Black point, colour management and Full Screen riddle

[tomczak]

I readjusted my screen a little, and everything seems to work reasonably well: black point, white saturation and gamma are about right.

With the new Gray Discrimination image (Black Test), the 1,2 and up pixel value rectangles are barely but discriminable from black (0) background in the PWP preview window both with colour management on or off.

However, in Fullscreen, when the colour management is on, the same 1 and 2 pixel value rectangles merge into black (i.e. no matter what I do with display contrast, brightness or gamma, which indicates to me that in full screen they are treated as having a value of 0). Interestingly, turning the colour management off, makes them display correctly even in full screen as well, just like they do in regular preview panel.

How do I make sense out of it?

[jsachs]

Will take a look when I get a chance - not for a while.

[tomczak]

Thanks. Just to add a clue to it. The original 8bit image has a Gray Gamma 2.2 profile attached to it. If I convert it to 24bit colour, sRGB, those very low-value greys become zeros, just like in Full Screen.

My working profile is sRGB and I suspect it has to do with the sRGB's gamma straight line segment for the lowest 10 or so values with higher slope than the regular 2.2 gamma curve.

[jsachs]

PWP should be converting images to the monitor color space whether full screen or not. Converting an 8-bit image to some other color space will almost always lose some gray levels.

This raises some questions about whether the gray discrimination test image has any value in a color managed environment.

[tomczak]

The thing is that with either 8 or 16 bit black discrimination image, colour management on, and all other settings the same, the regular preview panel shows what I expect, while the Full Screen blocks the lowest greys. It seems to me that the Full Screen must be doing something different colour-management wise than the regular preview panel.

When and if you have some time to look at it, I'll explain the settings that I use.

[Charles2]

If I understand what Maciej does, I duplicated the situation and had no problem. By Fullscreen whether you mean Windows | Maximize or PWP | Image screen only, I had no problem, PWP Color Management on or off.

[tomczak]

Thanks for trying to reproduce what I thought I was seeing, Charles.

I've spent some time this morning to determine if I wasn't dreaming. I still have to figure out how to support visually what I'm going to claim, but the issue is not about manually converting colour spaces in the colour-managed environment and thus losing some grey levels due to gamma curves differences or 8bit posterising.

Instead, it is about the difference in the automatically colour-managed, displayed image in the PWP regular Preview panel vs. PWP Full-Screen Preview (Edit/Full Screen or Ctrl+F), everything else being equal. For a given image, those two previews are not the same, and I couldn't figure out why (albeit the differences are slight and show up in special cases).

Here is my attempt at diagnosis (all with colour management on, i.e. using colour mgmt engine that does automated conversions, including B&W to colour):

1. In PWP regular Preview panel, the image with an embedded ICC profile (e.g. Grey Gamma 2.2) is converted to the monitor colour space for display. The working colour space is not involved (the image wasn't converted to it when opened).

2. In PWP Full Screen Preview, the image is converted first to the Working Colour Space, and only then to the monitor colour space for display. This seems to account for visible blocking v. dark greys in Full-Screen preview, which isn't the case in the regular PWP panel preview.

Is what I described possible?

[jsachs]

I did a quick check and indeed, that is what is happening. If the image is black and white, it is being converted to 24-bit color in the working color space before resizing and being converted to the monitor color space.

Fixed for next release.

[tomczak]

Many thanks!