Definition: Colour Depth

Colour Depth

Other terms which mean the same as colour depth include: “bit depth”, “pixel depth”, or “color depth” (different spelling).

What is it?

Colour depth describes the levels of colour found in a graphics display, or more specifically, each pixel. The basic colour range used for graphic displays is the RGB system – or Red, Green, Blue system. Together, these three colours can make all the colours necessary for screen displays. Colour depth can involve huge numbers of colours. Today, our computer systems can display billions of colours on-screen.

The relationship between ‘bits’ and the number of colours

Colour depth reflects the number of binary digits or ‘bits’ used to represent a colour in a computer system. We talk about colour depth as having a ‘bit-colour’ depth. The following information shows how the different colour depths match to the different colour graphics systems…

• 1-bit colour (2¹ = 2 colours): monochrome, few tones, often black and white.
• 2-bit colour (2² = 4 colours): CGA, gray-scale – early computers. Some colour displays for machine readouts.
• 3-bit colour (2³ = 8 colours): many early home computers with TV displays (ZX Spectrum and BBC Micro)
• 4-bit colour (2⁴ = 16 colours): Early colour displays (Enhanced Graphics Adapter – EGA) and by the VGA standard graphics systems.
• 7-bit colour (2⁷ = 128 colours): Pre-VGA systems (like EGA).
• 8-bit colour (2⁸ = 256 colours): early colour Unix workstations, VGA at low resolution, Super VGA and colour displays like TV and specialised monitors.
• 16-bit colour (2¹⁶ = 65,536 colours): XGA, (during the 1990’s)
• 24-bit colour “True Colour” (2²⁴ = a true red/green/blue colour scheme). Used in modern display systems like LCD screens, camera screens and plasma screens.
• 48-bit colour or “Deep Colour” (2⁴⁸ = 281,474,976,710,656 colours). Deep colour supports 30/36/48/64-bit for three RGB colours. Video cards with 10 bits per one colour (30-bit colour RGB). It is an established standard (2020’s). However, such colour depth is likely to require a high-end graphics card. This would tend to make the computer much more expensive.

Todays standard consumer display

Since 2018, nearly all standard graphics cards and monitors for most consumer computers, mobile phones use ‘True colour’ (24 bit colour) giving Red (256) x Green (256) x Blue (256) variations of colour… a total of 16,777,216 colours. For most purposes this is sufficient. The human eye can only distinguish about 10 million colours.

Colour depth and ‘bits per pixel’

The term, colour depth, relates to the number of bits per pixel. The more bits per pixel, the higher the colour variety. Generally, the quality of the monitor and the graphics card which drives the display also needs to be of higher quality to display ‘deep colour’ (48 bit colour depth). Consequently, few people will spend the extra to get a greater colour depth. Especially since they cannot see the difference. High-end designers, artists, gamers and possibly other specialist users may consider it an important as part of thier equipment.

Colour depth perception

As the the number of colours possible on a display exceeds the number we can see, the difference is not easily percieved by the human eye. However, some people report that deep colour (2⁴⁸ colours) is more restful to view. Others have suggested that greater colour depth enhances artistic creativity. These are opinions, and colour perception itself is not a science. We all see colours in slightly different ways. So, the debate on the perception of colour depth is likely to continue.

Colour and ‘gamut’

Color depth is only one aspect of color representation. The other aspect is the range of colours that can be expressed. The range of colours that may be displayed is the “gamut”. The definition of colour precision and gamut is described by a colour encoding specification. This assigns a digital code-value to a location in a “color space “. Consequently, different display technologies, printers and even papers, may have specialised colour space coding schemes. Thus, the user of technologies involving colour should be aware of both the colour depth and colour space of images and display equipment.

The RGB colour space is well known and used extensively around the world. It occurs in cameras, mobiles, computers, TV’s, image editiing and internet images and so on. However, different colour spaces crop up in printing (CMYK), various image editing apps (e.g. Adobe RGB) and in various custom formats (for example in photographic paper usage). The full description of the colour depth includes the not only the hues range, RGB itself, but also the other derivative colours which create the full gamut. So, the way a particular display or paper is used may demand its own colour space, and therefore a variant spcification for the colour depth coding.

Implications

Colour depth describes the extent to which display systems can express colours. However, that takes in a wide range of impacts on the viewer and encompasses many different technologies. While colour representations are good on modern screens, and in modern images, the understanding of colour perception is still undeveloped. Colour spaces help define how colour encoding can express certain colour ranges for particular uses on our computers.