Fixed|DisplayMode: Factor in the original color transfer curve

When Doomsday modifies the display's color transfer function, it
shouldn't override the monitor color calibration that the user has
done. Now the user's native color curve is factored in to the curve
that Doomsday specifies (which is based on a linear color space).

doomsday/engine/src/ui/displaymode.cpp

@@ -340,18 +340,40 @@ int DisplayMode_Change(const DisplayMode* mode, boolean shouldCapture)
     return DisplayMode_Native_Change(mode, shouldCapture || (originalMode != *mode));
 }
 
-void DisplayMode_GetColorTransfer(displaycolortransfer_t* colors)
+static inline de::duint16 intensity8To16(de::duint8 b)
 {
-    /// @todo  Factor in the original color transfer function, which may be
-    /// set up specifically by the user.
+    return (b << 8) | b; // 0xFF => 0xFFFF
+}
+
+void DisplayMode_GetColorTransfer(displaycolortransfer_t *colors)
+{
+    displaycolortransfer_t mapped;
+    DisplayMode_Native_GetColorTransfer(&mapped);
 
-    DisplayMode_Native_GetColorTransfer(colors);
+    // Factor out the original color transfer function, which may be set up
+    // specifically by the user.
+    for(int i = 0; i < 256; ++i)
+    {
+#define LINEAR_UNMAP(i, c) ( (unsigned short) (float(mapped.table[i]) / float(originalColorTransfer.table[i]) * intensity8To16(c) ) )
+        colors->table[i]       = LINEAR_UNMAP(i,       i);
+        colors->table[i + 256] = LINEAR_UNMAP(i + 256, i);
+        colors->table[i + 512] = LINEAR_UNMAP(i + 512, i);
+    }
 }
 
-void DisplayMode_SetColorTransfer(const displaycolortransfer_t* colors)
+void DisplayMode_SetColorTransfer(displaycolortransfer_t const *colors)
 {
-    /// @todo  Factor in the original color transfer function, which may be
-    /// set up specifically by the user.
+    displaycolortransfer_t mapped;
+
+    // Factor in the original color transfer function, which may be set up
+    // specifically by the user.
+    for(int i = 0; i < 256; ++i)
+    {
+#define LINEAR_MAP(i, c) ( (unsigned short) (float(colors->table[i]) / float(intensity8To16(c)) * originalColorTransfer.table[i]) )
+        mapped.table[i]       = LINEAR_MAP(i,       i);
+        mapped.table[i + 256] = LINEAR_MAP(i + 256, i);
+        mapped.table[i + 512] = LINEAR_MAP(i + 512, i);
+    }
 
-    DisplayMode_Native_SetColorTransfer(colors);
+    DisplayMode_Native_SetColorTransfer(&mapped);
 }