Does processing an image twice reduce the quality twice?

[andrewd72]

Some discussion on intermediate images here:

How to "ignore" intermediate images

I have a slightly complex render-image hook, it takes a large image, resizes, sharpens at the reduced size then resize to the same size to lower quality. Images look perfectly crisp, couldn’t do it server side, loving the power of Hugo. So, unsurprisingly, on top of the original image the processing chain creates three images for each final output size. Original Resized Resized and sharpened Final output (which gets the .RelPermalink) I’ve been trying to only have the final output get copied…

Your final quality maths won’t be as simple for two “quality 75” operations.

For best quality for multiple operations you could use a tiff intermediate or a higher quality jpg.
To resize to multiple sizes, you might as well start from the original source for each case.