Has anyone ever noticed that if you open a image in mspaint (.jpg, .jpeg) and then just save it, the image size is reduced by many folds. I use this method to reduce the file size.
However, I am not sure about the quality loss due to this. Can anyone please tell/explain the quality loss if any using this method?
Answer
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):
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