Add function for extracting single channel data from an RGB[A] image (backport #706)

graphics/include/gz/common/Image.hh

@@ -62,6 +62,19 @@ namespace gz
     /// \brief Encapsulates an image
     class GZ_COMMON_GRAPHICS_VISIBLE Image
     {
+      /// \brief Image channel
+      public: enum class Channel
+              {
+                /// \brief Red channel
+                RED = 0,
+                /// \brief Green channel
+                GREEN = 1,
+                /// \brief Blue channel
+                BLUE = 2,
+                /// \brief Alpha channel
+                ALPHA = 3
+              };
+
       /// \brief Pixel formats enumeration
       public: enum PixelFormatType
               {
@@ -222,6 +235,12 @@ namespace gz
       /// \return true if image has a bitmap
       public: bool Valid() const;
 
+      /// \brief Extract a single channel (red, green, blue, or alpha) from
+      /// an RGB[A] image and return a single channel 8 bit image data.
+      /// \param[in] _channel Channel to extract
+      /// \return 8 bit single channel image data
+      public: std::vector<unsigned char> ChannelData(Channel _channel) const;
+
       /// \brief Convert a single channel image data buffer into an RGB image.
       /// During the conversion, the input image data are normalized to 8 bit
       /// values i.e. [0, 255]. Optionally, specify min and max values to use

graphics/src/AssimpLoader.cc

@@ -572,7 +572,7 @@ std::pair<std::string, ImagePtr> AssimpLoader::Implementation::LoadTexture(
 
 std::pair<ImagePtr, ImagePtr>
     AssimpLoader::Implementation::SplitMetallicRoughnessMap(
-    const common::Image& _img) const
+    const common::Image &_img) const
 {
   std::pair<ImagePtr, ImagePtr> ret;
   // Metalness in B roughness in G
@@ -583,20 +583,26 @@ std::pair<ImagePtr, ImagePtr>
   std::vector<unsigned char> metalnessData(width * height * bytesPerPixel);
   std::vector<unsigned char> roughnessData(width * height * bytesPerPixel);
 
+  std::vector<unsigned char> metalnessData8bit =
+      _img.ChannelData(Image::Channel::BLUE);
+  std::vector<unsigned char> roughnessData8bit =
+      _img.ChannelData(Image::Channel::GREEN);
   for (unsigned int y = 0; y < height; ++y)
   {
     for (unsigned int x = 0; x < width; ++x)
     {
       // RGBA so 4 bytes per pixel, alpha fully opaque
-      auto baseIndex = bytesPerPixel * (y * width + x);
-      auto color = _img.Pixel(x, (height - y - 1));
-      metalnessData[baseIndex] = color.B() * 255.0;
-      metalnessData[baseIndex + 1] = color.B() * 255.0;
-      metalnessData[baseIndex + 2] = color.B() * 255.0;
+      unsigned int idx = y * width + x;
+      unsigned int colorB = metalnessData8bit[idx];
+      unsigned int colorG = roughnessData8bit[idx];
+      auto baseIndex = bytesPerPixel * idx;
+      metalnessData[baseIndex] = colorB;
+      metalnessData[baseIndex + 1] = colorB;
+      metalnessData[baseIndex + 2] = colorB;
       metalnessData[baseIndex + 3] = 255;
-      roughnessData[baseIndex] = color.G() * 255.0;
-      roughnessData[baseIndex + 1] = color.G() * 255.0;
-      roughnessData[baseIndex + 2] = color.G() * 255.0;
+      roughnessData[baseIndex] = colorG;
+      roughnessData[baseIndex + 1] = colorG;
+      roughnessData[baseIndex + 2] = colorG;
       roughnessData[baseIndex + 3] = 255;
     }
   }

graphics/src/Image.cc

@@ -22,6 +22,7 @@
 #include <cstdint>
 #include <cstring>
 #include <string>
+#include <vector>
 
 #include <gz/common/Console.hh>
 #include <gz/common/Util.hh>
@@ -806,3 +807,47 @@ FIBITMAP* Image::Implementation::SwapRedBlue(const unsigned int &_width,
 
   return copy;
 }
+
+//////////////////////////////////////////////////
+std::vector<unsigned char> Image::ChannelData(Channel _channel) const
+{
+  if ((this->dataPtr->imageType != FIT_BITMAP) ||
+      (this->dataPtr->colorType != FIC_RGB &&
+       this->dataPtr->colorType != FIC_RGBALPHA))
+  {
+    gzerr << "Extraction of channel data is only support for RGB[A] images"
+          << std::endl;
+    return {};
+  }
+
+  FIBITMAP* channelData = nullptr;
+  switch (_channel)
+  {
+    case Channel::RED:
+      channelData = FreeImage_GetChannel(this->dataPtr->bitmap, FICC_RED);
+      break;
+    case Channel::GREEN:
+      channelData = FreeImage_GetChannel(this->dataPtr->bitmap, FICC_GREEN);
+      break;
+    case Channel::BLUE:
+      channelData = FreeImage_GetChannel(this->dataPtr->bitmap, FICC_BLUE);
+      break;
+    case Channel::ALPHA:
+      channelData = FreeImage_GetChannel(this->dataPtr->bitmap, FICC_ALPHA);
+      break;
+    default:
+      break;
+  }
+
+  if (!channelData)
+  {
+    gzerr << "Invalid input channel: " << static_cast<int>(_channel)
+          << std::endl;
+    return {};
+  }
+
+  FIBITMAP *tmp = FreeImage_ConvertTo8Bits(channelData);
+  std::vector<unsigned char> data = this->dataPtr->DataImpl(tmp);
+  FreeImage_Unload(tmp);
+  return data;
+}

graphics/src/Image_TEST.cc

@@ -784,6 +784,55 @@ TEST_F(ImageTest, Color16bit)
   }
 }
 
+/////////////////////////////////////////////////
+TEST_F(ImageTest, ChannelData)
+{
+  // load image, extra data from each channel and verify values
+  common::Image img;
+  ASSERT_EQ(0, img.Load(kTestData));
+  ASSERT_TRUE(img.Valid());
+
+  std::vector<unsigned char> red =
+      img.ChannelData(common::Image::Channel::RED);
+  std::vector<unsigned char> green =
+      img.ChannelData(common::Image::Channel::GREEN);
+  std::vector<unsigned char> blue =
+      img.ChannelData(common::Image::Channel::BLUE);
+  std::vector<unsigned char> alpha =
+      img.ChannelData(common::Image::Channel::ALPHA);
+
+  EXPECT_FALSE(red.empty());
+  ASSERT_EQ(img.Width() * img.Height(), red.size());
+  ASSERT_EQ(red.size(), green.size());
+  ASSERT_EQ(red.size(), blue.size());
+  ASSERT_EQ(red.size(), alpha.size());
+
+  for (auto i = 0u; i < img.Height(); ++i)
+  {
+    for (auto j = 0u; j < img.Width(); ++j)
+    {
+      unsigned int idx = i * img.Width() + j;
+      unsigned int r = red[idx];
+      unsigned int g = green[idx];
+      unsigned int b = blue[idx];
+      unsigned int a = alpha[idx];
+
+      ASSERT_EQ(0u, g);
+      ASSERT_EQ(255u, a);
+      if (j < 80)
+      {
+        ASSERT_EQ(255u, r);
+        ASSERT_EQ(0u, b);
+      }
+      else
+      {
+        ASSERT_EQ(0u, r);
+        ASSERT_EQ(255u, b);
+      }
+    }
+  }
+}
+
 using string_int2 = std::tuple<const char *, unsigned int, unsigned int>;
 
 class ImagePerformanceTest : public ImageTest,