B&W Highlight Dithering

Update April 25th, 2019: Release Version 1.1 – Changed the method of final leveling of the dithered highlights into the full image. We now create a leveling ‘Level Dither’ adjustment layer that acts on just the ‘Dither’ highlight adjustment layer. Highlight leveling is not pre-adjusted – you will need to manually level the dithered highlights to your specific image and digital negative workflow process. To adjust the levels, double click on the histogram icon in the ‘Level Dither’ layer to reveal the Levels properties adjustment controls. Adjust the center control from its default of 1.0 up / down to get your highlights to better transition into your main image.


When working with B&W historic photographic printing processes and digitally created negatives, one quickly finds that highlight detail is hard to reliably come by. It is all about the digital negative, exposure correction curves, and the highlight details hiding out in the blacks of the printed negative itself. Try as you may, getting that last 8% of the exposure correction curve is at best frustrating and at worst you end up walking away from those precious highlight details and go for ‘high contrast’ instead.

Example Image Views


Download: Highlight Dither Photoshop Action

Included in the downloadable .zip file is this note as a PDF readable file and the Photoshop action itself that includes the three dithering options for 6%, 12%, and 18% highlight dither. This action has only been tested with the latest CC version of Photoshop as of 4/19/2019 and may work with PS6 and previous versions but has not been tested.

For verification on the dither gradients down to 0% against your specific digital negative workflow and the process specific exposure correction, I've been using this density grid from Freestyle Photographic Supplies. You can run the actions against the Freestyle image (make sure to adjust the image to 16-bit and AdobeRGB) and see the before and after of the highlight dithering and set the Levels on the Dither layer to adjust the dithering transition.
https://www.freestylephoto.biz/alternative-process/making-digital-negatives


Description and purpose

Reliably printing down to densities of 6% is doable, 5% and less is the hard part. It can be done, it can be done reliably, and can be done with some of the most persnickety printing processes, but not all, and not all variations within those processes. To help put an end to the never ending tweaking at the high end of exposure correction curves in Photoshop, I’ve put together an action sequence that will stochastically diffusion dither the highlights using a minimum density of 7%. If you can reliably print down to 7%, then you can now reliably print down to 1%. The trade off is that you will want a relatively high DPI image to work from as the highlights are getting dithered and we want that dithering noise to be below the threshold of visual acuity in the resultant print.

When printing digital negatives on professional grade Canon and Epson printers, a 720dpi image is as high as you might want to go based on these printer’s mechanical resolution, and might be more than the Epson hardware is capable of resolving. An image at 360dpi is just at the edge of being visible when viewed up close, yet is not discernible at a distance of a couple feet.

Three actions are provided, allowing for highlight dithering up through 6%, 12%, and 18% density ranges, giving options based on the specific needs of individual images.

The digital negative workflow for the action is simple: Create your B&W image or plate, apply the process exposure correction curve, then dither and level the highlight ‘Dither’ adjustment layer. Once you are happy with the dither transition, go ahead and finalize the negative creation process. Dithering before applying exposure correction will work and will need more of a level adjustment in the ‘Level Dither’ adjustment layer once you have added the process correction on top of the dithered highlights.

These actions do have working assumption prerequisites based on the imaging workflows used by Special Edition Art Project practitioners, principally Photoshop’s working color spaces must be setup in a specific way.


The Prerequisites

  • Image must be in 16-bit mode
  • Image must be have an Adobe RGB color profile
  • Image must be B&W (R=G=B) since we will be gray dithering the highlights
  • Photoshop’s Color Settings preferences must be set accordingly:
    • Working Spaces:
      • RGB: Pro Photo
      • Gray: Gray Gamma 2.2
    • Conversion Options:
      • Intent: Perceptual or Relative colormetric


Highlight Dithering in Three Easy Steps

Step One: Knockout the highlights (6%, 12%, or 18%) into a separate layer
Step Two: Diffusion dither the highlights (6%, 12%, or 18%) into a separate layer
Step Three: Knockout the old highlights and lay the new dithered highlights into the full image

The final Levels command in the action sequence is critical to getting your image created correctly. A leveling ‘Level Dither’ layer that acts on just the ‘Dither’ highlight adjustment layer is created for final levels adjusting. Highlight leveling is not pre-adjusted – you will need to manually level the dithered highlights to your specific image and digital negative workflow process. To adjust the levels, double click on the histogram icon in the ‘Level Dither’ adjustment layer to reveal the Levels properties adjustment controls. Adjust the center control from its default of 1.0 up / down to get your highlights to better transition into your main image.



Final Notes

When viewing a highlight dithered image, Photoshop and other applications will produce aliasing artifacts in the dithered areas when not viewed at 100%. Here is a comparison example viewed at 100% and 50% from within Photoshop.

Dithered View 50% vs.100%


Within Photoshop, using the wide multi-pixel eye-dropper tool capable of seeing the dithered densities will verify the correctness expected in the dithered highlights. Here are examples of 2% gray and 2% dithered with a 7% minimum density dither.

2% Gray Dither Example


These are the color lookup tables are used to knockout the highlights for 6% (last row of 245-255), 12% (last two rows of 225-255), and 18% (last three rows of 210-255) dithering. There is plenty of room for modification here.

6%-12%-18% Color Table



Action Step Details

Step One:

  1. Convert full image to Gray Gamma 2.2 with Flatten, Black Point Conversion, and Perceptual rendering intent
  2. Convert image Color Depth to 8 Bits/Channel
  3. Convert to Indexed Color
  4. Set Color Table to produce specific highlight ranges only. All values except highlights are set to ‘white’. The following highlight values are kept alive for knockout and dithering: 6% = 240-255, 12% = 225-255, or 18% = 210-255.
  5. Convert image to Gray Gamma 2.2 with Flatten, Black Point Conversion, and Perceptual rendering intent
  6. Convert image Color Depth to 16 Bits/Channel
  7. Convert to Adobe RGB, Black Point Conversion, Perceptual rendering intent, and Dither of the previous color space
  8. Select all (highlight knockout image)
  9. Copy (highlight knockout image)
  10. Backup history to beginning of this action step - beginning of Step One
  11. Create New Group called ‘Dither Highlights’
  12. Select layer ‘Dither Highlights’
  13. Move layer to front, making ‘Dither Highlights’ the topmost group
  14. Create layer ‘Knockout Highlights’
  15. Select layer ‘Knockout Highlights’
  16. Paste (highlight knockout image)
  17. Hide layer ‘Knockout Highlights’


Step Two:

  1. Convert full image to Gray Gamma 2.2 with Flatten, Black Point Conversion, and Perceptual rendering intent
  2. Convert image Color Depth to 8 Bits/Channel
  3. Convert to Indexed Color
  4. Set Color Table to produce specific highlight ranges only. All values except highlights are set to ‘white’. The following highlight values are kept alive for knockout and dithering: 6% = 240-255, 12% = 225-255, or 18% = 210-255.
  5. Image Size, resize image: Width 400%, 360dpi, Constrain Proportions, Interpolation Automatic. Result is a minimum 7% dither density based on an image resize of 4x%. To get a 4% minimum dither density value, resize / downsample at 5x% or use 3x% resize / downsample to get 11% minimum density value. Most processes can reliably produce a 7% visual density image and is where this action set is focused.
  6. Convert image to Gray Gamma 2.2 with Flatten, Black Point Conversion, and Perceptual rendering intent. We have to go through Gray from Indexed to get to Bitmap mode, and is why we need to have available the greatest color gamuts set in Photoshop’s Color Settings preferences
  7. Convert to Bitmap mode, 360dpi, Diffusion Dither
  8. Convert to Gray mode, 4x downsample dither. This creates the dithered highlight image
  9. Convert image Color Depth to 16 Bits/Channel
  10. Convert to Adobe RGB, Black Point Conversion, Perceptual rendering intent, and Dither of the previous color space
  11. Select all (dithered highlight image)
  12. Copy (dithered highlight image)
  13. Reset history to begin of this section - beginning of Step Two
  14. Select layer ‘Dither Highlights’ (the group)
  15. New layer called ‘Dither’
  16. Select ‘Dither’ layer
  17. Paste (dithered highlight image)


Step Three:

  1. Show layer ‘Knockout Highlights’
  2. Select layer ‘Dither’
  3. Set layer blending mode to ‘Darken’
  4. Select layer ‘Knockout Highlights’
  5. Set layer blending mode to ‘Divide’
  6. Select layer ‘Dither’
  7. Levels Adjustment Layer ‘Level Dither’ locked to the ‘Dither’ layer. Highlight leveling is not pre-adjusted. You will need to manually level the dithered highlights to your specific image and digital negative workflow process.
  8. Select group ‘Dither Highlights’

Fin!



About the Author
Eric Anderson

As a youth in the late Nineteen Hundred and Sixties, Eric started with a Kodak 126 Instamatic and moved in the ’70s to an Olympus OM-1 – photography became locked into his psyche. The ’80s & ’90s were a time of change with studies in the arts and humanities, the physiology of color and perception, and the applied sciences of math, physics, and chemistry. In parallel, a printmaking move to Cibachrome and the digital darkroom with pigmented quad-tone and color inks solidified the desire to mix analog and digital printmaking. An engineering career in the nascent late ’80s technology industry became priority over the next two decades; designing, patenting, and creating software & hardware systems for the creatives of the world, photographers and color designers in particular. In winding down his twenty-seven year effort to surprise and delight, it was time to again merge into the world of active creative printmaking – forming the Special Edition Art Project with its’ mission to inspire artists and create community for the exchange of ideas and techniques.




© Special Edition Art Project, LLC 2019