diff --git a/trunk/src/nvimage/nvtt/tools/imgdiff.cpp b/trunk/src/nvimage/nvtt/tools/imgdiff.cpp
index 9e1dcf3..a82eb6f 100644
--- a/trunk/src/nvimage/nvtt/tools/imgdiff.cpp
+++ b/trunk/src/nvimage/nvtt/tools/imgdiff.cpp
@@ -23,14 +23,88 @@
 
 #include <nvcore/StrLib.h>
 #include <nvcore/StdStream.h>
+#include <nvcore/Containers.h>
 
 #include <nvimage/Image.h>
 #include <nvimage/DirectDrawSurface.h>
 #include <nvimage/nvtt/nvtt.h>
 
+#include <nvmath/Color.h>
+
+#include <math.h>
+
 #include "cmdline.h"
 
 
+static bool loadImage(nv::Image & image, const char * fileName)
+{
+	if (nv::strCaseCmp(nv::Path::extension(fileName), ".dds") == 0)
+	{
+		nv::DirectDrawSurface dds(fileName);
+		if (!dds.isValid())
+		{
+			printf("The file '%s' is not a valid DDS file.\n", fileName);
+			return false;
+		}
+		
+		dds.mipmap(&image, 0, 0); // get first image
+	}
+	else
+	{
+		// Regular image.
+		if (!image.load(fileName))
+		{
+			printf("The file '%s' is not a supported image type.\n", fileName);
+			return false;
+		}
+	}
+
+	return true;
+}
+
+// @@ Compute per-tile errors.
+struct Error
+{
+	Error()
+	{
+		samples = 0;
+		mabse = 0.0f;
+		mse = 0.0f;
+		maxabse = 0.0f;
+	}
+
+	void addSample(float e)
+	{
+		samples++;
+		mabse += fabs(e);
+		maxabse = nv::max(maxabse, fabs(e));
+		mse += e * e;
+	}
+
+	void done()
+	{
+		mabse /= samples;
+		mse /= samples;
+		rmse = sqrt(mse);
+		psnr = (rmse == 0) ? 999.0f : 20.0f * log10(32767.0 / rmse);
+	}
+
+	void print()
+	{
+		printf("Mean absolute error: %f\n", mabse);
+		printf("Max absolute error: %f\n", maxabse);
+		printf("Root mean squared error: %f\n", rmse);
+		printf("Peak signal to noise ratio in dB: %f\n", psnr);
+	}
+
+	int samples;
+	float mabse;
+	float maxabse;
+	float mse;
+	float rmse;
+	float psnr;
+};
+
 
 int main(int argc, char *argv[])
 {
@@ -38,12 +112,12 @@ int main(int argc, char *argv[])
 	MyMessageHandler messageHandler;
 
 	bool normal = false;
+	bool alpha = false;
 
 	nv::Path input0;
 	nv::Path input1;
 	nv::Path output;
 
-
 	// Parse arguments.
 	for (int i = 1; i < argc; i++)
 	{
@@ -52,6 +126,10 @@ int main(int argc, char *argv[])
 		{
 			normal = true;
 		}
+		if (strcmp("-alpha", argv[i]) == 0)
+		{
+			alpha = true;
+		}
 
 		else if (argv[i][0] != '-')
 		{
@@ -65,21 +143,75 @@ int main(int argc, char *argv[])
 		}
 	}
 
-	if (input0.empty() || input1.empty())
+	if (input0.isNull() || input1.isNull())
 	{
 		printf("NVIDIA Texture Tools - Copyright NVIDIA Corporation 2007\n\n");
 		
-		printf("usage: nvimgdiff [options] fileA fileB\n\n");
+		printf("usage: nvimgdiff [options] inputA inputB [output]\n\n");
 		
 		printf("Diff options:\n");
 		printf("  -normal \tCompare images as if they were normal maps.\n");
+		printf("  -alpha  \tCompare alpha channels.\n");
 
 		return 1;
 	}
 
-	// @@ Open input0, input1
-	// Compute RMS error and other errors.
-	// 
+	nv::Image image0, image1;
+	if (!loadImage(image0, input0)) return 0;
+	if (!loadImage(image1, input1)) return 0;
+
+	const uint w0 = image0.width();
+	const uint h0 = image0.height();
+	const uint w1 = image1.width();
+	const uint h1 = image1.height();
+	const uint w = nv::min(w0, w1);
+	const uint h = nv::min(h0, h1);
+
+	// Compute errors.
+	Error error_r;
+	Error error_g;
+	Error error_b;
+	Error error_a;
+	Error error_total;
+
+	for (uint i = 0; i < h; i++)
+	{
+		for (uint e = 0; e < w; e++)
+		{
+			const nv::Color32 c0(image0.pixel(e, i));
+			const nv::Color32 c1(image1.pixel(e, i));
+
+			float r = c0.r - c1.r;
+			float g = c0.g - c1.g;
+			float b = c0.b - c1.b;
+			float a = c0.a - c1.a;
+
+			error_r.addSample(r);
+			error_g.addSample(g);
+			error_b.addSample(b);
+			error_a.addSample(a);
+			
+			error_total.addSample(r);
+			error_total.addSample(g);
+			error_total.addSample(b);
+		}
+	}
+
+	error_r.done();
+	error_g.done();
+	error_b.done();
+	error_a.done();
+	error_total.done();
+
+	printf("Image size compared: %dx%d\n", w, h);
+	if (w != w0 || w != w1 || h != h0 || h != h1) {
+		printf("--- NOTE: only the overlap between the 2 images (%d,%d) and (%d,%d) was compared\n", w0, h0, w1, h1);
+	}
+	printf("Total pixels: %d\n", w*h);
+
+	error_total.print();
+
+	// @@ Write image difference.
 	
 	return 0;
 }