| <Back |
|
Resolution isn't the whole story when it comes to file size. The other important factor is bit depth, also known as colourspace.
Bit depth sounds complicated, but it isn't really. It's simply to do with how many colours there are in a file.
Each pixel in a file has a certain amount of memory allocated to it, to define what colour it is. That amount of memory is the bit depth (or colourspace). And the more colours you want to allow for, the bigger that colourspace has to be (see diagram).
In a black & white line drawing, each pixel is either black (for lines) or white (for the "paper"). Since the value for black is 0, you only need a space of 1 to allow for the possibility of a pixel being white. This mode is called One-bit, also Line Art, and, confusingly, Bitmap mode too.
Greyscale mode is used for representing black and white photographs. Each pixel has eight bits attached to it, which allows 256 levels of grey in the file.
RGB mode is used by monitors, which mix red, green and blue light to create all the colours you see on screen. The colour information is divided into three channels, one for each colour, so that each can be adjusted separately. Each channel contains eight bits, giving a total bit depth of 24.
CMYK mode (discussed in more detail later) is used for files that are to be printed. It uses four colour channels which match the four printer's inks, and each channel again contains eight bits. Total bit depth: 36.
People forget the effect of bit depth on file size; for example, I once had to receive some black & white line drawings by e-mail. The problem was, the sender couldn't make the files smaller than 12mb each, far too big to e-mail. I checked how they were scanning the artwork, and it turned out they were scanning at 300dpi in RGB mode. I asked them to scan at 600dpi in Line Art mode. They were reluctant because they thought the higher resolution would boost the file size. But they'd forgotten that RGB mode has a bit depth of 24 bits per pixel, whereas Line Art only has one bit per pixel.
The result? The new scans came out over 20 times smaller at 600k each, small enough to e-mail.
| <Back |
|