Added (simple) gradient for (simple) phong illumination purposes

src/linalg/mat.h

@@ -1 +1,24 @@
 #pragma once
+
+__device__ Vec3 computeGradient(float* volData, const int d_volumeWidth, const int d_volumeHeight, const int d_volumeDepth, int x, int y, int z) {
+    // Finite difference for partial derivatives.
+    // For boundary voxels - clamp to the boundary. 
+    // Normal should point from higher to lower intensities
+
+    int xm = max(x - 1, 0);
+    int xp = min(x + 1, d_volumeWidth  - 1);
+    int ym = max(y - 1, 0);
+    int yp = min(y + 1, d_volumeHeight - 1);
+    int zm = max(z - 1, 0);
+    int zp = min(z + 1, d_volumeDepth  - 1);
+
+    // Note: Assuming data is linearized (idx = z*w*h + y*w + x) TODO: Unlinearize if data not linear
+    float gx = volData[z  * d_volumeWidth * d_volumeHeight + y  * d_volumeWidth + xp]
+             - volData[z  * d_volumeWidth * d_volumeHeight + y  * d_volumeWidth + xm];
+    float gy = volData[z  * d_volumeWidth * d_volumeHeight + yp * d_volumeWidth + x ]
+             - volData[z  * d_volumeWidth * d_volumeHeight + ym * d_volumeWidth + x ];
+    float gz = volData[zp * d_volumeWidth * d_volumeHeight + y  * d_volumeWidth + x ]
+             - volData[zm * d_volumeWidth * d_volumeHeight + y  * d_volumeWidth + x ];
+
+    return Vec3(gx, gy, gz);
+}