“CAP_IMAGES: Can't Find Starting Number” Error in Python with Resolution

The error "CAP_IMAGES: Can't Find Starting Number" typically occurs in the context of the OpenCV when attempting to open a video file or capture device. This error message indicates that OpenCV encountered difficulties in reading or parsing the video stream due to the missing or incorrect starting frame number.

Why “CAP_IMAGES: Can't Find Starting Number” Error Occurs?

The root cause of the "CAP_IMAGES: Can't Find Starting Number" error is often related to issues with the video file being accessed or the configuration of the capture device. Some common reasons for this error include:

• Incorrect file path: If the provided file path to the video is incorrect or the file does not exist OpenCV will be unable to locate the starting frame.
• Corrupted video file: If the video file is corrupted or in an unsupported format OpenCV may fail to read the frames properly.
• Invalid frame index: If an invalid starting frame index is specified OpenCV may not be able to find the specified frame in the video stream.

Example:

If the images are not named correctly the program won't be able to find the starting number.

import cv2

# Trying to capture images from a non-existing directory
cap = cv2.VideoCapture('non_existing_directory/image_%d.png')

if not cap.isOpened():
    print("Error: CAP_IMAGES: Can't Find Starting Number")

Output:

Error: CAP_IMAGES: Can't Find Starting Number

When is “CAP_IMAGES: Can't Find Starting Number” triggered?

The "CAP_IMAGES: Can't Find Starting Number" error can occur in various scenarios including:

• When attempting to open a video file using cv2.VideoCapture() function in the OpenCV.
• When specifying a frame index that is out of the range or does not exist in video stream.
• When working with the corrupted or unsupported video file format.

How to resolve the “CAP_IMAGES: Can't Find Starting Number” error?

Here's how to address this issue:

1. Ensure Correct Naming Convention

Ensure your images follow a consistent sequential numeric pattern. For example, img_001.jpg, img_002.jpg, ..., img_NNN.jpg.

2. Using VideoCapture with Correct Naming

Rename your images to follow a pattern like img_001.jpg, img_002.jpg, etc.

Example

import cv2

# Path to the image sequence (using %03d to match the pattern img_001.jpg, img_002.jpg, etc.)
image_sequence_path = 'path/to/images/img_%03d.jpg'

# Initialize the VideoCapture object with the image sequence path
cap = cv2.VideoCapture(image_sequence_path)

# Check if the VideoCapture object was successfully created
if not cap.isOpened():
    print("Error: Could not open image sequence.")
    exit()

# Read and display the images in the sequence
while True:
    ret, frame = cap.read()
    if not ret:
        break
    cv2.imshow('Image Sequence', frame)
    if cv2.waitKey(30) & 0xFF == ord('q'):
        break

# Release the VideoCapture object and close the display window
cap.release()
cv2.destroyAllWindows()

Explanation:

• image_sequence_path: The path to the image sequence with a placeholder %03d that matches the numeric pattern in the filenames (e.g., img_001.jpg).
• cap.isOpened(): Checks if the VideoCapture object was successfully created.
• cap.read(): Reads the next image in the sequence. If no more images are found, it returns False.
• cv2.imshow(): Displays each image in a window.
• cv2.waitKey(): Waits for a key press, and if 'q' is pressed, exits the loop.
• cap.release(): Releases the VideoCapture object.
• cv2.destroyAllWindows(): Closes all OpenCV windows.

3. Alternative Method: Reading Images Manually

If the sequential naming convention does not suit your needs, you can manually read the images using the glob module.

Example

import cv2
import glob

# Path to the image sequence
image_files = sorted(glob.glob('path/to/images/*.jpg'))

for image_file in image_files:
    image = cv2.imread(image_file)
    if image is None:
        print(f"Error: Could not read image {image_file}")
        continue
    cv2.imshow('Image Sequence', image)
    if cv2.waitKey(30) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

Explanation

• glob.glob(): Finds all files matching the specified pattern (e.g., all .jpg files).
• sorted(): Ensures the files are in sequential order.
• cv2.imread(): Reads each image file individually.
• cv2.imshow(): Displays each image in a window.
• cv2.waitKey(): Waits for a key press, and if 'q' is pressed, exits the loop.
• cv2.destroyAllWindows(): Closes all OpenCV windows.

Troubleshooting Steps for “CAP_IMAGES: Can't Find Starting Number”

To resolve the "CAP_IMAGES: Can't Find Starting Number" error consider the following troubleshooting steps:

• Verify the file path: Double-check the file path to ensure that it is correct and that the video file exists at the specified location.
• Check the file format: To Ensure that the video file is in the supported format and is not corrupted.
• Validate the frame index: Make sure that the specified starting frame index is within the valid range of the frames in a video stream.
• Test with a different video file: Try opening a different video file to determine if the issue is specific to the original file.
• Update OpenCV: Ensure that you are using the latest version of OpenCV as newer versions may include bug fixes and improvements that address issues with the video reading.

Conclusion

The "CAP_IMAGES: Can't Find Starting Number" error in the OpenCV can be resolved by verifying the file path checking the file format and frame index and testing with the different video files. By following the troubleshooting steps outlined above we can effectively diagnose and fix the issues related to the reading video streams in the OpenCV.