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I need halftone dot pattern exist together with diffusion dither pattern, how to do that?
There will be circumstances you encounter an image or a design you need both diffusion dither dot pattern & halftone dot pattern together on a single piece of film. For example : You encounter an oil painting design with wood texture and a smooth shadow under. You need diffusion dither dot pattern for the wood texture and halftone dot pattern for the shadow, (halftone can present a shadow well with only 1 gray color but diffusion dither dot pattern needs 2 grays). Is that possible? Yes, it can be done.

STEP 1 : Set the image resolution
It depends on how you want the result on t-shirt looks after printed.

STEP 2 : Separate wood and shadow to different layers
The wood and shadow have to be in separate layers. It will be great if you have the wood & shadow already in different layers.

If they are merged/flattened, you have to separate them accurately using path tool. Path along the border between the shadow and the wood. After finish path~ing, the path coil the shadow. Save the path and name it "border". Save the file.

STEP 3 : Prepare a selection channel for delete "diffusion dither shadow" use
Still on the wood & shadow file. Load the "border" path as selections, save the selections as channel, name it "delete shadow". We have to sharpen the selection. Click on "delete shadow", an eye appeared beside means this channel is active and now you can work on it.
Goto Image > Adjust > Level... , darken the shadow to around 179 and lighten the highlight to 181.(see the Input Levels : below this image))

This level process is to prepare a sharp & clear selection. We are going to use this "delete shadow" channel to delete the diffusion dither shadow and replace with halftone dot shadow later.

Save the file.

STEP 4 : Index the image
Choose the best set of color and start Index separation... Save the indexed name as "wood_shadow_index"

STEP 5 : Delete the diffusion dither pattern shadow
From the file wood & shadow, drag "delete shadow" channel and drop into "wood_shadow_index". Load channel as selection ( hold down Ctrl and click the channel), press delete. You have just deleted diffusion dither pattern shadow.

Convert "wood_shadow_index" into individual color bitmap files.

STEP 6 : Replace with halftone shadow
Decide what color you want the shadow to be. Normally is Gray. Let's use Light Brown in this example. Open the Light Brown bitmap file, convert to Grayscale mode,
Image > Mode > Grayscale, Ratio 1:1 ? Say OK.

Open the original wood & shadow (RGB) with layers. Load the "delete shadow" channel as selections at the Shadow Layer. Drag and drop the selected shadow to the Light Brown file which is now in Grayscale mode.( for the selection to locate precisely during drag and drop, hold down Ctrl+Shift during drag and drop ).

Save the file.

You are now having Diffusion Dither and Halftone pattern within one file.

The files can be printed directly from Photoshop or send to service bureau. Check the Output Films, what you see on monitor screen of the color separated files will be what you get on the films. It is very easy to check wether they are output correctly. Always make sure the sqaure pattern dots are all in perfect square (very important), and the halftone dots at its frequency (line/inch) you used to.

Article by : John Lean
john@teedesign.com

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