improve image quality of cropped jpg files by disabling chroma subsampling

[LaserBorg]

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• I have searched the YOLOv5 issues and found no similar feature requests.

Description

opencv was used to save ROI crops, but imwrite is limited at jpg quality since it uses 2x2 chroma subsampling, often reffered to as "4:2:2" (-> jpg is YUV color space) even when setting cv2.IMWRITE_JPEG_QUALITY to 100.
Quality loss is clearly visible when working on images with highly saturated colors or sharp color changes.

I propose using pillow to write jpg files, with subsampling flag set to False. It is already in the requirements and the existing numpy matrix can easily get converted to pil format.

Use case

image quality of the crops is crucial when cropped output files are further used as training data, while jpg compression still offers great benefit-cost ratio in filesize and compression speed compared to lossless (png) or even uncompressed (bmp) formats.

Additional

I have a PR ready.

Are you willing to submit a PR?

• Yes I'd like to help by submitting a PR!

[github-actions]

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[glenn-jocher]

@LaserBorg good news 😃! Your original issue may now be fixed ✅ in PR #7008. To receive this update:

• Git – git pull from within your yolov5/ directory or git clone https://github.com/ultralytics/yolov5 again
• PyTorch Hub – Force-reload model = torch.hub.load('ultralytics/yolov5', 'yolov5s', force_reload=True)
• Notebooks – View updated notebooks
• Docker – sudo docker pull ultralytics/yolov5:latest to update your image

Thank you for spotting this issue and informing us of the problem. Please let us know if this update resolves the issue for you, and feel free to inform us of any other issues you discover or feature requests that come to mind. Happy trainings with YOLOv5 🚀!

[Mob-Nikhil898]

The current save_one_box() is not saving anything in the desired folder.

[glenn-jocher]

@Mob-Nikhil898 saving crops works correctly.

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Thank you! 😃

[Mob-Nikhil898]

Thank you bro, I messed up my detect.py settings that's why it was not saving . My bad.

[glenn-jocher]

@Mob-Nikhil898 no worries at all! It happens to the best of us 😊. If you encounter any other issues or have further questions, feel free to ask. Happy to help anytime!