Bit Depth, Colors and Digital Photos

Abstract

Objective

The objective is to investigate the relationship between the number of bits in a digital image and the number of colors and file size of the image.

Credits

Sara Agee, Ph.D., Science Buddies

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Introduction

A bit is the smallest unit of measurement regarding computer data. Each bit indicates one of 2 different states, on (represented by a 1) or off (represented by a 0). A single bit does not convey much information, but you can string bits together in binary codes to represent numbers, words, music, pictures, or any other type of computer data. In fact, all computer data is coded in binary numbers, and looks like a series of zeros and 1's:

00101011100100100101110110110101000000110111101001101010

For a digital photo, bit depth is the number of colors that can be shown in the image. Because the bits can only indicate one of 2 possible states (0 or 1), the number of colors can only be powers of 2. Some examples of bit depths (and the calculation of the decimal number for those of you who know exponents) for image files are: 2-bit (2^2 = 4 colors), 4-bit (2^4=16 colors), 8-bit (2^8=256 colors), 16-bit (2^16=65,536 colors), and 24-bit (2^24=16,777,216 colors).

"Every color pixel in a digital image is created through some combination of the three primary colors: red, green, and blue.  Each primary color is often referred to as a "color channel" and can have any range of intensity values specified by its bit depth.  The bit depth for each primary color is termed the "bits per channel."  The "bits per pixel" (bpp) refers to the sum of the bits in all three color channels and represents the total colors available at each pixel. " (McHugh, 2005)

The larger the bpp is, the more colors can be used in the image. The smaller the bpp is, the less colors can be used in the image. Since images with more colors contain more information, they have larger file sizes. In this way, the bpp of an image is related to the file size and the number of colors.

In this experiment you will change the number of colors in a digital photo and measure the effect on file size and bpp and download time. Will the file size and download time always increase if the bpp increases? How will changing the bpp of an image change the number of colors that are possible? Is the relationship linear?

Terms and Concepts

To do this type of experiment you should know what the following terms mean. Have an adult help you search the internet, or take you to your local library to find out more!

• bit
• binary code
• bit depth (bits per channel)
• bits per pixel (bpp)
• file size
• byte

• What is a bit?
• How many bits will make a good quality image?
• How will the number of bits change the number of colors and the file size?

Bibliography

• 2005a. "Color/Bit Depth and Image Resolution," Adobe Web Tech Curriculum, Adobe Systems, Inc. San Jose, CA. [accessed: 4/24/06]
  http://www.adobe.com/education/instruction/webtech/CS2/unit_graphics1/gb_res_bitdepth_id.htm
• 2005b. "Adobe Digital Kids Club." Adobe Systems, Inc. San Jose, CA. [accessed: 4/24/06]
  http://www.adobe.com/education/digkids/main.html
• Sean McHugh, S., 2005. "Tutorials: Bit Depth," Cambridgeincolour.com, Cambridge, UK. [accessed: 4/24/06]
  http://www.cambridgeincolour.com/tutorials/bit-depth.htm
• Wilson, Tracy V., Nice, K., and Gurevich, G. 2005. "How Digital Cameras Work." HowStuffWorks, Inc. Atlanta, GA. [accessed: 4/24/06]
  http://www.howstuffworks.com/digital-camera.htm

Materials and Equipment

• computer
• Adobe Photoshop
• your favorite photo
• color printer
• photo quality printing paper

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Experimental Procedure

1. On your desktop, create a new folder called "My Photo Experiment" and place a copy of your favorite photo in the folder (like puppy.jpg).
2. Open the photo in Adobe Photoshop.
3. Click on "File" and "Save for Web..." from the file menu. A dialog box should appear showing your image in a new window. Choose the "4-Up" tab from the top of the window:

4. Four copies of your photo should appear in the window. One copy will be your original, and there will be three more copies of your photo in GIF file format.
5. Next you will make a series of changes to the settings in each frame. You will change the number of colors in each frame to a different value by clicking on the drop down menu on the right side of the screen:

6. Each time you change the number of colors, you will see the information below the image change. Make sure you can locate the number of colors, file size and download speed:

7. Write all of the information in a data table:

   File Name	Number of Colors	Bits Per Pixel	File Size (Kb)	Download Time (sec)	Speed (Kbps)
   puppy.jpg
   puppy1.gif
   puppy2.gif
   puppy3.gif

8. You will need to calculate how many bits per pixel are in each image by using the number of colors in the image. Use this table to match up the bits per pixel with the number of colors chosen:

   Bits Per Pixel	Number of Colors Available
   1	2
   2	4
   3	8
   4	16
   5	32
   6	64
   7	128
   8	256