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Read image to an already allocated Mat

asked 2019-07-24 05:20:22 -0500

Przemek D gravatar image

updated 2019-07-24 07:43:27 -0500

modules/imgcodecs/src/loadsave.cpp defines a function imread_ that allows the caller to read image directly to some already existing Mat, potentially having an already allocated memory. However, neither the cv::imread in the C++ API, nor cv2.imread in Python API seem to allow reading images this way, forcing the user to allocate a new buffer every time. I'm using OpenCV version 4.1.0.

If I am wrong and this is indeed possible, please let me know what should I read up on to get it to work. If not, how complex would it be to implement this functionality? I'm particularly interested in exposing it as a Python binding.

What I'm trying to do is to have a master process doing IO and several worker processes to do some heavy work on the images. To cut on the inter-process communication time, I'd like to allocate some amount of shared memory, where the master would put images it reads, allowing workers to access it. I reckon it would be faster if master could store images directly into this shared memory, instead of allocating a new buffer, reading to it, and copying data from this buffer to shared mem.

I've asked a similar question on StackOverflow.

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Interesting question. It doesn't look like there is a way to do this in the current code. It looks like imread_ calls mat.create on the mat parameter, and mat.create seems to check the current size/dimensions and returns if no new allocation is needed. So could you make a custom build with this imread_ exposed?

Chris gravatar imageChris ( 2019-07-24 11:02:39 -0500 )edit

imread_ alocates a new image

If you want to make a PR, I think it's easier to work on imdecode and read file using python function

LBerger gravatar imageLBerger ( 2019-07-24 11:07:25 -0500 )edit

@LBerger Notice that mat.create does not allocate if it finds that the existing mat already has memory of the right shape. @Chris I might be able to rig together something like this, but I don't think I know enough about how Python bindings to C++ code work in OpenCV. Would you recommend something I could read to get started on this?

Przemek D gravatar imagePrzemek D ( 2019-07-25 04:14:07 -0500 )edit

@Przemek D img is always empty in imread

LBerger gravatar imageLBerger ( 2019-07-25 05:36:14 -0500 )edit

@LBerger My point exactly: in imread a new image is created, but with imread_ one can pass an existing one to act as a preallocated buffer. What I want is to have access to this function from Python API, so that I can put my newly read image in a specific memory region.

Przemek D gravatar imagePrzemek D ( 2019-07-25 05:47:00 -0500 )edit

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answered 2019-07-25 06:28:49 -0500

LBerger gravatar image

updated 2019-07-25 06:30:59 -0500

Ok let's try:

Python 3.6.6 (v3.6.6:4cf1f54eb7, Jun 27 2018, 03:37:03) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> import cv2 as cv
>>> img = np.zeros((512,512,3),np.uint8)
>>> img.__array_interface__['data']
(2902581210560, False)
>>> x=cv.imread('g:/lib/opencv/samples/data/lena.jpg',img)
>>> img.__array_interface__['data']
(2902581210560, False)

It looks ok (in c++ it works)

now if you want to make a PR you can clone my code

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Asked: 2019-07-24 05:16:08 -0500

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Last updated: Jul 25