What you see is not what you get.. image on screen is not correct when working in 16 bits image mode

[Molland photography]

I came across an error is the way an image is displayed; this used to be OK in previous versions, but now the image shown is incorrect.

The problem can easily be re-produced following the follwing steps.

1. Open a good quality raw image of a sunset (every one has one in her/his collection). See screenprint 1
2. Set the image mode to RGB 16 bits
3. Duplicate the background image.
4. Add 2 times a brightness/contrast adustment layer on top, set both to maximum brightness +150
5. Add 2 more brightness/contrast adustment layer on top, set both to minimum brightness -150
6. These 4 layers should cancel each other out.. So with perfect color depth and flawless calculations you should see an image that is the same (or very close to the original). However, I see on screen an image that is siginicantly different in colour, brightness etc. See screenprint 2
7. Now the strange thing.. If you force Photoshop to calculate, the image 'pops' back to be close to the original. You can do this (preferably) to merge the copied back-ground with the 4 adjustment layers, or to flatten the image. see screenprint 3
   
   

The image after step 7 is how it should look before step 7. After all, flatting an image should not change the way it looks. Now if you repeat the same steps with the image mode set to RGB 8 bits and do exactly teh same steps I find that the image after step 6 looks exactly the same as when I started with 16 bits. however, after step 7, it stays that way.

So this inicates that even when I work in 16bits mode, the image I see on the screen is calculated in 8 bits mode. This seems to be a serious bug, as we the image we see on the screen should reliably show all the modifications we have made to an image.. Withoug merging thos layers, I would have no clue how the final image would look like..

I work on a new macbook air with teh M1 chip set running big sur 11.6.

My photoshop version is 23.0.0

[PECourtejoie · Accepted solution]

Hi, is it the case also at 100% magnification?

[D Fosse · Accepted solution]

PEC has the right answer. This is because you're zoomed out to less than 100%.

All adjustment previews are calculated on the basis of the on-screen image. Zoomed out to less than 66.7% these previews are 8 bit depth and all calculations are 8 bit precision.

This is done for performance reasons. When you commit the adjustment by merging/flattening, the calculation is done on the full data at full bit depth.

IOW, it's not a bug, it's by design for a specific reason. You could argue that modern hardware should be able to handle this at full size - but then again, file sizes tend to increase in step with hardware, and there will always be a big part of the user base with low-spec machines.

[Molland photography]

Thanks guys, was not aware of that.

I would still prefer it to always show the result correctly on the screen, but if it is done by design to lower the hardware load it makes sence...

[PECourtejoie]

Photoshop has been undergoing many deep level architectural changes recently, as we can see by the "legacy compositing" and the "native canvas" options in recent preferences, and the switch from OpenCL/GL towards DirectX and Metal.

Maybe could you enter a feature request to have a "Full" 16bits preview at lower magnification, and hope that rally your request so that the team prioritizes that change over others, if there are lots of support.

[Molland photography]

That is a great idea.
The optimum in my opinion would be that this is an option one can select in the settings.

People with fast hardware can opt to be 16 bits all the time, others only when zoomed in.

Personally, I often work on the image full screen (i.e. 25% zoom) to do the larger area adjustmetnst and it would be good to be sure that the image I see on the screen is correct.

[D Fosse]

Yep, an option would be great, for several scenarios in addition to this one. Another case in point is noisy and "binary" images where the previews are misleading because it's done on a smaller, downsampled version. With all those sharp, pixel-level edges, actual pixel values are extensively changed with downsampling. And so the preview is wrong, but snaps into the correct version when flattening. People get confused over this no end.

But to be clear, the safe way around all these pitfalls is to view at 100%. Then you get the absolutely correct version all the way. With experience, ctrl+1 gets deeply embedded in your muscle memory. I do it almost on reflex now, without even thinking.