Interfaces: PC and MAC and the screenGamma property...

[Dar Scott]

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