On Saturday, January 3, 2004, at 12:38 AM, Chipp Walters wrote:
Dar, perhaps you can tell me what the internal gamma setting is for a Pshop
PC generated PNG vs a PShop Mac generated PNG...my guess it they are the
same, as I couldn't get a PNG and trans GIF to ever work correctly on my Mac
unless setting the gamma to the PC value.
Your guess turns out better than mine.
The one you sent me has a gamma of 2.222222.
The one I created with my Photoshop on OS X has a gamma of 2.200026, and it also has a primary chromaticities chunk that might override some of that. Maybe I don't have my photoshop set up right.
Setting screenGamma to 2.2 still might not be a solution. I saved my test stack last night with your test PNG image on it, the one with PNG written in the corner. When I loaded it this morning I found the gamma in both the text property and the export showed 1.7. Based on what I saw yesterday and what I see with the test box you sent, this _was_ 2.2. Maybe, setting screenGamma to 2.2 is still OK because, I suspect the pixel data is still for 2.2.
I set my test image to 80,80,80 (hex) grey in Photoshop. I don't know what values were actually stored in the PNG file. When I imported it into Revolution the pixels became 69,69,69 hex; well, they were 69 as soon as I could read them. Why I saw 86,86,86 (hex) on your png (the other one with the png label) beats me. I thought it was created as 50% grey, also. Oh, maybe those were 68s. Rats. Sorry.
I made a rectangle with a 50% background and it looks a lot lighter.
put (baseConvert("69",16,10)^(1/2.2))/(255^(1/2.2)) && (128^(1/1.7))/(255^(1/1.7))
==> 0.668099 0.666689
I _think_ this means that x69 on 2.2 will look the same as x80 on 1.7.
Maybe Rev is not converting the 69s to 80s as it should. Well, I don't know enough to say should.
One experiment might be to fiddle with gamma on images and see if Rev respects those.
The more I think on this, the less I know.
Dar Scott
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