Photoshop: rgb to cmyk conversion dithering

  • 1
  • Problem
  • Updated 6 years ago
  • Solved
  • (Edited)
I have been using Photoshop CS6 and noticed a problem. I checked with CS5 to compare only to find I am correct.

--------------
Workflow
--------------
I create a document in RGB mode.
I fill the document with squares in solid colours using the paint bucket tool.
I convert the document to CMYK mode.

I select the 'Magic Wand' tool and adjust it's settings:
'Anti Alias' unchecked
'Tolerance' 0
'Sample All Layers' unchecked
'Contiguous' checked

I click on one of the squares: it does not select the entire square. Instead, it selects in a strange pattern telling me the colours are not the same. This pattern sometimes seems to be at a 45 degree angle.



This is a big deal to me and I can not imagine other users not suffering from this same issue: if this is a bug it is quite serious.

-------------
Additional
-------------
The point is that if pixels are created that are identical in colour, they should be converted the same way. The document is not scaled: only the colour mode is changed. The conversion to the other mode should be applied the same to all pixels that are filled with the same colour.

If I inspect the pixel colour with the 'Eye Dropper' tool it tells me the colours are indeed different. A tolerance of 2 sometimes is not big enough.

The same happens if I start with a CMYK document and then convert that to RGB mode.

------------
Checked
------------

To make sure the squares are solidly filled I went back and redid this many times, checking the 'Magic Wand' tool in the original state and then in the converted state. The original state selects the solid squares without a problem. As soon as the colour mode is changed: things go bad.
This does not happen in Photoshop CS5: this functions perfectly.

------------------
Workarounds
------------------
1. Higher 'Tolerance'
This is not a great solution though: it will select pixels that are not alike. What I want is to simply select that same square (or even that formerly so solidly coloured background).

2. '3x3' or even '5x5' average wand setting
Instead of using the extremely powerful point sample setting, I can resort to the 3x3 or 5x5 average setting. Drawback of course is that colours may be selected that should not be.

I hope I either am overlooking something (although I have been using Photoshop for about 18 years) or that this is a bug that will be fixed shortly.
Photo of davidros

davidros

  • 3 Posts
  • 0 Reply Likes
  • confused

Posted 6 years ago

  • 1
Photo of Chris Cox

Chris Cox

  • 20280 Posts
  • 812 Reply Likes
Not a bug.

You have dithering enabled in color settings -- which is the appropriate choice for the vast majority of images. For test patterns and calibration targets it may not be the best choice because it will cause values to vary by plus or minus one after conversion -- that's why there is a preference to disable dithering.
Photo of davidros

davidros

  • 3 Posts
  • 0 Reply Likes
I used the automatic 'Image> Mode' menu item to switch modes.
In the 'Edit> Convert To Profile' window, the 'Use Dither' option is not checked.

If I convert the document using 'Edit> Convert To Color Profile' and do not check 'Use Dither', the result is still the same.

Still confused. And I do not think this is now solved...
Photo of Andrew Rodney

Andrew Rodney

  • 496 Posts
  • 86 Reply Likes
>I used the automatic 'Image> Mode' menu item to switch modes.

Check the color settings. Dither is there and that is accessed for Mode Change.
Photo of davidros

davidros

  • 3 Posts
  • 0 Reply Likes
Oh god, I never checked that one, it's one of the first things I turn off when I synch the products... I must have forgotten that during the trials with Photoshop CS6, I should have unchecked it as well... This is a big 'oops' for me... Consider this one solved!
Although one thing strikes me as odd. If I specify I do *not* want dithering when using the 'Convert To Color Profile' option, it still dithers. This should not be the case I think. I wonder why this is.