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Author Topic: "Use Dither" in Photoshop Settings affects 8 bit printer drivers!  (Read 2891 times)

Doug Gray

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"Use Dither" in Photoshop Settings affects 8 bit printer drivers!
« on: September 28, 2017, 11:16:35 am »

I've long assumed, incorrectly, that the "Use Dither in 8 bit/channel RGB images," a global setting in Edit->Settings, only sets the default conversions between different colorspaces with 8 bit images as a way to minimize banding. This is a small effect even going from ProPhoto RGB to sRGB. When converting colorspaces the "dither" option only shows up with 8 bit images and is grayed out, unchecked, with 16 bit images. That makes sense when you consider the conversion errors with 16 bit images are on the order of .01 dE.

I just found out, via measurement and noticing unexpected results, that it also affects images when printing. Even though it is not listed as an option in the printer dialog. Further, when printing it affects both high bit and 8 bit images. When that option is selected in Edit->Settings, printing to 8 bit printer drivers is dithered. This is because the image, after conversion to printer space, is always 16 bits.

The dithering occurs when the driver space RGB values have fractional values such as RGB 100,110,120.3, in which case the printed pixels will be varied between 100,110,120, and 100,110,121 so as to average out the fractional part.

This has no affect on profile targets because their fractional parts are 0 and are already in device space.

However, the dither option does affect 16 bit images already in device space if they have fractional parts not accurately represented in 8 bits.

And it also affects printing of specific colors. For instance if one wishes to print the Cyan value Lab (50,-70,-20), the result after printer profile conversion will have fractional components. Dithering will produce slightly more accurate printing, roughly similar to that produced by a true, 16 bit printer driver. These are tiny effects of less than .5 dE even on the most sensitive colors. Most of the printer space is only affected .1 to .2 dEs.

This is from measurements. I have never seen this impact a print visually but if it does, it is most likely to show up in deep shadow areas with smooth, non-noisy gradients.

Edit: Added plots of measured L* for RGB values ranging from RGB(150,150,150),  RGB(150.125,150.125,150.125),  RGB(150.375,150.375,150.375) .... RGB(161,161,161).

The blue line is without "dither" checked, Orange is with. All else is unchanged. Attached is the ISIS image that was printed to get the measurements.

« Last Edit: September 28, 2017, 08:32:07 pm by Doug Gray »
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Bart_van_der_Wolf

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Re: "Use Dither" in Photoshop Settings affects 8 bit printer drivers!
« Reply #1 on: September 28, 2017, 11:56:58 am »

I've long assumed, incorrectly, that the "Use Dither in 8 bit/channel RGB images," a global setting in Edit->Settings, only sets the default conversions between different colorspaces with 8 bit images as a way to minimize banding. This is a small effect even going from ProPhoto RGB to sRGB. When converting colorspaces the "dither" option only shows up with 8 bit images and is grayed out, unchecked, with 16 bit images. That makes sense when you consider the conversion errors with 16 bit images are on the order of .01 dE.

I just found out, via measurement and noticing unexpected results, that it also affects images when printing. Even though it is not listed as an option in the printer dialog. Further, when printing it affects both high bit and 8 bit images. When that option is selected in Edit->Settings, printing to 8 bit printer drivers is dithered.

Thanks for sharing, Doug.
Good find.

Cheers,
Bart
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Doug Gray

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Re: "Use Dither" in Photoshop Settings affects 8 bit printer drivers!
« Reply #2 on: November 25, 2017, 04:53:22 pm »

Works great on ABW too!

Just checked this out on ABW as well and Photoshop dithers 16 bit B&W images with the 8 bit driver. The ABW mode is exceptionally smooth on the 9800 so printing 16 bit Tiffs produces incredibly smooth B&Ws.
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