[Updated] Web Designer’s Nightmare: GIMP PNGs darker on MacOS X 10.5?

I absolutely don’t know where that problem comes from, but there seems to be a rendering issue under MacOS 10.5 Leopard with PNGs created using the GIMP. I stumbled upon this while I was working on a website design I created some time ago. I don’t know if this might be a feature of one of the involved tools but I don’t think so.

Since I switched GIMP and MacOS X versions simultaneously, I’ll mention the versions here to ensure the difference is reproducible. Although you may not see it (depending on your monitor), there is a subtle difference.

Steps to reproduce:

  1. Take a PNG file created with MacOS X 10.4 Tiger and GIMP 2.4.0
  2. Open it with GIMP 2.4.2 under MacOS X 10.5 Leopard
  3. While still using GIMP 2.4.2, save it to another file. Keep the same export options as the original file.
  4. Now compare the 2 files with quicklook and see the difference.

You might say “ok that’s a GIMP Bug/Feature”, but I’d definitely attribute it to MacOS X’s rendering rather than the GIMP. Here’s why:

The problem gets even more obvious when you upload the newly created file on a web server and look at it using Safari and Firefox at the same time. Since they use 2 differend rendering engines, you can see 2 different renderings of the same file in the same OS. While working in the GIMP, I see the brighter version. MSIE 6 and 7 on Windows also behave like firefox and GIMP, so the brighter version must be the right one.

MacOS X 10.5 PNG Rendering issue

Pretty scary…

If you had the same or similar experiences, feel free to comment.

Update

While talking to my colleague Martin Kneißl, who’s into photography and that kinda stuff, he suggested that the difference may come from unwanted color profile evaluation. After some Google’ing and explanation tryouts, I stumbled upon this site which talks about exactly that problem. Especially the following part is interesting:

List of Color-Managed Browsers:

Apple’s SAFARI for Mac
(Assumes monitor profile on untagged files)

[…]

List of Un-Managed Browsers:

Mozilla FIREFOX (Mac & Windows)

That makes it pretty clear: Either Apple added color profile evaluation in Leopard (which is unlikely), or the newer GIMP versions handle PNG export differently (which is more likely). Probably, the new GIMP didn’t tag the files with sRGB, so MacOS X’s PNG rendering engine used my monitor’s color profile. And since Firefox’s rendering engine doesn’t care about color profiles at all, it explains why the image renders differently in that other browser.

Nevertheless, the result is really unexpected and even though I understand what’s happening now, I still find the end result scary from a Web developer point of view.

Update 2

Okay, just got back from work and checked the color profile of the file. GIMP pretends it’s sRGB. Would this be a bug somewhere after all?

Gimp sRGB Color profile

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