Photoshop

Digital Graphics

Bit Depth and Colour Depth

Colour depth describes the maximum number of colours an image can contain. This is dependent on the bit depth of its pixels. The higher the bit depth value, the more colour information each pixel can display and the greater the number of colours the image can contain.

Bit Depth

Bit depth is a value that describes the number of colours that an individual pixel can display. A bit can either be on or off. Therefore, a 1-bit pixel can display two colours: black and white. An 8-bit pixel displays 256 colours. Each bit can be on or off (2 states). When combined in eight different ways (2x2x2x2x2x2x2x2 or 2⁸) it can display up to 256 colours.

Pixels are usually one of five standard bit-depths. Rarely is anything above 8 bits necessary.

Bit Depth	No. Colours	Calculation
1	2	2¹
8	256	2⁸
16	65,536	2¹⁶
24	16,777,216	2²⁴
32	16,777,216 plus 8-bit alpha channel	24 + 8 bits

Colour Depth

Sometimes the terms bit depth and colour depth are used interchangeably. We're going to use the term colour depth to describe the overall ability of an image to display colours. The distinction lies in the fact that some colour modes use more than one colour 'channel'. For example, The RGB colour mode contains 8 bits in each of its three colour channels (red, green, blue) giving an RGB image a colour depth of 24 bits (8 bits per channel x 3 channels). Below are examples of an image displayed in various colour modes to demonstrate how image quality and file size are affected.

Colour Modes and Bit Depth
	Colour Mode: Bitmap Colour Depth: 1-bit (1 channel x 2¹) Colours: 2 File Size: 4 kb (as GIF)
	Colour Mode: Indexed Colour Colour Depth: 4-bit (1 channel x 2⁴) Colours: 16 File Size: 8 kb (as GIF)
	Colour Mode: Greyscale Colour Depth: 8-bit (1 channel x 2⁸) Colours: 256 File Size: 24 kb (as GIF)
	Colour Mode: Indexed Colour Colour Depth: 8-bit (1 channel x 2⁸) Colours: 256 File Size: 16 kb (as GIF)
	Colour Mode: RGB Colour Depth: 24-bit (3 channels x 8 bits per channel) Colours: 16,777,216 File Size: 12 kb (as JPEG)

The increase in file sizes theoretically should be proportional to the number of bits used in an image; however, the difference in GIF and JPEG compression resulted in a smaller-than-expected 24-bit image file size.

Photoshop supports 16 bits per channel. This provides for slightly better colour rendering (if your monitor supports it) but increases the file size dramatically. A 16-bit pixel will display 65,536 shades of colour. For Web graphics, this would be overkill.

Bit Depth and Colour Depth Summary

Bit depth is a value that describes the number of colours that an individual pixel can display.
The higher the bit depth, the more colour information it can display.
Colour depth describes the maximum number of colours an image can display.
An image's colour depth is dependent on the bit depth of the image's pixels and the number of channels it contains.
The file size of an image is proportional to its colour depth.

Coming Soon:

Feb 13: ISSD 24 - XML

Feb 14: Web Design Level 4: XHTML Advanced

Feb 21: ISSD 23 - Web 2.0 Technology

Feb 23: Facebook for Business
Hours: 2
Cost: Free!

Feb 25: Search Engine Optimization (SEO)
Hours: 2
Cost: Free!

Feb 25: Search Engine Marketing (SEM)
Hours: 2
Cost: Free!

Feb 27: ISSD 24 - OLDB (Online Databases with MySQL and PHP)

Mar 05: ISSD 23 - Ajax

Mar 20: Web Design Level 5: DHTML Introduction

Photoshop - TOC - Introduction - Books -
Digital Graphics - Links - Questions -
1 - 2 - 3 - 4 - 5 - [ 6 ] - 7 - 8 - 9 - A -