Reimplement drawing system with vertex buffers.

Now with 10000% more performance.

src/FlatLayerVisualisation.cpp

@@ -0,0 +1,93 @@
+#include <glog/logging.h>
+#include <GL/gl.h>
+
+#include "FlatLayerVisualisation.hpp"
+
+using namespace std;
+using namespace fmri;
+
+static inline void computeColor(float intensity, float limit, float* destination)
+{
+    const float saturation = min(-log(abs(intensity) / limit) / 10.0f, 1.0f);
+    if (intensity > 0) {
+        destination[0] = saturation;
+        destination[1] = saturation;
+        destination[2] = 1;
+    } else {
+        destination[0] = 1;
+        destination[1] = saturation;
+        destination[2] = saturation;
+    }
+}
+
+FlatLayerVisualisation::FlatLayerVisualisation(const fmri::LayerData &layer) :
+        faceCount(layer.numEntries() * 4),
+        vertexBuffer(faceCount * 3),
+        colorBuffer(faceCount * 3),
+        indexBuffer(faceCount * 3)
+{
+    auto& shape = layer.shape();
+    CHECK_EQ(shape.size(), 2) << "layer should be flat!" << endl;
+    CHECK_EQ(shape[0], 1) << "Only single images supported." << endl;
+
+    const int limit = static_cast<const int>(layer.numEntries());
+    auto data = layer.data();
+    const auto [minElem, maxElem] = minmax_element(data, data + limit);
+
+    auto scalingMax = max(abs(*minElem), abs(*maxElem));
+
+    int v = 0;
+    int j = 0;
+    for (int i = 0; i < limit; ++i) {
+        const float zOffset = -2 * i;
+        const int vertexBase = i * 4;
+        // Create the 4 vertices for the pyramid
+        vertexBuffer[j++] = -0.5f;
+        vertexBuffer[j++] = 0;
+        vertexBuffer[j++] = 0.5f + zOffset;
+        vertexBuffer[j++] = 0;
+        vertexBuffer[j++] = 0;
+        vertexBuffer[j++] = -0.5f + zOffset;
+        vertexBuffer[j++] = 0;
+        vertexBuffer[j++] = 1;
+        vertexBuffer[j++] = 0 + zOffset;
+        vertexBuffer[j++] = 0.5;
+        vertexBuffer[j++] = 0;
+        vertexBuffer[j++] = 0.5 + zOffset;
+
+        // Define the colors for the vertices
+        for (int c = 0; c < 4; ++c) {
+            computeColor(data[i], scalingMax, &colorBuffer[12 * i + 3 * c]);
+        }
+
+        // Create the index set for the faces
+        // Simply connect all vertices in ascending order and it works.
+        indexBuffer[v++] = vertexBase;
+        indexBuffer[v++] = vertexBase + 1;
+        indexBuffer[v++] = vertexBase + 2;
+        indexBuffer[v++] = vertexBase;
+        indexBuffer[v++] = vertexBase + 1;
+        indexBuffer[v++] = vertexBase + 3;
+        indexBuffer[v++] = vertexBase;
+        indexBuffer[v++] = vertexBase + 2;
+        indexBuffer[v++] = vertexBase + 3;
+        indexBuffer[v++] = vertexBase + 1;
+        indexBuffer[v++] = vertexBase + 2;
+        indexBuffer[v++] = vertexBase + 3;
+    }
+    assert(v == faceCount * 3);
+    assert(j == faceCount * 3);
+}
+
+void FlatLayerVisualisation::render()
+{
+    glEnableClientState(GL_VERTEX_ARRAY);
+    glEnableClientState(GL_COLOR_ARRAY);
+
+    glVertexPointer(3, GL_FLOAT, 0, vertexBuffer.data());
+    glColorPointer(3, GL_FLOAT, 0, colorBuffer.data());
+    glDrawElements(GL_TRIANGLES, faceCount, GL_UNSIGNED_INT, indexBuffer.data());
+
+    glDisable(GL_VERTEX_ARRAY);
+    glDisable(GL_COLOR_ARRAY);
+}