I count myself among those who normally don’t even think that changes do or even should occur when routinely flattening a multi-layered file in Photoshop. Well too be 100% technically accurate – the tones in the file itself are unlikely to change in any perceptible way – but what you see on screen certainly can.
It can be disconcerting if you are not aware of what is going on and are taking measures to compensate for it.
Here is an instance where the difference was significant and worth sharing. I posted an animated interactive example. Click the colored square at the top see the before an after.
The images which were used to create were screen grabbed before and after pressing ‘flatten’. Note that I was viewing this image zoomed out to fit the screen. Photoshop caches images which it uses for screen display and therein lies the problem.
There are 2 strategies for dealing with this problem and they may provide some clues as to what the causes are.
- View the image at 100% and when you flatten the image there is no perceptible change.
- Set the image cache in Photoshop preferences to 1 and restart Photoshop and reopen the file. The zoom out and flatten the image and there is no longer a perceptible change. But having said that – the entire image now looks very different zoomed out than it did with a normal cache setting)
The people at Adobe know the cause - and meantime I can only guess – - but it is conceivable that this is a ‘detail/scaling/cache’ issue rather than a straightforward tonal accuracy issue (a ’rounding up’ or down of layer tonal values for fast interactivity – which, initially is all I thought was was happening).
The reason I say this is because of the change in the appearance of those areas in the image which change tonally when I instituted the second strategy outlined above. Those sections of the image appeared way less anti aliased (i.e. more granular and gritty).
Whatever algorithm employed in creating and displaying the multi-cached images is smoothing and averaging those areas tonally. Smoothing over the extreme fine high contrast detail contained in those areas may have resulted in a the tone change which was evidenced.
All this is neither here nor there, but one practical take-away: If the image changes on flattening – undo and then check the tones first at 100% or reduce cache settings to be sure you have the tonal values you need before flatten button is depressed.