Does re-saving a JPEG in MS Paint reduce image quality?
Asked 2/19/2013
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If I open an existing JPG/JPEG in MS Paint and save it again without editing, the file often becomes much smaller. I sometimes use this to reduce file size, but I’m unsure how much image quality is lost. Does simply re-saving a JPEG in Paint degrade it, and why?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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The fact that the image file gets smaller tells you that you are losing quality. The JPEG format is optimised for a size vs. quality compromise, so the file size is more or less a direct measure of the quality.
If you view the image and zoom to 1:1 scale or more, you can usually see the artifacts caused by the JPEG compression.
The compression works by making 8x8 pixel blocks with a color gradient to resemble the original data as close as possible, then the difference between that mosaic and the original data is stored with the amount of precision that corresponds to the quality level chosen. The higher the compression, the more of the mosaic is visible.
Here is an example of how the compression artifacts are visible around the edges of an object (a maple leaf) against a smooth background (the sky):
Originally by user149. Source · Licensed CC BY-SA 4.0
user149
13y ago
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Yes. Re-saving a JPEG in MS Paint will usually reduce image quality, even if you make no visible edits.
JPEG uses lossy compression, which throws away some image data to make the file smaller. If the saved file is much smaller, that usually means more compression was applied and more data was discarded. Re-encoding a JPEG also causes generational loss: each save can add more artifacts and further reduce quality.
Typical JPEG artifacts become easier to see at 1:1 zoom or higher, especially around edges and smooth backgrounds. Because JPEG compresses the image in small blocks, heavier compression can make those blocky patterns and edge distortions more visible.
MS Paint is likely saving with its own default JPEG quality setting, which may be lower than the original file’s setting. That’s why the new file can be much smaller.
If you want smaller JPEGs with better control over quality, use software that lets you choose the JPEG quality/compression level yourself. And if you need to re-save repeatedly while editing, use a lossless format during editing and export to JPEG only at the end.
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