Scaling on 4K monitor in OS X (Mac)

[Hans Kruse]

I recently got a 4K display (Dell 32" UP3214Q) and was a bit surprised about how the scaling options in OS X affect how Lightroom scales images displayed as well as zooming in to 1:1. The scaling option are

the middle one is scaled to look like a 27" monitor of 2560x1440. Desktop resolution as seen from a screen dump is 5120x2880.
the next one to the right scales to look like 3008x1692. Desktop resolution as seen from screen a dump is 6016x3384.
the last one which is also default for the display is full 4K resolution of 3840x2160. Desktop resolution as seen from a screen dump is 3840x2160 (of course).

I had so far used the scaling to 3008x1692 as text on the 32" monitor looked good at that scaling. Little did I realize that zooming in to 1:1 does not show 1:1 on the screen!! Actually quite a bit less. Not that 1:1 looked blurry, but the 1:1 view in the default scaling looked sharper and crisper.

Attached are 3 screen dumps of full screen view of a picture scaled to 3236x2160 (since aspect ration is 3:2 and not 16:9 as the screen). The order of the screen dumps are like the scaling options described earlier. Given the desktop size it is understandable that the 4K sized picture will show less on the screen with the scaling in effect.

In fact the retina screen on the MacBook Pro (to which the Dell monitor is connected) does something similar, but only in the default 1440x900 scaling option does a picture output for full resolution of the monitor of 2880x1800 pixels fill the screen. The more space options decrease the size the image displayed. That also means that 1:1 zoom only works as intended using the default scaling option. Little did I know this either!! As I had been using the more space option of 1680x1050 as I liked that scaling better.

Maybe I have overlooked something but no Google searches did reveal these things to me

So the take away for me from this is the following:

On the 4K display and using Lightroom change the scaling to default. When I use a browser or e-mail etc. I will change the scaling to one of the other scaling options to be able to better read the text.
On the MacBook Pro retina display I will use the default scaling with Lightroom and probably using the more space option for other applications.

[Hans Kruse]

I'm surprised nobody had commented on this

[Hans Kruse]

There you go, commented. Better? 

I'm sorry, what comment?

[sbay]

I just checked on my rMBP late 2013 and I can confirm that the "1:1" scaling in lightroom appears differently depending on the scaling set in the system preferences. This is counter to how I expected it would work.

I generally use spaces and have multiple tabs / programs open. Switching back to default scaling whenever I use lightroom is going to be a pain. I'm thinking that I will "batch-up" my detail panel adjustments. On the other hand, I didn't notice this affect before, so maybe in practice the scaling doesn't have that much impact on my final adjustments (even though visually they look slightly different).

[Hans Kruse]

I just checked on my rMBP late 2013 and I can confirm that the "1:1" scaling in lightroom appears differently depending on the scaling set in the system preferences. This is counter to how I expected it would work.

I generally use spaces and have multiple tabs / programs open. Switching back to default scaling whenever I use lightroom is going to be a pain. I'm thinking that I will "batch-up" my detail panel adjustments. On the other hand, I didn't notice this affect before, so maybe in practice the scaling doesn't have that much impact on my final adjustments (even though visually they look slightly different).

I'm glad to see others be surprised as well. At least the 1:1 view for critical sharpness checking makes on the 4K display quite some difference from what I have seen. Maybe less on the MBP screen since it is so much smaller and high PPI.

It was also different than I had expected since I was under the impression that pictures used the full resolution of the display contrary to scaled text and interface elements. But clearly pictures are scaled to and only in default scaling there is a 1:1 mapping from display pixels to screen pixels.

[KarlGohl]

Hans

Does this happen only in full screen (F) mode?  If you are in Loupe or Develop with at least some panels, filmstrip, etc. showing, when you go to 1:1 zoom mode, are objects in the image the same physical size on the screen across the different Display scaling options?  (I'm wondering if there this is a bug in full screen mode that causes LR to use the pre-scaled window dimensions.)

Karl

[Hans Kruse]

Hans

Does this happen only in full screen (F) mode?  If you are in Loupe or Develop with at least some panels, filmstrip, etc. showing, when you go to 1:1 zoom mode, are objects in the image the same physical size on the screen across the different Display scaling options?  (I'm wondering if there this is a bug in full screen mode that causes LR to use the pre-scaled window dimensions.)

Karl

No, it is not related to full screen mode. It shows exactly the same in both loupe mode and in the develop module. The same on the MacBook Pro retina screen but with a difference size order from default up to the highest scaling. It's because of the relative size of the picture to the size of the space that the application (Lightroom) writes into and this space is then mapped to the physical screen resolution. It's actually (almost) all explained there https://developer.apple.com/library/mac/documentation/GraphicsAnimation/Conceptual/HighResolutionOSX/Explained/Explained.html#//apple_ref/doc/uid/TP40012302-CH4-SW1

Using the retina display on the MacBook Pro 1:1 works correct with the default scaling and the rest of the applications also look good although one would like a bit more space, but when a higher scaling is chosen the 1:1 gets smaller on the screen. But when using the 4K screen default is the full resolution of the screen which means very small fonts in Lightroom and other apps are unusable. Therefore the situations is not as good with 4K screens as the MacBook retina display. I have to scale back and forth depending on wether I use Lightroom or other apps. Something that I did not know before i got the 4K screen. The 4 screen is gorgeous in many ways, but I would have considered it worth while or not to get 4K if I had known this before I got it. So as I see it there is not a bug in Lightroom! Photoshop works the same way.

[KarlGohl]

Hans

Thanks for your post.  It made me realize that in my research for the possible future purchase of high res display, I had misunderstood what a "retina aware" app can do.  In particular, I misinterpreted this explanation:
http://www.anandtech.com/show/6023/the-nextgen-macbook-pro-with-retina-display-review/6

I now realize:
1. With the "scaled" Display Prefs settings, there are three resolutions of concern: the "Looks Like" resolution,  the Backing Store resolution (2x the LL res), and the actual Physical Display resolution.  For example LL=3008x1692, BS=6016x3384, PD=5120x2880
2. Normally, apps draw in terms of LL resolution.  OSX scales the drawing up 2x to BS res.  In some cases, the OS is able to generate a higher res version of what the app drew.  For example, for text it can generate a 2x more detailed version. For items like icons, it can look to see if there is a higher res version of the icon that it can use. Otherwise, the OS has to up-res what the app drew.  For example, if the app sets a pixel in LL space to white, the OS sets the corresponding four pixels in the backing store to white.  This is what normally happens to raster images like photos displayed by "retina naive" apps.
3. A "retina aware" app can set the scale factor for a particular UI element (e.g. the part of LR's Loupe screen where the image is displayed) to 1.0 (instead of the default 2.0).  It then draws in BS res and the OS sets the corresponding pixels in the backing store
4. The OS then resizes the contents of the backing store down to PD res and updates the display

Previously, I thought that a retina aware app was able to draw to a specific UI element using PD res.  For example, it could tell the OS "for the part of the display screen that corresponds to this UI element, here are exact pixel values to show there".  That is: "when you update that part of the display, just put these exact pixels there instead of the pixels from (the down-res'd version) of the backing store. 

So a retina aware app is able to avoid the up-res from LL res to BS res, but it can't avoid the down-res from BS res to PD res.  So LR can't implement the 1:1 zoom semantic that you and I would prefer (1 pixel in the image maps to one physical pixel on the display). In short, LR's ratio based zoom settings are w.r.t. BS res.  [In theory, LR could determine exactly which image pixels should be displayed to give image-to-display ratios and up-res the pixels the right amount to cancel out the OS's down-res.  This would make items in the image appear at the same size at 1:1 zoom for all Display Prefs resolutions.  However, after going through the up-res and down-res, the pixel values might change slightly, especially since the resizes are non-integral and use different algorithms. But this still might be the best option for evaluating sharpening.]   
 
The realization that with a high DPI display, LR's ratio-specified zoom levels aren't image pixels to display pixels ratios was initially disconcerting to me.  However, since I seldom make prints, I doubt that it will be a problem for me.  Also, when I get a high DPI display, I can keep a 100 DPI display as a secondary display to use for critical sharpening.

-- Karl

[Hans Kruse]

Hans

Thanks for your post.  It made me realize that in my research for the possible future purchase of high res display, I had misunderstood what a "retina aware" app can do.  In particular, I misinterpreted this explanation:
http://www.anandtech.com/show/6023/the-nextgen-macbook-pro-with-retina-display-review/6

I now realize:
1. With the "scaled" Display Prefs settings, there are three resolutions of concern: the "Looks Like" resolution,  the Backing Store resolution (2x the LL res), and the actual Physical Display resolution.  For example LL=3008x1692, BS=6016x3384, PD=5120x2880
2. Normally, apps draw in terms of LL resolution.  OSX scales the drawing up 2x to BS res.  In some cases, the OS is able to generate a higher res version of what the app drew.  For example, for text it can generate a 2x more detailed version. For items like icons, it can look to see if there is a higher res version of the icon that it can use. Otherwise, the OS has to up-res what the app drew.  For example, if the app sets a pixel in LL space to white, the OS sets the corresponding four pixels in the backing store to white.  This is what normally happens to raster images like photos displayed by "retina naive" apps.
3. A "retina aware" app can set the scale factor for a particular UI element (e.g. the part of LR's Loupe screen where the image is displayed) to 1.0 (instead of the default 2.0).  It then draws in BS res and the OS sets the corresponding pixels in the backing store
4. The OS then resizes the contents of the backing store down to PD res and updates the display

Previously, I thought that a retina aware app was able to draw to a specific UI element using PD res.  For example, it could tell the OS "for the part of the display screen that corresponds to this UI element, here are exact pixel values to show there".  That is: "when you update that part of the display, just put these exact pixels there instead of the pixels from (the down-res'd version) of the backing store.  

So a retina aware app is able to avoid the up-res from LL res to BS res, but it can't avoid the down-res from BS res to PD res.  So LR can't implement the 1:1 zoom semantic that you and I would prefer (1 pixel in the image maps to one physical pixel on the display). In short, LR's ratio based zoom settings are w.r.t. BS res.  [In theory, LR could determine exactly which image pixels should be displayed to give image-to-display ratios and up-res the pixels the right amount to cancel out the OS's down-res.  This would make items in the image appear at the same size at 1:1 zoom for all Display Prefs resolutions.  However, after going through the up-res and down-res, the pixel values might change slightly, especially since the resizes are non-integral and use different algorithms. But this still might be the best option for evaluating sharpening.]  
  
The realization that with a high DPI display, LR's ratio-specified zoom levels aren't image pixels to display pixels ratios was initially disconcerting to me.  However, since I seldom make prints, I doubt that it will be a problem for me.  Also, when I get a high DPI display, I can keep a 100 DPI display as a secondary display to use for critical sharpening.

-- Karl

Yes, that's it. I should have thought about it when I did the examples that setting LL=1920x1080, Lightroom will display the correct 1:1!!! So there are two LL resolutions that will work on a 4K display 1920x1080 and the full resolution og 3840x2160. Using the LL=1920x1080 also makes the Lightroom text much more readable. In this case the BS=3840x1920 which is equal to the PD=3940x2160. Apple did not choose to have the default set to 1920x1080 for 4K. Using a 32" monitor with LL=1920x1080 gives rather large text. But for Lightroom it actually is not bad at all.

If you want to use a larger LL resolution than 1920x1080 for having enough resolution on the display (having space for multiple windows etc. ) then the LL=2560x1440 displays the next best 1:1 in Lightroom where LL=3008x1692 gives the smallest 1:1 display. I would say that these two LL resolutions does not make the Lightroom 1:1 unsharp as such, just slightly less sharp than the ones that will map 1:1 to the PD resolution. These are 1920x1080 and 3840x2160.

Thanks for leading the horse to the water

[KarlGohl]

Yes, that's it. I should have thought about it when I did the examples that setting LL=1920x1080, Lightroom will display the correct 1:1!!! So there are two LL resolutions that will work on a 4K display 1920x1080 and the full resolution og 3840x2160. Using the LL=1920x1080 also makes the Lightroom text much more readable. In this case the BS=3840x1920 which is equal to the PD=3940x2160. Apple did not choose to have the default set to 1920x1080 for 4K. Using a 32" monitor with LL=1920x1080 gives rather large text. But for Lightroom it actually is not bad at all.

If you want to use a larger LL resolution than 1920x1080 for having enough resolution on the display (having space for multiple windows etc. ) then the LL=2560x1440 displays the next best 1:1 in Lightroom where LL=3008x1692 gives the smallest 1:1 display. I would say that these two LL resolutions does not make the Lightroom 1:1 unsharp as such, just slightly less sharp than the ones that will map 1:1 to the PD resolution. These are 1920x1080 and 3840x2160.

Thanks for leading the horse to the water

Good point!  When LL res is exactly 1/2 of PD res, then BS res equals PD res and there is no down-res when copying the pixels from the backing store to the display. Thus, the image pixels LR draws into the backing store map 1:1 to display pixels.  IIRC, on the retina 15" MBP, the retina display had twice the pixels (in each dimension) of the non-retina display, so they set the default LL res to half the PD res, so many people didn't see the problem you pointed out.