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How to generate 'format: "32FC1; compressedDepth png"' Image in python from float array?

asked 2022-10-09 11:43:39 -0500

lucasw gravatar image

updated 2022-10-09 15:58:48 -0500

I think there's an answer that involves converting C++ code ( to python- does anyone have a block of code where this is already done? Or do the right arguments to CvBridge.cv2_to_compressed_imgmsg work for this?

This goes the other direction

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answered 2022-10-09 17:22:59 -0500

lucasw gravatar image

updated 2022-10-09 17:25:02 -0500

Here is what I came up with- I haven't yet checked to make sure the depth values when decompressed are correct, they just seemed reasonable as seen in rqt:

import cv2
import numpy as np
from sensor_msgs.msg import CompressedImage

def encode_compressed_depth_image_msg(depth_image: np.array,
                                      depth_min=1.0, depth_max=10.0,
    depth_image[depth_image > depth_max] = np.nan
    depth_image[depth_image < depth_min] = np.nan
    depth_z0 = depth_quantization
    depth_quant_a = depth_z0 * (depth_z0 + 1.0)
    depth_quant_b = 1.0 - depth_quant_a / depth_max
    inv_depth = (depth_quant_a / depth_image + depth_quant_b).astype(np.uint16)
    depth_encoded = cv2.imencode(".png", inv_depth)[1]
    header = np.array([0.0, depth_quant_a, depth_quant_b], np.float32)

    compressed_depth_msg = CompressedImage()
    compressed_depth_msg.format = "32FC1; compressedDepth png" = header.tobytes() + depth_encoded.tobytes()
    return compressed_depth_msg

The payload of the CompressedDepth "32FC1; compressedDepth png" is 12 header bytes which contains the conversion scaling numbers quant a & b, then an encoded 16-bit grayscale png follows (if you lop off those first 12 bytes and save the rest of the bytes to a file you can display it in a png viewer and see the depth values). The scaling numbers fixed above but could be changed per-frame if there was a reason to, by manipulating depth_max and depth_quantization (or refactor so depth_quant_a and b are set through some other means).

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Asked: 2022-10-09 11:43:39 -0500

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Last updated: Oct 09 '22