Tag Archives: CMYK

grab colour – use it

colour pen

Many of you will have seen the Scribble Pen which uses a colour sensor to detect colours. The sensor is embedded at the end of the pen opposite the nib. The pen then mixes the required coloured ink (cyan, magenta, yellow, white and black) for drawing, using small refillable ink cartridges that fit inside its body. The device can hold 100,000 unique colours in its internal memory and can reproduce over 16 million unique colours.

But wait. Don’t think that means you will be able to use the pen to write in 16 million different colours. You won’t. A typical phone screen can display about 16 million unique combinations of RGB (red, green and blue). But many of the RGB combinations are indistinguishable. Open up powerpoint and make two squares. Set the RGB values of one to [10 220 10] and of the other to [10 220 11]. I would be amazed if you could really tell the difference between them. And anyone who has read much of my blog will know that I believe that if two colours look the same then they are the same. So the pen might be able to create 16 million combinations of cyan, magenta, yellow, white, and black – but that doesn’t mean 16 million different colours.

The second problem is that just because your pen can grab a colour (using its sensor) doesn’t mean it can create it. There are lots of colours out there in the world that are outside the colour gamut of an ink-based system (even one using five primaries – cyan, magenta, yellow, white and black).

Read more: http://www.dailymail.co.uk/sciencetech/article-2647129/Forget-crayons-Multicolour-pen-lets-pick-colour-draw-16-million-shades.html#ixzz35gJ0racJ
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colour management for beginners

Colour displays are now affordable and enjoyed by consumers at work, at home, on mobile displays and in cinemas. Consumers often take it for granted that there is good colour fidelity as images are transferred between different devices. So, for example, a red object in an image appears to be approximately the same red when the image is displayed on different computer displays, when it is printed, and when it is viewed on a mobile phone.

This colour fidelity is not easy to achieve. Different devices use very different technology to display colour images. For example, a computer display will mix together light from three primaries (red, green and blue) to generate a range (gamut) of colours. On the other hand, a printer uses completely different technology and typically uses mixtures of cyan, magenta, yellow and black inks to create the gamut of colours. Even computer monitors use a variety of different technologies (from CRT displays, which are becoming obsolete, to LCD, LED, and plasma technologies) each of which may use quite different red, green and blue primaries. Colour management is required to compensate for differences between the technologies (colour primaries, colour mixing, colour gamuts) between different image-display devices. This necessitates that the companies that produce image-display devices must cooperate so that the devices are able to talk to each other; this is achieved through the International Color Consortium (ICC) . The ICC is an industry consortium that was established in 1993 by eight industry vendors (including Microsoft and Apple). Today approximately 70 companies are members of the ICC whose goals are to “create, promote and encourage evolution of an open, vendor-neutral, cross-platform colour management system architecture and components”. The ICC system is implemented in terms of device profiles and colour management system. The device profile is a computer file that is associated with each device (printer, camera, monitor, etc.) that essentially contains information to allow colour to be managed. In the case of a computer monitor, for example, the device profile would include information about the monitor’s primaries that would allow the colour image to be adjusted to compensate for the properties of the monitor so that the colours are displayed correctly. The colour management system is software that manages how these device profiles interact with each other and is normally part of the operating system of the computer.

Thus, when users capture, view, or print images they are using colour management all the time even though they may be unaware of it. Though this level of colour management is built into software and device drivers and is broadly invisible to the user it does enable colour consistency for images when they are captured, viewed and printed throughout the world. However, this level of default colour management is far from perfect. It does not, for example, generally account for changes in settings for a device (for example, a user may change the contrast, brightness, or colour temperature of a display) so that colour fidelity is, in practice, only approximate. This level of colour fidelity is probably sufficient to satisfy about 90% or more of consumers for whom colour is not a critical issue. However, for professionals working in industries where colour is a major concern (e.g. design, retailing) a higher level of colour management is often required. For these users, it is possible to obtain systems (typically low-cost colour-measurement devices and associated software) that allow a user-defined profile to be generated for a particular device with particular settings. This user-defined profile then over-rides the default profile and should enable a better level of colour fidelity to be achieved. Nevertheless, colour fidelity is always likely to be an imperfect issue. It is difficult for colour-management systems to perfectly compensate for the fact that, for example, different devices may generate quite different colour gamuts (typically, the bright red on a computer screen cannot be achieved by a CMYK consumer-level printer).

For ICC see www.color.org

RYB primaries

There are two phrases I keep seeing written down all over the internet that cause my blood pressure to increase.

The first is that the colour primaries are red, yellow and blue (RYB). And the second is that the primaries are colours that cannot be made by mixing other colours. Neither of these statements are true, of course.

The first statement makes no distinction between additive colour mixing (of lights) and subtractive colour mixing (of paints and inks) but subtractive colour mixing is normally implied. However, RYB is a relatively poor choice for three colour primaries. The range of colours that can be produced is actually quite small. For most painters and artists it doesn’t matter because very few work in just three primaries – if they did so they would probably be frustrated by the small gamut of colours achievable. Many artists (painters) will use 10 or more basic colours to mix their palette. However, there is a group of people who care passionately about the gamut of colours that can be obtained by mixing three colour primaries – that is the people who work for companies such as HP and Canon. These companies make CMYK printers for the consumer market and their jobs depend upon consumers liking their printers. They understand that the largest gamut (in subtractive mixing) can be obtained if the primaries are cyan, magenta and yellow (CMY). The teaching of RYB as the (subtractive) primaries should be stopped. It’s already gone on for far too long.

One reason I don’t like the teaching of RYB as being the subtractive primaries, in addition to the fact that it is wrong, is that it confuses people who are trying to learn colour theory. This is because red, yellow and blue seem to be quite pure colours and this encourages people to hold the second belief I don’t like which is that the primaries are pure colours that cannot be mixed from other colours. If people understood that the primaries were CMY it would be less tempting to hold this belief about the purity of the primaries. Of course, if you make a palette of colours of three primaries then it is true that no mixture of two or more colours from that palette can match any of the primaries. However, there are other colours (that are outside the gamut of the primary system) that could be mixed together to match the primaries. This false notion of purity confuses the real issue – that is, that the subtractive primaries are cyan, magenta and yellow because the additive primaries are red, green and blue. Look at this picture below:

The additive primaries are red, green and blue and the secondaries are cyan, magenta and yellow. Correspondingly, the subtractive primaries are cyan, magenta and yellow and the subtractive secondaries are red, green and blue. Simple.

I wrote about this before so for a slightly different perspective see my earlier post.

Perhaps I am so agitated about it today because I am just watching England getting trounced by Ireland at rugby when the Grand Slam was so tantalisingly close. Or maybe I will feel just the same tomorrow.

subtractive mixing – why not RGB?

In a previous post I spoke about the difference between additive and subtractive mixing and why the additive primaries are red, green and blue or RGB for short – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/

The chromaticity diagram – see http://colourware.wordpress.com/2009/09/28/colourchat-audiovisual-guide-to-the-chromaticity-diagram/ – has a very useful property. If you plot the chromaticities of two lights, then the straight line that joins the two points on the chromaticity diagram show you the additive mixtures that can be obtained by mixing together the two lights. If we take three lights, then the additive mixtures that can be obtained are defined by the triangle that is formed if the chromaticities are the vertices of the triangle. Ok – that’s a bit of a mouthful so let’s have a practical example. The triangle in the diagram below shows the gamut that can be achieved when we have three additive primaries that are positioned at the corners of the triangle.


 From this diagram it should become obvious why the additive primaries are RGB. Say, we chose, two reds and a cyan as the three additive primaries – well, the triangle would be tiny. In other words, the gamut would not be very big. The biggest triangle in the chromaticity is one whose vertices are formed by a red, a green and a blue. WhichRGB will give the biggest triangle? I don’t know – it’s been something that has been puzzling me for the last few days and I’ll come back to this in a later post. But certainly any RGB triangle is pretty large as long as the red, green and blue primaries chosen are reasonably saturated.

So what happens if we choose RGB as the subtractive primaries? Subtractive colour mixing describes how inks and paints mix together to form colours. The first thing to point out is that subtractive colour mixing is not additive and linear – you remember I said that when you mix two lights together the colour mixtures all fall on the straight line that joins the  two points in the chromaticity diagram that represent the two lights? Well, this is only true for additive colour mixing. So to work out the gamut for subtractive systems is not an easy thing to do. However, if you do select the three subtractive primaries as RGB you’ll get a gamut that looks something like this:


Notice that the gamut is concave. Mixing red and green lights produces a nice yellow. You can test this by going into your colour-picker in software such as Photoshop or Powerpoint and setting the RGB values to be 255:255:0. You’ll get a nice yellow. But mixing red and green paints – it will give you a similar hue to yellow but you’ll get something quite desaurated; most likely you’ll get a brown. So using RGB as the subtractive primaries would not be a very good thing at all.

It turns out that additive and subtractive colour mixing are very related. The best subtractive primaries are the ones that control the amount of red, green and blue light reflected. A yellow dye applied to textiles, for example, mainly absorbs short wavelengths in the blue section of the spectrum, allowing the other wavelengths to be reflected by the textile. The “other wavelengths” that are reflected give yellow. But the important point is that the yellow dye absorbs blue. Similarly, a magenta dye absorbs green and a cyan dye absorbs red. This leads to the idea of the optimal subtractive primaries being those that are cyan, magenta and yellow or CMY. This leads to a gamut somewhat like this:


The biggest gamut for subtractive mixing is obtained by using CMY as the primaries. But weren’t you taught at school that the subtractive primaries are red, blue and yellow? Almost certainly you were – and this is because it is accepted dogma at most art colleges and in many art and design textbooks. But it is quite easy to show that the optimal primaries – those giving the largest gamut – are CMY not RBY. If you were building a colour-reproduction system using only three colours such as a printer you would come to the conclusion – as companies such as HP, Xerox, and Epson have done – that you get the largest colour range with CMY. So why has it become commonplace for artists to refer to red, yellow and blue as the primaries? Could it be a colour naming and language issue – that they really mean cyan when they say blue and it’s just a naming error. Possible, but not likely in my opinion.  I think it is more likely that most artists are not overly concerned that RYB gives a smaller gamut than CMY because they rarely restrict themselves to three primaries. An artist would typically use 6 or more primaries. For example, they might use two blues (one that is reddish and one that is greenish), two reds (one that is yellowish and one that is bluish) and two yellows (one that is greenish and one that is reddish) in order to easily be able to mix a wide range of colours. The (mis-)identification of RYB as the subtractive primaries has much to do with colour wheels. I like to keep each of these blog posts reasonably concise – if I start writing about the problems of colour wheels now I will be writing for another 2 hours. And it’s nearly midnight now so colour wheels will need to wait for another day!