python - OpenCV:压缩后图像大小增加

Question

我正在尝试使用 quality=90 比较压缩前后一堆图像的大小OpenCV 的参数。但在压缩之前，我想将它们全部裁剪成固定大小。但是，我不明白为什么裁剪后图像的平均大小小于裁剪+压缩后的图像大小？

这是我正在做的:

import cv2
import PIL
from pathlib import Path

image_paths = [...]

cropped_imgs_size = 0
compressed_imgs_size = 0

# crop images
for orig_img_path in image_paths:
    cropped_img_path = "cropped_" + orig_img_path
    PIL.Image.open(orig_img_path).crop((0,0,256,256)).convert('RGB').save(cropped_img_path)
    cropped_imgs_size += Path(cropped_img_path).stat().st_size

    # compress cropped image
    dest_path = "q90_" + cropped_img_path
    cv2.imwrite(dest_path, cv2.imread(cropped_img_path), [int(cv2.IMWRITE_JPEG_QUALITY), 90])
    compressed_imgs_size += Path(dest_path).stat().st_size

• 预计:compressed_imgs_size < copped_imgs_size
• 实际:compressed_imgs_size > copped_imgs_size

我错过了什么？

Answer 1

首先，您要使用 PIL.save() 保存裁剪。正如您在 documentation 上看到的那样, 默认的 quality=75:

The save() method supports the following options:

• quality: the image quality, on a scale from 1 (worst) to 95 (best). The default is 75. Values above 95 should be avoided; 100 disables portions of the JPEG compression algorithm, and results in large files with hardly any gain in image quality.

• ...

然后，您使用 cv2.imwrite() 传递 quality=90。因此，您得到的是预期的行为。