentralDifferences)
     };
 
     ProcessorDecoratorShading::ProcessorDecoratorShading(const std::string& lightUniformName /*= "_lightSource"*/)
         : AbstractProcessorDecorator()
         , _enableShading("EnableShading", "Enable Shading", true, AbstractProcessor::INVALID_SHADER | AbstractProcessor::INVALID_RESULT)
-        , _gradientMethod("GradientMethod", "Gradient Computation Method", gradientOptions, 3, AbstractProcessor::INVALID_SHADER | AbstractProcessor::INVALID_RESULT)
+        , _gradientMethod("GradientMethod", "Gradient Computation Method", gradientOptions, 4, AbstractProcessor::INVALID_SHADER | AbstractProcessor::INVALID_RESULT)
         , _lightPosition("LightPosition", "Light Position", tgt::vec3(-100.f), tgt::vec3(-500.f), tgt::vec3(500.f), tgt::vec3(1.f))
         , _ambientColor("AmbientColor", "Ambient Light Color", tgt::vec3(0.4f), tgt::vec3(0.f), tgt::vec3(1.f))
         , _diffuseColor("DiffuseColor", "Diffuse Light Color", tgt::vec3(0.75f), tgt::vec3(0.f), tgt::vec3(1.f))
@@ -85,13 +86,16 @@ namespace campvis {
 
         switch (_gradientMethod.getOptionValue()) {
             case ForwardDifferences :
-                toReturn.append("#define computeGradient(tex, texParams,texCoords) computeGradientForwardDifferences(tex, texParams, texCoords)\n");
+                toReturn.append("#define computeGradient(tex, texParams, texCoords) computeGradientForwardDifferences(tex, texParams, texCoords)\n");
                 break;
             case CentralDifferences :
-                toReturn.append("#define computeGradient(tex, texParams,texCoords) computeGradientCentralDifferences(tex, texParams, texCoords)\n");
+                toReturn.append("#define computeGradient(tex, texParams, texCoords) computeGradientCentralDifferences(tex, texParams, texCoords)\n");
                 break;
             case FilteredCentralDifferences :
-                toReturn.append("#define computeGradient(tex, texParams,texCoords) computeGradientFilteredCentralDifferences(tex, texParams, texCoords)\n");
+                toReturn.append("#define computeGradient(tex, texParams, texCoords) computeGradientFilteredCentralDifferences(tex, texParams, texCoords)\n");
+                break;
+            case SobelFilter :
+                toReturn.append("#define computeGradient(tex, texParams, texCoords) computeGradientSobel(tex, texCoords)\n");
                 break;
             default:
                 tgtAssert(false, "Invalid enum!");

core/pipeline/processordecoratorshading.h

@@ -44,7 +44,8 @@ namespace campvis {
         enum GradientMethod {
             ForwardDifferences,
             CentralDifferences,
-            FilteredCentralDifferences
+            FilteredCentralDifferences,
+            SobelFilter
         };
 
         explicit ProcessorDecoratorShading(const std::string& lightUniformName = "_lightSource");