Add support for alpha pngs by adding per stream type decode functions (backport #106)

include/web_video_server/image_streamer.hpp

@@ -60,6 +60,7 @@ class ImageTransportImageStreamer : public ImageStreamer
   virtual void start();
 
 protected:
+  virtual cv::Mat decodeImage(const sensor_msgs::msg::Image::ConstSharedPtr & msg);
   virtual void sendImage(const cv::Mat &, const rclcpp::Time & time) = 0;
   virtual void restreamFrame(double max_age);
   virtual void initialize(const cv::Mat &);

include/web_video_server/png_streamers.hpp

@@ -21,6 +21,7 @@ class PngStreamer : public ImageTransportImageStreamer
 
 protected:
   virtual void sendImage(const cv::Mat &, const rclcpp::Time & time);
+  virtual cv::Mat decodeImage(const sensor_msgs::msg::Image::ConstSharedPtr & msg);
 
 private:
   MultipartStream stream_;
@@ -47,6 +48,7 @@ class PngSnapshotStreamer : public ImageTransportImageStreamer
 
 protected:
   virtual void sendImage(const cv::Mat &, const rclcpp::Time & time);
+  virtual cv::Mat decodeImage(const sensor_msgs::msg::Image::ConstSharedPtr & msg);
 
 private:
   int quality_;

src/image_streamer.cpp

@@ -108,6 +108,26 @@ void ImageTransportImageStreamer::restreamFrame(double max_age)
   }
 }
 
+cv::Mat ImageTransportImageStreamer::decodeImage(
+  const sensor_msgs::msg::Image::ConstSharedPtr & msg)
+{
+  if (msg->encoding.find("F") != std::string::npos) {
+    // scale floating point images
+    cv::Mat float_image_bridge = cv_bridge::toCvCopy(msg, msg->encoding)->image;
+    cv::Mat_<float> float_image = float_image_bridge;
+    double max_val;
+    cv::minMaxIdx(float_image, 0, &max_val);
+
+    if (max_val > 0) {
+      float_image *= (255 / max_val);
+    }
+    return float_image;
+  } else {
+    // Convert to OpenCV native BGR color
+    return cv_bridge::toCvCopy(msg, "bgr8")->image;
+  }
+}
+
 void ImageTransportImageStreamer::imageCallback(const sensor_msgs::msg::Image::ConstSharedPtr & msg)
 {
   if (inactive_) {
@@ -116,22 +136,7 @@ void ImageTransportImageStreamer::imageCallback(const sensor_msgs::msg::Image::C
 
   cv::Mat img;
   try {
-    if (msg->encoding.find("F") != std::string::npos) {
-      // scale floating point images
-      cv::Mat float_image_bridge = cv_bridge::toCvCopy(msg, msg->encoding)->image;
-      cv::Mat_<float> float_image = float_image_bridge;
-      double max_val;
-      cv::minMaxIdx(float_image, 0, &max_val);
-
-      if (max_val > 0) {
-        float_image *= (255 / max_val);
-      }
-      img = float_image;
-    } else {
-      // Convert to OpenCV native BGR color
-      img = cv_bridge::toCvCopy(msg, "bgr8")->image;
-    }
-
+    img = decodeImage(msg);
     int input_width = img.cols;
     int input_height = img.rows;
 

src/png_streamers.cpp

@@ -1,6 +1,12 @@
 #include "web_video_server/png_streamers.hpp"
 #include "async_web_server_cpp/http_reply.hpp"
 
+#ifdef CV_BRIDGE_USES_OLD_HEADERS
+#include "cv_bridge/cv_bridge.h"
+#else
+#include "cv_bridge/cv_bridge.hpp"
+#endif
+
 namespace web_video_server
 {
 
@@ -20,6 +26,17 @@ PngStreamer::~PngStreamer()
   boost::mutex::scoped_lock lock(send_mutex_); // protects sendImage.
 }
 
+cv::Mat PngStreamer::decodeImage(const sensor_msgs::msg::Image::ConstSharedPtr & msg)
+{
+  // Handle alpha values since PNG supports it
+  if (sensor_msgs::image_encodings::hasAlpha(msg->encoding)) {
+    return cv_bridge::toCvCopy(msg, "bgra8")->image;
+  } else {
+    // Use the normal decode otherwise
+    return ImageTransportImageStreamer::decodeImage(msg);
+  }
+}
+
 void PngStreamer::sendImage(const cv::Mat & img, const rclcpp::Time & time)
 {
   std::vector<int> encode_params;
@@ -64,6 +81,17 @@ PngSnapshotStreamer::~PngSnapshotStreamer()
   boost::mutex::scoped_lock lock(send_mutex_); // protects sendImage.
 }
 
+cv::Mat PngSnapshotStreamer::decodeImage(const sensor_msgs::msg::Image::ConstSharedPtr & msg)
+{
+  // Handle alpha values since PNG supports it
+  if (sensor_msgs::image_encodings::hasAlpha(msg->encoding)) {
+    return cv_bridge::toCvCopy(msg, "bgra8")->image;
+  } else {
+    // Use the normal decode otherwise
+    return ImageTransportImageStreamer::decodeImage(msg);
+  }
+}
+
 void PngSnapshotStreamer::sendImage(const cv::Mat & img, const rclcpp::Time & time)
 {
   std::vector<int> encode_params;