Add depth map export (closes #371)

src/main/java/com/replaymod/render/processor/GlToAbsoluteDepthProcessor.java

@@ -0,0 +1,33 @@
+package com.replaymod.render.processor;
+
+import com.replaymod.render.frame.BitmapFrame;
+
+import java.nio.FloatBuffer;
+
+public class GlToAbsoluteDepthProcessor extends AbstractFrameProcessor<BitmapFrame, BitmapFrame> {
+    // absolute depth is 2 * near * far / (far + near - (far - near) * (2 * z - 1))
+    // precomputed:     [      a       ]  [    b     ]  [    c     ]
+    private final float a;
+    private final float b;
+    private final float c;
+
+    public GlToAbsoluteDepthProcessor(float near, float far) {
+        a = 2 * near * far;
+        b = far + near;
+        c = far - near;
+    }
+
+    @Override
+    public BitmapFrame process(BitmapFrame frame) {
+
+        FloatBuffer buffer = frame.getByteBuffer().asFloatBuffer();
+        int len = buffer.remaining();
+        for (int i = 0; i < len; i++) {
+            float z = buffer.get(i);
+            z = a / (b - c * (2 * z - 1));
+            buffer.put(i, z);
+        }
+
+        return frame;
+    }
+}