Lesson 02b: Key Points

The Nature of Channels

• There are two fundamental building blocks of color:
  • luminosity values
  • color channels
• Black-and-white photos are in grayscale, in which each pixel is recorded as a number.
  • This number is called a luminosity value or level.
  • A value of 0 means the pixel is black.
  • A value of 256 means the pixel is white.
• In the RGB color model, an image is divided into red, green and blue channels.
  • If red, green and blue light are mixed together at full brightness, they mix to form white light.
  • If each of these colors is divided into a channel, each can each have its own grayscale image.
  • Each of these grayscales can have a strength of color from 0 to 256.
  • A value of 0 means a pixel has none of that channel's color.
  • A value of 256 means a pixel has full brightness of that channel's color.
  • Multiplying 256 luminosity values for red by 256 for green by 256 for blue equals 16,800,000 possible colors (256³).
• When looking at an individual channel (such as red), you can tell where in an image that channel is contributing a lot of its color. Look at its brightest spots.
• Where a channel has dark spots, that color is not contributing much to the composite image.
• When the red, green and blue channels are mixed together, you get the full-color image.

 

How the Auto Commands Work

Photoshop offers three commands that correct the brightness and contrast of an image automatically. These commands don't always do a great job of it - far from it - but they do the job without any guidance from you. They are available from the Image > Adjustments menu.

Here's a comparison of the main differences between Auto Levels, Auto Contrast and Auto Color.

Command	How It Affects Luminosity	How It Affects Color
Original Photo
Auto Contrast His skin's a bit too red.	Makes the darkest color as dark as it can be and the lightest color as light as it can be without changing the color of either.	The color balance remains unchanged.
Auto Levels Now his skin's a bit too green.	Analyzes each color channel independently; makes the darkest color in each channel black and the lightest color white.	Makes no attempt to preserve colors. Often results in a color shift, replacing one color cast for another.
Auto Color Now his skin is peachy and the fabrics are green.	Like Auto Levels, deepens shadows and lightens highlights channel-by-channel, but tries to change them to neutral grays.	Color casts are usually corrected.

 

The Brightness/Contrast Command

The Brightness/Contrast command was one of the worst functions in earlier versions of Photoshop. In Photoshop CS3 it is vastly improved. It lets you correct the luminance of an image by using a Brightness slider and a Contrast slider. It is available from the Image > Adjustments menu.

 

Source: "Adobe Photoshop CS3 One-On-One" by Deke McClelland