Digital Photos and PCs

What is color depth?

Color depth is the number of colors that are supported by an image format. The color depth is set by the number of bits used to store the color of each pixel.

  • 1-bit file supports two colors (usually black and white)
  • 8-bit file supports 256 colors
  • 24-bit file supports 16 million colors
  • 32-bit file supports 4 billion colors

24-bit, 16 million colors, incidentally, is considered true color as it provides a greater number of colors and shades than the human eye can distinguish.

The higher the color depth that is supported by an image format, the more space the files take up on your hard drive.

With digital photography color depth and resolution are the two important facets of image quality, you would rarely reduce the number of colors in an image if you wanted to retain quality.

Note: In computer terminology 8-bits make up one byte.

How big are digital images?

In the resolution FAQ we provided the example:

Image Size Uncompressed Size
2 mega-pixels 8 Mb
1024 x 768 3 Mb
3000 x 2200
(10″ x 8″ x 300dpi)
26.4 Mb

How do you calculate the size of an image?

To calculate the uncompressed size use the formula:

 [Horizontal Pixels] * [Vertical Pixels] * n = Number of bytes

Where:

n =  4 (for 32-bit color, 4 billion colors)
n =  3 (for 24-bit color, 16 million colors)
n =  2 (for 16-bit color, 65 thousand colors)

How much memory do I need in my PC?

Regardless of whether you use compressed images or not, an image is expanded to its full size in memory.

To work out the minimum memory you need, you must first calculate the size of your largest image using the preceding formula. You need as a minimum enough memory to:

  1. Load an image editing program (32Mb including operating system)
  2. Open an image to manipulate (size of largest image)
  3. Open a second image or create a mask for use in a transformation (size of largest image)
  4. Create an output image (size of largest image)

 32Mb + 3 * [Largest image size] = Minimum Memory

How can I tell if I have too little memory?

Computer systems use a technique called virtual memory to extend the physical memory installed in your PC. Virtual memory create a memory overflow area on disk in a swap file. This technique enables you to run more programs at once on your PC than actually would fit in the installed memory.

It works because not all of these programs want to run at once, and even for active programs they do not access all the memory locations they have allocated to them. If however you run something like a graphics editor where all the images you want to manipulate at once won’t fit into memory then suddenly the PC is having to continuously swap memory back and forward from disk.

In really bad situations the PC will put out a block of memory to disk and then a few milliseconds later have to retrieve this same block again. This condition is called thrashing with the PC doing little else but reading and writing memory to disk.

You know you have too little memory when you get severe performance degradation and your hard disk is being continuously accessed.