Create depth map image as 24-bit (Carla)

Question

I have a depth map encoded in 24 bits (labeled "Original"). With the code below:

carla_img = cv.imread('carla_deep.png', flags=cv.IMREAD_COLOR)
carla_img = carla_img[:, :, :3]
carla_img = carla_img[:,:,::-1]
gray_depth = ((carla_img[:,:,0] + carla_img[:,:,1] * 256.0 + carla_img[:,:,2] * 256.0 * 256.0)/((256.0 * 256.0 * 256.0) - 1))
gray_depth = gray_depth * 1000

I am able to convert it as in the "Converted" image. As shown here: https://carla.readthedocs.io/en/latest/ref_sensors/

How can I reverse this process (Without using any larger external libraries and using at most openCV)? In Python I create a depth map with the help of OpenCV. I wanted to save the obtained depth map in the form of Carla (24-bit).

This is how I create depth map:

imgL = cv.imread('leftImg.png',0)
imgR = cv.imread('rightImg.png',0)
stereo = cv.StereoBM_create(numDisparities=128, blockSize=17)
disparity = stereo.compute(imgL,imgR)

CameraFOV = 120
Focus_length = width /(2 * math.tan(CameraFOV * math.pi / 360))
camerasBaseline = 0.3
depthMap = (camerasBaseline * Focus_length) / disparity

How can I save the obtained depth map in the same form as in the picture marked "Original"?

Answer 1

Docs say :

normalized = (R + G * 256 + B * 256 * 256) / (256 * 256 * 256 - 1)
in_meters = 1000 * normalized

So if you have a depth map in_meters , you do the reverse, by rearranging the equations .

You need to make sure your depth map (from block matching) is in units of meters. Your calculations there look sensible, assuming your cameras have a baseline of 0.3 meters.

First variant

take the calculation apart, using division and modulo operations.

Various .astype are required to turn floats into integers, and wider integers into narrow integers (assumption for pictures).

normalized = in_meters / 1000
BGR = (normalized * (2**24-1)).astype(np.uint32)
BG,R = np.divmod(BGR, 2**8)
B,G = np.divmod(BG, 2**8)
carla_img = np.dstack([B,G,R]).astype(np.uint8) # BGR order

Second variant

One could also do this with a view , reinterpreting the uint32 data as four uint8 values. This assumes a little endian system, which is a fair assumption but one needs to be aware of it.

...
reinterpreted = BGR.view(np.uint8) # lowest byte first, i.e. order is RGBx
reinterpreted.shape = BGR.shape + (4,) # np.view refuses to add a dimension

carla_img = reinterpreted[:,:,(2,1,0)] # select BGR
# this may require a .copy() to get data without holes (OpenCV may want this)

Disclaimer

I could not test the code in this answer because you haven't provided usable data.