PS CS5 8-bit processing with adjustment layers

<p>Hi all,</p>

<p>I've been hard at work editing some of my back-log of photos and noticed something rather troubling about my PhotoShop CS5 Extended. When using adjustment layers, all processing of 16 bit images seems to be done in 8 bit. This is particularly noticeable when applying a curve adjustment layer, and it is manifested by "combing" and "binning" in the histogram. If I apply the same adjustment without making it a layer (e.g. Image/Adjustments/Curves), I see the processing is done in 16 bit, and the histogram is smooth.</p>

<p>Do other PS users see the same thing? Perhaps there's something wrong with my installation? Perhaps this issue is fixed in subsequent versions? Any insights would be appreciated.</p>

<p>Thanks!<br>

Sarah</p>

 

<p>I see quantization error whether its an adjustment layer curves, image adjustments curves or even apply layer. Its almost like an aliasing artifact as pixels gets mathematically pushed into another bin. I am not sure how you surmise the processing is in 8-bit mode. </p>

<p>If I do the same exercise with an 8-bit image, I see no difference in the histogram between a curve adjustment layer and the application of a curve. Either will produce the same "combing" or "binning." But with a 16 bit image, the curve adjustment layer will produce exactly the same combing as on the 8 bit image, and the application of a curve will instead produce a smooth histogram.</p>

<p>Let's just say this 8 bit processing issue looks very familiar to me. It was the reason I jumped ship from PaintShop Pro to PhotoShop. PSP dragged their feet implementing 16 bit engines throughout their application. I'm disappointed to see what appears to be the same issue with the adjustment layers feature of PS. I'm hoping there is simply something wrong with my installation or settings or something.</p>

<p>Hi Sarah - When I added 1% of noise to the 1st 16bpc test image that I tried, I saw just about the same amount of combing / jaggedness in the histogram both before and after I apply a substantial curve (either as an adjustment layer, or directly). </p>

<p>However, if I don't add the noise, then I see effects like you describe.<br>

<br />What I didn't mention was that my first 16 bpc test image actually started out life as an ordinary 8 bpc file to which I did a simple conversion to 16 bpc. In contrast, if I start out with a "real" 16 bpc image (say from ACR), it acts just like the image after adding 1% noise.</p>

<p>My guess if one's starting image really was 8bpc and then was artificially up'ed to 16bpc, the numerical precision of the curves math being done in 16 bpc mode is so good that it effectively retains the 8bpc character of the starting image. In other words, when viewed on a 16 bpc level of resolution on the horizontal axis, a histogram that started out as a picket fence, stays a picket fence, just that the pickets get moved around a bit so that when they are graphed on the 256 bins of the histogram display, some of the moved spikes will line up with a bin, while others will not, and hence you see a comb like display.</p>

<p>Just my $0.02,<br>

<br />Tom M</p>

<p>PS - I hope that was understandable. I wrote it being half asleep, LOL.</p>

<p>The question you must ask yourself when you see banding is this: Is it in the data or the display path? If you're working in high bit and see banding, it's most likely (surely) the display, not the data. </p>

<p>Hey Tom!</p>

<p>I'm actually starting out with some 16 bit TIFF files that I had converted from RAW (in DPP) quite a while ago. I had thought the same thing, that they were really 8 bit files. But when I checked (Image/Mode) I saw that I was working in 16 bits. My next thought was that these files had been 8 bit, saved as 16. However, I found this was not the case, replicating the problem with freshly converted files. To rule out any issues with DPP (my preferred conversion software), I also opened a raw file in ACR, making sure to use the 16 bit color depth, and the issue with adjustment layers was exactly the same. So I'm certain I'm working with true 16 bit files. If I were not, I'd see the combing with a simple application of a curve (not through the adjustment layers), which I do not.</p>

<p>Do you have CS5? If so, could you confirm whether your copy does the same thing as mine? Just open a RAW file in ACR in 16 bit, and manipulate it with a curve adjustment layer (should produce combing) vs. application of a curve (Image/Adjustments/Curves -- should not produce combing). I am wondering whether there is a setting somewhere that will force 16 bit processing of the adjustment layers, perhaps the default being 8 for speed. (I've looked, but I can't find one.)</p>

<p>Hi Andrew,</p>

<p>I see the issue in the histogram. I don't actually see the problem on the monitor. My concern is with cumulative error. I often do a lot of editing on a photo, and the cumulative errors from 8 bit processing have sometimes resulted in (barely) noticeable noise or banding by the time I'm finished. I prefer doing my editing with more significant bits of resolution to avoid this cumulative error problem. But FAIW, I'm running an NEC PA241W monitor, calibrated with XRite's ColorMunki. My graphics card allows me to display in 10 bits of resolution, although the driver sometimes misbehaves and drops me back to 8 bits. However, as I said, I'm observing this issue in the histogram, which becomes quite jagged when I apply a curve adjustment layer.</p>

<p>Histograms don't operate in high bit and as such you can't and shouldn’t be concerned, <br>

Try this: Duplicate the entire image and flatten. See banding? <br>

And yes, layers are going to apply data loss once flattened or if you make a print. They are not really non destructive IF your definition is 'no data loss' (that's mine).</p>

<blockquote>

<p>Try this: Duplicate the entire image and flatten. See banding?</p>

</blockquote>

<p>Ah ha! No! It goes away! [light bulb moment]</p>

<p>It also goes away if I save the ACR-opened 16-bit image with comb-patterned histogram in 16 bit TIFF format, close, and then reopen. The combed histogram remains if I save to a 16 bit PSD file and then reopen. I assume this means the adjustment layers give you a low resolution (8 bit) preview until you actually "apply" them in some way, which would make sense.</p>

<p>FAIW, I'm not trying to <strong>eliminate </strong>data loss, but rather trying to minimize it. (I realize it can't be eliminated.)</p>

<p>Thanks, Andrew! I feel much better now about what's happening.</p>

<p>And Tom, now see you found the same thing with your installation of PS. Sorry I didn't read it correctly the first time.</p>

<p>Thanks, guys!</p>

Hi Sarah - Sorry about the poorly worded earlier response, but it looks like it was at least somewhat decypherable, LOL.

BTW, unfortunately, I only have the latest version of PS installed, so my observations only apply to it.

 

All the best,

 

Tom

<p>More a matter of it being poorly read, Tom. My brain doesn't completely quite wake up until noon'ish. ;-)</p>

<p>Anyway, I am marching forward on my edits with renewed confidence, using adjustment layers. (Such a nice feature!) Hopefully I'll cough up a substantial bolus of finished photos into my portfolio in a week or so. It seems one of my biggest talents is procrastination, and it takes my getting sick to motivate me to sit down in front of the 'puter and get a few things done.</p>