Flattening layers in PS

[Norm Buchanan]

I have been wondering something about the way that Photoshop flattens an image. My understanding with layers is that they are applied to the image when flattened in the order, one at a time. Is this correct? For example, If I have two curves layers, one on top of the other, does Photoshop figure out the combined adjustment of the two layers and then apply a single function to the pixels, or does it apply each layer adjustment to the pixels individually, which is my understanding? I realize that this is in the bowels of the PS code but I thought someone may know how this works. I am using CS2 if that matters.

Thanks,
N.

[Jeremy Moore]

does it apply each layer adjustment to the pixels individually, which is my understanding?

Thanks,
N.

This is correct, it will apply them sequentially starting at the bottom.

[Sheldon N]

It works layer by layer, taking each one as it stands and applying it to the image.

On thing to keep in mind it that even with adjustment layers you can do destructive things to an image by creating multiple curves that work against each other. For example if you do a curve layer to open up shadows, then another curve layer to darken shadows later in the editing process you are more prone to introduce posturization than if you had backed off the original curve layer to a more appropriate point.

Multiple curve layers aren't necessarily terrible, but it's a good practice not to work against yourself when stacking layers.

[Norm Buchanan]

Thanks for verifying this guys. The reason I asked was that I have seen some authors writing about PS claiming that layers are less destructive than simply applying the adjustments to the base without using layers. I didn't think this was the case but it made me think layers might be a bit "smarter". I don't see a reason why the adjustments, or functions can't be applied to each other before the pixels themselves. This means that any working against yourself would be taken care of before touching the pixels themselves.

[Marko]

Norm, of course flattening represents a definitive change!

But the entire idea behind using adjustment layers instead of applying modifications directly to the image is based on saving a layered "master" file while keeping it smaller and keeping any edits applied in that way completely reversible and, well, editable.

Flattening an image prior to saving cancels any such advantage and it should be done only to copies made for output, print or screen, as the very last step and never on an original.

[Joanna Carter]

Norm, working with layers is far superior to adjusting the background image. The main difference is that, with adjustments made to the original image, you can't undo, say, the third of five adjustments without also undoing the fourth and fifth. I also find that altering the order of adjustment layers can change the combined effect. In short, always maintain a totally unaltered background, copying it if you need to apply destructive adjustments; then you can always revert to the original image, even if you have saved the file.

[Ed Richards]

One exception to the unaltered base layer - I spot it, figuring I have no reason to return to the dust spots. Usually I add a curve to enhance the contrast so I can see the dust better.

[Joanna Carter]

Ed, I quite agree about spotting the base layer, but I assume you mean to add a contrast curve layer temporarily just for spotting ? Something that is very helpful.

[Sideshow Bob]

On the subject of layers, I have a question. On some of my images I make several adjustment layers and on occasion (not always) when I flatten the image, for printing, the color balance and/or lightness will shift. Does anyone else have this problem or know what’s causing it?

Gale

[Ed Richards]

Joanna - right, a temporary layer just to highlight the crap.

Bob - Makes me crazy on black and white conversions done as a layer. For these, I merge the base layer and the conversion layer, then flatten the whole thing. I do not print color, so I do not know what the right sequence is for preserving color.