selmslie Loc: Fernandina Beach, FL, USA

RGB (red, green, blue) is used for digital capture and display and CMYK (cyan, magenta, yellow and black) is used for printing.

RGB is additive. An 8-bit JPEG has a range of 0-255 for each color. Black is when all three values are 0, white when they are all 255 and middle gray is about half way in between. As brightness is increased the image becomes lighter.

CMYK is subtractive. If you put no ink on the paper you end up with paper white. A little cyan, magenta and yellow in the right proportions can produce a neutral light gray. But it’s not practical to reach a really dark black by simply adding more color ink. That’s where K comes in. It supplements the darkness of the other three colors.

You can print a non-photographic image with only the four basic CMYK colors. But for a quality image you need more colors.

A good photo printer might use three* neutral intensities – black, light black and very light black. Black may be different for matte and non-matte paper. It can also use a regular and light cyan and magenta. Only a single yellow is needed because it’s already light. You can end up with eight or more different inks.

Red (complementary to cyan) is made up primarily of magenta and yellow. Green (complementary to magenta) is made up primarily of cyan and yellow. Blue (complementary to yellow) is made up primarily of cyan and magenta.

There is room for more ink colors. Adding red, green and blue inks can reduce the use of the colors from which they are made. Purple or orange can make it easier to faithfully reproduce a slightly wider gamut of colors and reduce the need for complex mixtures of the other colors.

* Dedicated B&W printers can use from six to eleven shades of gray for fine art prints. For more information Google 'Piezography'.

selmslie Loc: Fernandina Beach, FL, USA

Or click on Piezography.

Toment Loc: FL, IL

Interesting
Thanks

BebuLamar

Ink Jet printers need more colors because they can't print CYMK with varying intensity. So they have to use a combination of many dots to render a pixel. Having more colors reduce the number of dots needed and thus can product higher PPI with the same DPI.
Dye Sub generally do not need more than 4 colors. Printers that print on RA-4 paper with laser beam or light pipe only have 3 channels.

selmslie Loc: Fernandina Beach, FL, USA

With a digital image, DPI comes from PPI. DPI is usually an order of magnitude higher than PPI for quality photography.

For example, 300ppi can be used to print at 1440dpi and up depending on the printer.

Dye sublimation printers at 300 or 600 dpi do not have the potential to make good small prints to be viewed close up. But they are OK for large prints at much lower PPI when viewed from a much greater distance.

RA4 paper is a color negative paper. It doesn't have a black or white emulsion.

Strodav Loc: Houston, Tx

You can't talk about CMYK without talking about screening. Unlike mixing R,G, and B light to make colors, if you mix a bit of C, M, and Y ink you get get an ucky brown color. The CMYK inks are not perfectly transparent, many are made with pigments, and can only be applied in a very thin layers of varying size, so they need to be laid down next to each other, not on top of each other. Traditional screening is a process of converting color intensity (say, 0-255 for 8 bits / color) to dot sizes proportional to intensity and then placing the dots near each other. Even then you cannot make black with CMY ink like you can make white with RGB light, hence the need for K, or black ink.

Fun post, thanks.

selmslie Loc: Fernandina Beach, FL, USA

Exactly. That's where 1440 or 5760 dpi comes in. We can't get there with watercolors.

profbowman Loc: Harrisonburg, VA, USA

"selmslie," can you please explain what you mean by the example. How are you measuring ppi that is different than dpi? My photos at 6000 x 4000 pixels have no size until I decide to print them. So, give me any pixel dimensions and I can print them at 1440 dpi. But if I do that with my example of 6000 x 4000 pixels, the picture on paper will be a 4.2 x 2.8 inch picture. Not sure why I would want to do that. --Richard

selmslie Loc: Fernandina Beach, FL, USA

Pixels have no size until you print them. The the dots created by your printer are the same size regardless of the number of pixels

If you print an image that is 6000 pixels wide to make a print 5, 10 or 20 inches wide you end up with 1200, 600 and 300 pixels per inch respectively.

But each image at 1440 dpi will and up with 7200, 14400 and 28800 dots on the paper.

Strodav Loc: Houston, Tx

A dot, as opposed to a pixel, has only two states, on or off, so only two intensities. What do you do if you have a pixel with, say, 256 levels of intensity (8 bits) and you want to print it on a printer that only prints dots? You group dots together side by side. I could use a matrix of 16 x 16 dots (16^2 = 256) starting with no dots (paper white) adding one dot at a time until they are all on giving you, say for B&W, a black pixel. The size of the pixel would be the printer's dot size times 16 in one direction, and the printer's dot size times 16 in the other direction.

Printer manufacturer's use much more sophisticated patterns of dots to make pixels, but the concepts is the same. The smaller the dot size, the more pixels you can make per inch.

selmslie Loc: Fernandina Beach, FL, USA

Each dot can be Cyan, Magenta, Yellow or blacK or simply not there.

That gives you a full range of colors from black to white with all of the gray and RGB color values in between.

The ratio of dots to pixels is variable, not fixed.

A printed pixel can be represented by any number of dots depending on the print size.

anotherview Loc: California

Thanks for rundown on printer inks and their use.