I'm an architect student and do a lot of visualizations with photoshop. Working with mostly 5000 to 6000 pixel horizontally to get a fine large print.
I have two questions to that Retina-Photoshop issue:
1. If I tell photoshop to create a 6000 px by 3000 px document/canvas, how big will the exported image be at the end of the day? Horizontally 6000 px or 12.000 px?
2. If I zoom in close in photoshop, I get to see every single pixel in the grid view. What pixel will I see on the rMBP? One pixel equals one pixel of the image or one pixel equals 4 pixel of the image?
I get really confused thinking about that stuff. I'm really worried that the future visualizations, that I will create, will never look good enough on that rMBP and I will be wasting too much time on details, no one will see later on!!!!
Thanks
Today, before it's optimized, your Photoshop works just like you are using it on a 1440x900 regular computer. Only difference is that, the image will look blurry and pixelated.
So to answer your question, 3,000 pixels are just 3,000 pixels. They even look the same size (roughly) as 3,000 pixel on another 15 inch laptop w/ 1440x900 resolution.
Once when Adobe upgrades, then of course this will change. What will it become? It really depends on what Adobe does.
If Photoshop does what iPhoto does, which is to utilize 2880x1800 resolution, then a 3,000 pixel image will now look on the screen only half of the size as before. Notice, even if your screen is using 1440x900 resolution, iPhoto will still display images at 2880x1800 resolution, so you will see the true sharpness of your photo. This is what iPhoto does, and maybe Adobe will do it too for Photoshop. This makes the most sense: split the application so that the interface is using a 1440x900 resolution while the content (in this case images) are displayed in 2880x1800 resolution. (You can argue that since it's retina enabled, then even the application is in 2880x1800. While you are technically right, for the purpose of this argument, let's say the interface of the application and the content of the application are on two different resolutions. Here is a better way to understand. Today, using best for retina resolution, the application icon will look size N inch, and a 1,000 pixel image will look width W inch. Once optimized, again using the "best for retina" resolution, the application interface icon STILL has a size N inch, but the same 1,000 pixel image will have a W/2 inch width! (Increasing quality/sharpness of the image while not losing usability of the application interface.)
But to answer your question, regardless of how the image appear on the screen, when you print, the same 3,000 pixel width print at 300dpi from a retina MacBook Pro will output the same size of print as a 3,000 pixel width print at 300dpi from a PC, because printer has its own dpi, irrelevant of the resolution of your screen.
When you said "output" I assumed you meant "print." If you meant "Save as", then it depends again on the device you open the file with.
To answer your question 2.
Today, before optimized, on Photoshop, the smallest unit of your image (a pixel) will actually utilize 4 physical pixel on your retina screen. If you zoom in 200%, then that pixel will take 16 pixels (4x4). If that pixel is pink, then u will see 16 pink pixel (4x) when you zoom in at 200%.
Once it's optimized, the smallest unit of your unit (a pixel) will utilize only 1 pixel, just like a non-retina screen will behave. Once you zoom in 200%, that pixel will take 4 pixels (2x) just like a non-retina screen will behave. Only differnece is that, every pixel is only 1/4 the size of a non-retina pixel, so a 200% image on retina looks to be the same size to your eyes as a 100% on a non-retina screen.