This lesson covers an explanation of RGB and Colour Depth.
RGB – RedGreenBlue – Additive Colour Mode
An old CRT (Cathode Ray Tube) showing how the image is derived from 3 electron guns.
The Red/Green/Blue process is known as an additive system – the more colour you add, the closer you get to white. This is the basis of the Cathode Ray Tube guns in older monitors and TVs, the LEDs and LCD displays in newer monitors and the CCDs in digital cameras and scanners. A wide range of tones can be displayed using RGB, although some very dark colours can be difficult to reproduce. If you go back to your science classes at school, you will surely recall the experiment with 3 torches or 3 projectors:
Shining Red Green and Blue lights over each other gives a white(ish) light
When the Red light is projected over the Green light, the result is a Yellow light. Add some blue light and the result is a dirty white.
The RGB values of any colour are expressed as 3 integers ranging from 0 to 255. One number for Red, One for Green and one for Blue. There are 256 integer between 0 and 256, so there are 256 different amounts (or intensities) of colour. The larger the number, the more intense colour is present.
So, a value of 255 for Red, a value of 255 for Green and a value of 255 for Blue would mix together with each colour adding more light and it would wind up being white.
And a value of 0 for Red, a value of 0 for Green and a value of 0 for Blue would simply be no green light, no red light and no blue light – i.e., a net result of black.
It gets more interesting when the values are between 0 and 255. If the red, the blue and the green have equal numbers, then we wind up with a shade of grey. (The bigger the number triplet, the lighter the grey)
Some examples:
RGB values
This system gives a total of 16.7 million different colours (256 x 256 x 256) and is called 24-bit colour or “True Colour” (each set of 256 possibilities requires 8 bits). This is known as the colour depth.
Scanners and Digital Cameras often use a larger bit depth which allow the hardware to “Choose” the “best” 16.7 million tones according to the configuration preferences that you might have set.
There are also extra bits that can be used for special features like transparency (another 8 bits); or High Dynamic Range Imaging (HDRI) that uses 30 bits, 36 bits and 48 bits giving up to 281 Trillion different colour possibilities (but we’ll look at these colour depths in a more advanced course).
What about Less than 24-bit Colour Depth?
Some images don’t have a 24-bit colour depth. These kind of images are used on the Web because a lower bit depth = less bits = smaller images. It is actually amazing what you can get away with:
Here are 3 images.
Look closely at them:
This image is a 24-bit true colour image (usually a JPG or a TIFF or some other image format).
This image is an 8-bit colour depth image (usually a GIF or a PNG file). It is an image which only contains 256 possible colours.
This image is an 8-bit dithered image (usually a GIF or a PNG file). The dithered image is nearly as good as the 24-bit image, but with a much smaller file size. This is achieved by simulating missing colours using complex patterns of dots.
We will learn how to create these images a little later on. Today it’s just theory.
Next Lesson (also theory): CMYK
