Ask Your Question

Display CompressedDepth Image Python cv2

asked 2016-12-09 16:06:47 -0600

Pototo gravatar image


What is the correct way to subscribe to a CompressDepth (32FC1, plus, no regular depth image available) image and display (e.g., cv2.imshow) in Python? I can do this easily in roscpp, but I get an empty array rospy, even if I use something like:

depth_image = cv_bridge.CvBridge().compressed_imgmsg_to_cv2(msg)


depth_image = np.array(cv_bridge.CvBridge().compressed_imgmsg_to_cv2(msg))

And even:

np_arr = np.fromstring(, np.uint8)
image_np = cv2.imdecode(np_arr, cv2.CV_LOAD_IMAGE_UNCHANGED)

Nothing works

edit retag flag offensive close merge delete

1 Answer

Sort by ยป oldest newest most voted

answered 2017-07-16 08:54:25 -0600

Christian Rauch gravatar image

The right way to decode compressedDepth is to first remove the header from the raw data and then convert the remaining data.

This is documented in image_transport_plugins/compressed_depth_image_transport/src/codec.cpp. On my machine the header size is 12 bytes. This might however be different on other architectures since the size of an enum is not defined.

The following python code snippet exports compressed 16UC1 and 32FC1 depth images as png file:

# 'msg' as type CompressedImage
depth_fmt, compr_type = msg.format.split(';')
# remove white space
depth_fmt = depth_fmt.strip()
compr_type = compr_type.strip()
if compr_type != "compressedDepth":
    raise Exception("Compression type is not 'compressedDepth'."
                    "You probably subscribed to the wrong topic.")

# remove header from raw data
depth_header_size = 12
raw_data =[depth_header_size:]

depth_img_raw = cv2.imdecode(np.fromstring(raw_data, np.uint8), cv2.CV_LOAD_IMAGE_UNCHANGED)
if depth_img_raw is None:
    # probably wrong header size
    raise Exception("Could not decode compressed depth image."
                    "You may need to change 'depth_header_size'!")

if depth_fmt == "16UC1":
    # write raw image data
    cv2.imwrite(os.path.join(path_depth, "depth_" + str(msg.header.stamp) + ".png"), depth_img_raw)
elif depth_fmt == "32FC1":
    raw_header =[:depth_header_size]
    # header: int, float, float
    [compfmt, depthQuantA, depthQuantB] = struct.unpack('iff', raw_header)
    depth_img_scaled = depthQuantA / (depth_img_raw.astype(np.float32)-depthQuantB)
    # filter max values
    depth_img_scaled[depth_img_raw==0] = 0

    # depth_img_scaled provides distance in meters as f32
    # for storing it as png, we need to convert it to 16UC1 again (depth in mm)
    depth_img_mm = (depth_img_scaled*1000).astype(np.uint16)
    cv2.imwrite(os.path.join(path_depth, "depth_" + str(msg.header.stamp) + ".png"), depth_img_mm)
    raise Exception("Decoding of '" + depth_fmt + "' is not implemented!")
edit flag offensive delete link more

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

Question Tools



Asked: 2016-12-09 16:06:47 -0600

Seen: 1,545 times

Last updated: Jul 16 '17