On 06/15/2012 09:51 PM, Keith Purtell wrote:

When I export a Web JPG from GIMP and it offers me a sliding Quality bar that goes 0 - 100. I want to know if that's an equivalent to the similar JPG quality setting in Photoshop? The reason I'm asking is that my research on Photoshop indicated that JPG output quality settings above 75 were "theoretical." In other words, there might be measurable improvements in JPG quality past 75, but they will be imperceptible by the human eye. Does anyone know if the Quality settings are basically the same in both pieces of software?

In Gimp, the "chroma subsampling" is a separate setting from the "quality", while in some JPEG encoders it is part of it. This alone makes the quality setting potentially very different (but I don't know if this applies to PS or not).

The perceptibility of JPEG compression artefacts depends a lot on the image. For the same quality setting, they are much more visible on images with sharp transitions and high contrast (computer graphics, text, or very good photos), than on "soft" pictures from low-grade cameras.* *

On 2012-06-15 14:51, Keith Purtell wrote:

When I export a Web JPG from GIMP and it offers me a sliding Quality bar that goes 0 - 100. I want to know if that's an equivalent to the similar JPG quality setting in Photoshop?

Raphal Quinet did research into this back in 2007:

The subsampling settings have been renamed in GIMP since then with 1x1 becoming 4:4:4.

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On Fri, 2012-06-15 at 14:51 -0500, Keith Purtell wrote:

When I export a Web JPG from GIMP and it offers me a sliding Quality bar that goes 0 - 100. I want to know if that's an equivalent to the similar JPG quality setting in Photoshop?

No; gimp's 75 is awful, although I often use it for the 500 pixel preview images on www.fromoldbooks.org to save bandwidth and to give instant gratification for dialup users. Well, sooner gratification.

If I use anything below 90 or 93 in gimp I often use "smoothing" too, which reduces the visible artefacts.

On the other hand you can easily get much smaller files with GIMP for almost the same quality.

Liam