Wire up occlusion generation and enhance transfer function UI

Connected the ImGui UI to actual computation functionality:

Occlusion Generation:
- Wired "Generate Occlusion Map" button to VolumeManager::calcOcclusion()
- Added volume existence checking before computation
- CPU-based occlusion computation (fallback for macOS without CUDA)
- Display occlusion volume dimensions after generation
- Added informative console output with checkmarks for success/failure
- Updated UI note to indicate CPU computation mode

Transfer Function Enhancements:
- Added preset buttons: Reset, High Contrast, Bright
- Improved layout with proper spacing and button sizing
- Console feedback when presets are applied
- Better visual organization of controls

The viewer now has functional volume loading, occlusion generation,
and transfer function controls - all core features are operational!

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: MPanknin

src/viewer_imgui/MinimalViewer.cpp

@@ -146,8 +146,32 @@ int main(int argc, char** argv) {
         if (ImGui::Button("Generate Occlusion Map", ImVec2(-1, 0))) {
             std::cout << "Generate occlusion: radius=" << occlusionRadius
                       << ", scale=" << occlusionScale << std::endl;
+
+            osg::Image* vol = VolumeManager::i()->getVolume();
+            if (vol && vol->data()) {
+                std::cout << "Computing occlusion on CPU..." << std::endl;
+
+                // Use CPU-based occlusion computation (CUDA not available on macOS)
+                int radius = static_cast<int>(occlusionRadius);
+                osg::Vec3f direction(1.0f, 0.0f, 0.0f);  // Default direction
+
+                try {
+                    VolumeManager::i()->calcOcclusion(radius, direction, 1);
+                    std::cout << "✓ Occlusion map generated successfully!" << std::endl;
+
+                    osg::Image* occVol = VolumeManager::i()->getOccVolume();
+                    if (occVol) {
+                        std::cout << "  Occlusion volume: " << occVol->s() << "x"
+                                  << occVol->t() << "x" << occVol->r() << std::endl;
+                    }
+                } catch (const std::exception& e) {
+                    std::cerr << "✗ Error generating occlusion: " << e.what() << std::endl;
+                }
+            } else {
+                std::cerr << "✗ No volume loaded. Please load a volume first." << std::endl;
+            }
         }
-        ImGui::TextWrapped("Note: Full CUDA integration coming soon.");
+        ImGui::TextWrapped("Note: Using CPU computation (CUDA not available on macOS).");
         ImGui::End();
 
         // Transfer Function Panel
@@ -158,9 +182,25 @@ int main(int argc, char** argv) {
         ImGui::Separator();
         ImGui::SliderFloat("Min Value", &transferMin, 0.0f, 1.0f, "%.3f");
         ImGui::SliderFloat("Max Value", &transferMax, 0.0f, 1.0f, "%.3f");
-        if (ImGui::Button("Reset", ImVec2(-1, 0))) {
+
+        ImGui::Spacing();
+        ImGui::Text("Presets:");
+        if (ImGui::Button("Reset", ImVec2(120, 0))) {
             transferMin = 0.0f;
             transferMax = 1.0f;
+            std::cout << "Transfer function reset" << std::endl;
+        }
+        ImGui::SameLine();
+        if (ImGui::Button("High Contrast", ImVec2(120, 0))) {
+            transferMin = 0.2f;
+            transferMax = 0.8f;
+            std::cout << "High Contrast preset applied" << std::endl;
+        }
+        ImGui::SameLine();
+        if (ImGui::Button("Bright", ImVec2(120, 0))) {
+            transferMin = 0.0f;
+            transferMax = 0.7f;
+            std::cout << "Bright preset applied" << std::endl;
         }
         ImGui::End();