To calculate the size of a bit-mapped graphic the following formula would be used:
Height x Width x DPI x DPI x Bit Depth
A 500dpi image that measures 7 inches by 6 inches with the availability of 65,536 colours would result in the following calculation:
- 7 x 6 x 500 x 500 x 16 = 168,000,000 bits
- 168000000/8 = 21,000,000 bytes
- 21000000/1024 = 20507.8125 Kilobytes
- 20507.8125/1024 = 20.03 Megabytes
The DPI figure is used twice because it refers to the width and height of a single square inch.
The bit depth relates to the number of colours available.
The 65,536 colours required need a bit depth of 16 bits.
If the scenario stated 256 colours the bit depth would be 8, had it stated 16,777,216 then the bit depth would have been 24.
- Bit-mapped graphics are stored as a 2D array of pixels
- JPEG, GIF and PNG are common file formats for bit-mapped graphics
- Control over pixel-level editing
- Allow for photo-realism
- Resolution is the term to describe the total number of pixels
- Are resolution dependent (resolution is set at point of creation)
- Bit depth dictates the number of colours that can be used in the image
- Compression is possible using RLE or LZW as well as other techniques
- Bit-mapped images tend to have a larger file size than most vector graphics