how to get value of depth image from *.bag file

edit

A great place to start is: this question

If the tutorial linked there is insufficient for your needs, the bag_to_video code linked to there should give you the appropriate template for creating a tool to suit you.

Edit

Actually more informative code can be found here. See in particular the lines opening the bag and looping over its messages:

00068         // Load input bag file
00069         rosbag::Bag bag;
00070     bag.open(bag_in_name, rosbag::bagmode::Read);
00071         ROS_INFO("Opened %s", bag_in_name.c_str());
00072 
00073         // Create output bag file
00074         rosbag::Bag bag_out;
00075     bag_out.open(bag_out_name, rosbag::bagmode::Write);
00076         ROS_INFO("Created %s", bag_out_name.c_str());
00077 
00078         // Get topics
00079     rosbag::View view(bag);
00080 
00081         // Loop over messages
00082         BOOST_FOREACH(rosbag::MessageInstance const m, view) {
00083 
00084                 // Get data
00085         sensor_msgs::ImageConstPtr image_msg = m.instantiate<sensor_msgs::Image>();

The part converting this to an OpenCV image:

00091         if (image_msg != NULL) {
00092 
00093             cv::Mat image;
00094 
00095             if (image_msg->encoding == "32FC1") { // depth image
00096                 cv_bridge::CvImagePtr depth_image_ptr;
00097                 try {
00098                     depth_image_ptr = cv_bridge::toCvCopy(image_msg);
00099                 } catch (cv_bridge::Exception& e) {
00100                     ROS_ERROR("cv_bridge exception: %s", e.what());
00101                 }

And the part reading the pixels off this image:

    00105                 for(int y = 0; y < depth_image_ptr->image.rows; y++) {
    00106                     for(int x = 0; x < depth_image_ptr->image.cols; x++) {
    00107                         float distance = depth_image_ptr->image.at<float>(y, x);