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index separation

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What is Index Color Separation?
This is a process of choosing a few colors to represent millions of colors that built up an image/design. It breaks down millions of colors into a few colors that BEST represent an image when they are printed back together on a t-shirt. Choosing the best set of colors is extremely crucial in order to produce an excellent print. The more colors allowed the better an image looks. 12 index colors image looks twice as smoother than a 6 index color image. Index color separation converts a design/image into a Diffusion Dither random pixel pattern that almost totally keep you away from the moiré pattern problem (waveform pattern cause by interference between half-tone dot angle and screen mesh).

Why use Index Color separation?

Index Color dot pattern : Diffusion dither
CMYK dot pattern : Halftone

Halftone dots form an image by varying the size of the dots. Different sizes produce different shading, many different shadings form an image/design.
Diffusion dither random dots are all same size, it form an image by varying the distance between the dots. Different distance produce different shading, many different shadings form an image/design.

Below are the comparison between Index Color and traditional CMYK halftone to see why Index Color stands out as a better solution for t-shirt printers.

1. moiré
> Different angle of Cyan 15°, Yellow 75°, Magenta 45°, Black 75° easily lead screen printers into the moiré trouble.
> The random dot pattern in Index Color automatically prevent screen printers from trapping in moiré trouble.

2. color shift
> In CMYK process printing, CMYK inks overlapping each other to reproduce millions of other colors. Any uneven controls of squeeze or flood bar pressure, ink opacity or viscosity, bad color separation job, loose screen tension, will easily cause color shift.
> In Index Color printing, every single pixel is printed precisely side by side. Pixels are lining up with 4 edges touching each other, no overlapping, no spacing. If overlapping occur, this means the registration is off. Therefore, no such case as color shift.

3. over saturate
> Printed dots always smear 30% to 50% larger than the dots on film. This circumstances called "Dot Gain". CMYK inks are transparent, more smear means more overlapping thereby over saturate the printed image.
> In Index Color printing. Although Dot Gain does occur, there is no room/space between pixels minimize the smear, additionally inks use are opaque. Therefore over saturation never occur.

4. color fade out
> CMYK printed image tend to fade out quickly after a few washing.
> Index color print won't fade.
[ it usually crack. Cracks are due to uncompleted fusion (heating process transforms liquid plastisol into solid elastic plastic). Plastisol fused at 320°F or 160°C, duration 1.5 minute.]


Dark side of Index Color
There is always a 'minimum number of color requirement' for each design to look correct when printing in Index Color. The biggest problem to perform a good index color printing is lack of printing head, not enough color. 8 printable colors (white base and black included) machine will be good enough, 10 colors will be excellent! 14 colors will be award winning. I'd say 6 printable colors is the minimum (of course depends on design).


When use/don't use Index Color?
Index Color works

EXCELLENT on : Art painting, wood/tree texture, small but clear wording in a design, detail illustrated surf wear t-shirt art, wildlife art with detail fur... almost everything, but...

POOR on : smooth gradient, shadows, transparency effect, critical photo realistic images. (Not to say that Index Color won't works, only that you need more colors to perform well compare to using the traditional halftone dot pattern).

What happen if you're going to index a detail illustrated smoking gorilla with small but clear wording attach on the cigar and an obvious shadow fade out behind?
>gorilla with fur need index color
>small clear wording need index color
>fading shadow need halftone

NO PROBLEM! You can make both the index color Diffusion Dither dot pattern and the halftone dot pattern exist simultaneously in one color/film. How? Please click here to find out.


Index Color Separation step by step guidance :
Adobe Photoshop is the only software you need. Basically the process in Photoshop 3.0, 3.0.5, 4.0, 5.0, 5.5 is quite similar. I'll guide you along with a demo [motorcycle image], its just as simple as 123.


Step 1 :
Open a design you wish to index.
Action : File> Open > (browse into the directory where the file locate) select the file, click Open.

Make sure the design is in RGB mode.
(Photoshop 5.0)Action : image > mode > RGB color
(Photoshop 3.0)Action : mode > RGB color

Set the image resolution (pixel/inch) depends on how you want the result on t-shirt looks after printed.
Action : image > image size, set to the actual print size on t-shirt, set the pixel/inch value as below guide line ~

80 - 100 pixel/inch
rough, stipple look, dots clearly seen after printed on t-shirt.
135 - 155 pixel/inch
standard resolution for most designs
175 - 195 pixel/inch
very fine tone, no pixel is visible after printed on t-shirt


Step 2 :
Choosing the best set of color to represent the entire design is the most crucial factor in color separation.

Analyze the design, look for primary colors in the designs (Red, Yellow, Blue are good example). These are colors that CANNOT be reproduces by mixing any other colors together. Leave secondary colors behind, these are colors that can be reproduced by mixture of primary colors.

motorcycle image :

motorcycle RGB


These are the Primary Colors for this design, Not any of these color can be reproduced by mixing any 2 among themselves

Pick the secondary colors only if you still have extra places after choosing all the primary colors.

Now you have the primary color concept clear. Okay, here is a more precise one : "Always pick the MIDTONE primary colors". Never pick dark primary colors. Dark primary colors are consider secondary colors because they can be reproduced by mixing themselves with black or darker primary colors.

midtone BLUE
black/darker purple
dark BLUE
(real primary)
(unreal primary)

Keep in mind that any color which can be reproduced by mixture of any 2 other colors is NOT a REAL primary color.

You are now well equip to pick the best set of color for any design. Of course with a few trial and true instead of error.

(Photoshop 5.0)Action : image > mode > Indexed Color...
- uncheck "Preview"
- check "Preserve Exact Colors" (if the design have a lot of solid colors)
- Amount : 100% (best quality setting)
- Dither : Diffusion
- click the triangle at "Palette :" from the pull down menu choose "Custom..."

Photoshop index color table


(Photoshop 3.0)Action : mode > Indexed Color...
- choose "Custom", check "Diffusion Dither"


Step 3 :
Now you are entering the most important part of the entire Index Color Separation process : Choosing the BEST set of colors to represent the design/image.

Color Table pop-up, make sure all the small color boxes are in complete white. If not, select all the boxes (click the most upper left box & drag to the most lower right box), Color Picker pop-up, pick the complete white color (R=255, G=255, B=255), click OK, click OK again.

color table_blank

Click the first box, base on what you have learn, pick your first index color from the design with the Eye Dropper tool, you can move around searching for the best choice, after confirm, click on the spot, click OK.

Continue building the color palette until the last color which is usually the black.

color table_ color set pick

Click OK at the Color Table, click OK again at the Indexed Color pop-up to see the result. (You only click OK once at the Color Table for Photoshop 3.0)

Zoom to 100%(Photoshop 5.0) or 1:1 (Photoshop 3.0) or bigger to study the color picked. The image/design looks cracked below this size.

looks cracked motorcycle
image looks cracked below size

8 color index saparation
8 color indexed at 80 dpi, size 100% or 1:1

- The more colors allowed the better an image looks
- The higher the dpi, the smoother an image looks

Move around, analyze the image part by part, keep UNDO (command-Z/Ctrl-Z for PC) and REDO (command-Z/Ctrl-Z for PC) between RGB and Indexed Color mode to study the changes and you will see whether you chose the correct colors and which color you should rechoose or which color you should drop (if you chosen the wrong one). This is the most direct and effective way to improve your separation.

Nobody will satisfy with the first try.

Tips : Don't ever think that the Index Color separation process looks too complicated and making decision to purchase a color separation software to solve this. This is a wrong perception. Separation software is a system. You need a good color concept to backup a system. You still have to decide which color to pick in the end, automated color picking still not that intelligent... yet, to pick the correct set of color.

Once you are satisfied with your final set of colors, save the color palette (CLUT-Color Look Up Table). Image > Mode > Color Table..., click "Save" at the Color Table, enter a name for the palette, click "Save".

At the Color Table, click OK. Save the indexed mode file with a new file name in Bitmap format. File > Save as... > enter new "Filename:" > select BMP format> click OK

BMP Options menu pop-up, choose the file format and click OK. Now you have an Index separated design saved.


Step 4 :
Save the separation into individual color file ready for film output.

Open the Index separated (bitmap) file, goto Image > Mode > Color Table, the color table pop-up...

Assume that you are now going to convert BLUE color into an individual file for film output.

Leave the BLUE color box, select & drag all the other colors, set them to complete white (R=255, G=255, B=255), click OK, click OK again.

color table left blue

Click the BLUE color box, set the color to complete Black (R=0, G=0, B=0), click OK.

blue convert to black

Click OK again at the Color Table window.

bitmap Blue

Goto Image > Mode > Gray scale > Discard color information? OK
Goto Image > Mode > Bitmap > don't change the resolution, leave it as the value set during STEP 1, check Diffusion Dither, OK.

Save the file as bitmap and name it BLUE.

Continue this "convert to Black" process for each individual color until you have all the colors saved individually in Bitmap. (This process consume less than 5 minutes)


Step 5 :
Output the Bitmap files
Diffusion dither pattern files need NOT specify Frequency or Angle during output. Simply print out directly from Photoshop or pass to your Service Bureau. (normally they will output from QuarkExpress)


Related topic :
Choosing the correct color, how to…?

Choosing the best set of color to represent an image is extremely crucial to obtain good print result. Have the primary & secondary color concept clear and index color separation will be as easy as 123. Light blue, midtone blue and dark blue, which blue should I choose?...

Article by : John Lean

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