Page name: The non repro blue trick [Logged in view] [RSS]
2008-07-02 21:55:45
Last author: Yncke
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The non repro blue trick

Or 'How to avoid erasing sketch lines'

A little history

This is by no means a new trick. In the good old days, when 'photoshop' was a kind of store where you went with your pictures, a specific shade of blue was used for sketching and similar. The more advanced equipment ignored this colour when photographing the image to sent it to the printing department.
Nowadays, with graphic software, the same trick can be used easily without advanced equipment or even having to use the genuine non repro blue pencils.

Needed equipment

- A scanner
- A working computer with a program that can split channels to RGB
- A colour printer

The principle

Colours on a computer screen consist of a certain combination of red light, green light and blue light. Black is just the absence of light, white is the full power of the combination of red light, green light and blue light. When you split in RGB channels, the blue channel gives white for the areas that need a lot of blue, and black where you need none, hence blending blue lines into the white background, and keeping your back lines intact, since they also consist of red and green.

The trick

Assume you have a pose you now want to get dressed. The pencil's deep in the paper, which has had a hard time from the eraser. You pity it. You scan it.


Then, you make these lines blue. There are several ways to do this, for example the "Colour to target" option for the "Retouch" tool in "Paint Shop Pro".


Depending on your printer and personal taste, you can make these lines lighter before printing. It's on this print that you now can easily clothe the pose without having to work on the battered paper of your initial drawing, or mess around with light boxes and such.

When you've finished drawing, you scan it.


Just brighten the whole drawing up a little.


Use the "Colours" > "Split Channels" > "Split to RGB" option in "Paint Shop Pro" or similar in your favourite graphic program, and throw away the red and green version. You'll notice that, if you've brightened the scan enough, that the blue lines have vanished.


The dark lines look a bit peeky though, so pump them up a bit (for example with "Curves") and you have the clothed sketch. Now you can reuse this trick to ink this, if you wish.



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2007-08-31 [Zab]: Finally I found this! I've read it before but since I didn't have any printer, I just wrote it on a note in the back of my I have a printer and I'm gonna try it :P

2007-09-01 [Yncke]: :) I'm looking forward to your experiences with it.

2007-09-01 [Zab]: ^___^ It worked well:P I used it for the fox picture in my house.^^

2007-09-02 [Yncke]: I'm glad to read that. :) It's a lovely picture!

2007-09-02 [Zab]: Thanks.^__^
Hm..this should work with red too? *thinks of when the blue ink is used up on the printer*

2007-09-02 [Yncke]: Of course. And green too. The principle's the same. (At the moment, I'm doing red, because I want to use up the printer inks at the same rate. :) )

2007-09-02 [Zab]: *lmao* I guessed so:P

2007-09-02 [Yncke]: Yeah. It would be a very expensive trick, otherwise. :)

2007-09-02 [Zab]: well..not for me, I buy the colors separate anyway, so it wouldn't matter if it's the blue or red that gets empty first..O_o

2007-09-02 [Yncke]: You've got a good printer for it, then. :) I've been wondering if I could still find a black only printer and just put a red cartridge in....

2007-09-02 [Zab]: You think there are red ink thingies that would fit a black only printer?
The one I bought, a HP, wasn't very expencive tho..and it's really good (990 SEK should be about $150-200 I guess without checking that..x) And it sucks left over ink back into the inkthingies to let it be used again, instead of letting it fall down on some filter and go to waste like most printers..after what the guy in the store said..XD

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