CS2: 16bit layers and posterization

MS
Posted By
michael_shaffer
Nov 19, 2006
Views
516
Replies
12
Status
Closed
I was working towards B/W conversion with a 16bit color image and using "channel mixer" as an adjustment layer along with 3-4 other layers that employed blending and gray scale layer masks. The merged image was showing rather drastic disappointing posterization until I noticed the posterization disappeared if the zoom was closer to 100%.

I hadn’t noticed this before and I wonder if it is normal, or if it might be a problem associated with my PC’s graphics card?

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C
chrisjbirchall
Nov 19, 2006
the posterization disappeared if the zoom was closer to 100%.

Images should always be evaluated on screen at 100% magnification.
MS
michael_shaffer
Nov 19, 2006
Images should always be evaluated on screen at 100% magnification.

For many methods I’d agree … but this amount of posterization affects how anyone would evaluate the overall composition. It also affects layered documents only. The link below includes an example of the image in question and also its "copy merged" 16bit equivalent.

<http://www.michael.shaffer.net/decor/posterization.jpg>

cheerios 🙂
RK
Rob_Keijzer
Nov 19, 2006
Michael,

I don’t know your excact steps, but I did this:

Took a 16 bit colour image with sky gradient, Used Channel Mixer (Monochrome; 150% Red; -50% Blue) as an Adjustment Layer.

Then I added a Levels Adjustment Layer, Darkened the mid-point.

With all Layers intact I see no posterisation at 25, 50 and 100%, also not merged or flattened.

Does this approximates what you did?

Rob
MS
michael_shaffer
Nov 19, 2006
hello Rob, and thanx for taking a stab at it … but I need admit that a number of layers are in play and the posterization may be a simple effort of PS to reduce refresh computations.

Above the RGB base, there is a curves adj layer, a levels (mode multiply), the channel mixer, and a final adj layer( levels, mode multiply with a gradient mask for the sky).
CC
Chris_Cox
Nov 19, 2006
I think what you’re seeing is just the fact that your display is limited to 8 bits per channel.
JJ
John Joslin
Nov 19, 2006
In the end, the printed result is the only final way to judge.
RK
Rob_Keijzer
Nov 20, 2006
I think what you’re seeing is just the fact that your display is limited to 8 bits per channel.

But then we wouldn’t see the difference within his posted example, would we?

Rob
MS
michael_shaffer
Nov 20, 2006
I think what you’re seeing is just the fact that your display is limited to 8 bits per channel.

Rather a strange response from one who is usually right-on, and who apparently hadn’t seen my example in post#2(?)

Still … if I am to take Mr Cox’s response correctly, then what I am seeing is not normal behavior, when I assumed CS2 was simply employing a computational shortcut. That is, I really didn’t expect PS to keep up with many 16bit adjustment layers, more a question of how many could I expect to see an accurate display from?

cheerios 🙂
D
deebs
Nov 20, 2006
I think one important concept is not to assume all the time that onscreen representation/degradation of data with actual degradation of data.

I think someone recently posted about how jaggies appear at odd steps (eg 66.6% and 33.3% zoom levels as opposed to 25%, 50% 75%).

While the effect may not be obvious all of the time there are likely to be occasions when funny zoom levels may the data look worse than it really is?
CC
Chris_Cox
Nov 20, 2006
OK, that example doesn’t look normal.

But I’d have to see exactly what adjustments you’re doing to figure out why you’re getting banding.

(and sorry if I’m off my game, I’ve been out sick for a little too long now…)
MS
michael_shaffer
Nov 21, 2006
I’d have to see exactly what adjustments you’re doing to figure out why you’re getting banding …

A cropped version of the file (6.7Mb) can be downloaded from here:

<http://www.michael.shaffer.net/posterization.tif>

TIA 🙂
CC
Chris_Cox
Nov 22, 2006
The posterization you’re seeing is due to 2 things:

1) Photoshop uses pyramid levels for previewing at 50% and below. For a 16 bit document, those pyramid levels are only 8 bits/channel (because it’s just a preview). Adjustment layers will apply to the pyramid levels for your preview.

2) You applied several _extreme_ adjustments to make those clouds visible. If you had done this to an 8 bit image, you would get results similar to what you see below 50% zoom.

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