Скачать или смотреть Solving the Expected Ptr cv::UMat for argument 'src' Error in OpenCV

Discover how to successfully append frames from a video to a list and write them back using OpenCV, resolving the common error with clear coding examples.
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Solving the Expected Ptr<cv::UMat> for argument 'src' Error in OpenCV: A Step-by-Step Guide

When working with video processing in OpenCV, you may encounter the frustrating error: Expected Ptr<cv::UMat> for argument 'src'. This error usually arises when you're trying to append frames to a list and then write them back to a video file. In this guide, we will explore the problem in detail and provide you with the right solutions to tackle it effectively.

Understanding the Problem

Imagine you have a video file, and your goal is to read this video frame by frame, store those frames in a list, and eventually write some of those frames back into a new video file. However, while executing this process, you might face the Expected Ptr<cv::UMat> for argument 'src' error, indicating that the input to the writer.write function is incorrect.

Breaking Down the Solution

To address the issue, we will follow these organized steps:

Step 1: Reading the Video Frames

The first step involves capturing and reading the video frames properly. Instead of relying on a successful read flag, we can check if the video is actually open. Here’s the recommended code:

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Check if video is opened: vidcap.isOpened() ensures that we are only trying to read frames if the video file is accessible.

Resizing frames: Each frame needs to be resized to the necessary dimensions (in this case, 512x512 pixels) to maintain consistency when writing the video.

Step 2: Creating the VideoWriter Object

Before writing the frames, you'll need to initialize the cv2.VideoWriter. It’s crucial to ensure that the dimensions and color settings are correct. Here's the proper initialization:

[[See Video to Reveal this Text or Code Snippet]]

Codec choice: Always use "mp4v" instead of "MP4V" to avoid potential warnings.

Step 3: Writing the Frames to Video

Once we have the VideoWriter set up, we can write the processed frames back to the new video file. Remember, instead of adding frames from a list directly, we'll loop through the selected frames.

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Complete Code Example

Putting it all together, here’s a complete code snippet to read a video, process it, and write the frames back to a new video file:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By following these detailed steps, not only have you learned how to properly manage the reading and writing of video frames in OpenCV, but you have also resolved the common mistakes that lead to the Expected Ptr<cv::UMat> for argument 'src' error. Happy coding in your video processing projects!