From 160fbb5e39791de6384f58f118a80940463f698d Mon Sep 17 00:00:00 2001
From: Pavel Krajcevski <pavel@cs.unc.edu>
Date: Thu, 3 Oct 2013 14:12:41 -0400
Subject: [PATCH] Add method for downscaling by averaging nearby pixel values.

---
 PVRTCEncoder/src/Image.cpp      | 60 +++++++++++++++++++++++++++++++++
 PVRTCEncoder/src/Image.h        |  4 +++
 PVRTCEncoder/test/ImageTest.cpp | 23 +++++++++++++
 3 files changed, 87 insertions(+)

diff --git a/PVRTCEncoder/src/Image.cpp b/PVRTCEncoder/src/Image.cpp
index 09b256f..07a24d8 100644
--- a/PVRTCEncoder/src/Image.cpp
+++ b/PVRTCEncoder/src/Image.cpp
@@ -225,6 +225,66 @@ void Image::BilinearUpscale(uint32 xtimes, uint32 ytimes,
   m_Height = newHeight;
 }
 
+static Pixel AveragePixels(const ::std::vector<Pixel> &pixels) {
+  if(pixels.size() == 0) {
+    return Pixel();
+  }
+
+  uint32 sum[4] = {0};
+  ::std::vector<Pixel>::const_iterator it;
+  for(it = pixels.begin(); it != pixels.end(); it++) {
+    for(uint32 c = 0; c < 4; c++) {
+      sum[c] += (*it).Component(c);
+    }
+  }
+
+  Pixel result;
+  for(uint32 c = 0; c < 4; c++) {
+    result.Component(c) = sum[c] / pixels.size();
+  }
+
+  return result;
+}
+
+void Image::AverageDownscale(uint32 xtimes, uint32 ytimes, EWrapMode wrapMode) {
+  const uint32 w = GetWidth();
+  const uint32 h = GetHeight();
+
+  const uint32 newWidth = w >> xtimes;
+  const uint32 newHeight = h >> ytimes;
+
+  Pixel *downscaledPixels = new Pixel[newWidth * newHeight];
+
+  uint8 bitDepth[4];
+  m_Pixels[0].GetBitDepth(bitDepth);
+
+  uint32 pixelsX = 1 << xtimes;
+  uint32 pixelsY = 1 << ytimes;
+
+  ::std::vector<Pixel> toAvg;
+  toAvg.reserve(pixelsX * pixelsY);
+
+  for(uint32 j = 0; j < newHeight; j++) {
+    for(uint32 i = 0; i < newWidth; i++) {
+      uint32 newIdx = j * newWidth + i;
+
+      toAvg.clear();
+      for(uint32 y = j * pixelsY; y < (j+1) * pixelsY; y++) {
+        for(uint32 x = i * pixelsX; x < (i+1) * pixelsX; x++) {
+          toAvg.push_back(GetPixel(x, y, wrapMode));
+        }
+      }
+
+      downscaledPixels[newIdx] = AveragePixels(toAvg);
+    }
+  }
+
+  delete m_Pixels;
+  m_Pixels = downscaledPixels;
+  m_Width = newWidth;
+  m_Height = newHeight;
+}
+
 void Image::ContentAwareDownscale(uint32 xtimes, uint32 ytimes,
                                   EWrapMode wrapMode, bool bOffsetNewPixels) {
   const uint32 w = GetWidth();
diff --git a/PVRTCEncoder/src/Image.h b/PVRTCEncoder/src/Image.h
index e908495..7f63f57 100644
--- a/PVRTCEncoder/src/Image.h
+++ b/PVRTCEncoder/src/Image.h
@@ -82,6 +82,10 @@ class Image {
                              EWrapMode wrapMode = eWrapMode_Wrap,
                              bool bOffsetNewPixels = false);
 
+  // Downscales the image by using a simple averaging of the neighboring pixel values
+  void AverageDownscale(uint32 xtimes, uint32 ytimes,
+                        EWrapMode wrapMode = eWrapMode_Wrap);
+
   void ComputeHessianEigenvalues(::std::vector<float> &eigOne, 
                                  ::std::vector<float> &eigTwo,
                                  EWrapMode wrapMode = eWrapMode_Wrap);
diff --git a/PVRTCEncoder/test/ImageTest.cpp b/PVRTCEncoder/test/ImageTest.cpp
index 527e530..785d77e 100644
--- a/PVRTCEncoder/test/ImageTest.cpp
+++ b/PVRTCEncoder/test/ImageTest.cpp
@@ -279,6 +279,29 @@ TEST(Image, BilinearUpscaleWrapped) {
   }
 }
 
+TEST(Image, AverageDownscale) {
+  PVRTCC::Image img(8, 8);
+  for(uint32 j = 0; j < img.GetHeight(); j++) {
+    for(uint32 i = 0; i < img.GetWidth(); i++) {
+      if((i ^ j) & 1) {
+        img(i, j) = PVRTCC::Pixel(0xFF000000);
+      } else {
+        img(i, j) = PVRTCC::Pixel(0xFFFFFFFF);
+      }
+    }
+  }
+
+  img.AverageDownscale(1, 2);
+  EXPECT_EQ(img.GetWidth(), static_cast<uint32>(4));
+  EXPECT_EQ(img.GetHeight(), static_cast<uint32>(2));
+
+  for(uint32 j = 0; j < img.GetHeight(); j++) {
+    for(uint32 i = 0; i < img.GetWidth(); i++) {
+      EXPECT_EQ(PixelPrinter(0xFF7F7F7F), PixelPrinter(img(i, j).PackRGBA()));
+    }
+  }
+}
+
 TEST(Image, ContentAwareDownscale) {
   PVRTCC::Image img(8, 8);
   for(uint32 j = 0; j < img.GetHeight(); j++) {