Mapping Pixel Values to Screen Intensity

Summary:

• To change the Scaling Function (how pixel values are binned into color levels):
  • Click on the Scale Pull-down Menu and select the desired scaling function, or
  • Click on one of the scaling function command buttons (e.g., Linear, Log, Sqrt, or None).

• To change the Color Map (assignment of colors to color levels):
  • Click on the Color Pull-down Menu and select the desired color map, or
  • Click on one of the color map command buttons (e.g., Grey, A, B, I8, or Invert).

• To adjust contrast, click and drag the right mouse button vertically in the display window.

• To adjust bias, click and drag the right mouse button horizontally in the display window.

Contents of This Section

• Binning Pixel Values by Scaling
• Mapping Scaled Values to Colors
• Contrast and Bias

Binning Pixel Values by Scaling

In order to display a large range of pixel values with a limited number of color levels (typically 200 for common 8-bit color monitors), image data must be binned according to some scaling function. A number of different functions are supported:

• Linear Scaling
  The range of pixel values is divided by the number of color levels to determine the range of pixel values for each color level. The fixed range is applied to mapping all pixel values between the minimum and maximum pixel value.

• Log Scaling
  The distribution of pixel values to color levels is based on the distribution of exp(n) from 0 to X, where n is a parameter (the default is 10.0) and X is determined by n and the two ranges (pixel and color level). Positive values of n favor the lower pixel values, while negative values of n favor the higher pixel values.

• Square Root Scaling
  The distribution of pixel values to color levels is based on the distribution of X^{1/n} from 0 to 1, where n is a parameter (the default is 2.0) and X varies from 0 to 1 in steps determined by the number of color levels. Values of n greater than 1 favor the lower pixel values, while values of n less than 1 favor the higher pixel values (negative values and 0 are not allowed).

• Wrapped Linear Scaling
  Wrapped linear scaling divides the range of pixel values into n sub-ranges (the default is 10) and applies linear scaling in each sub-range. The color levels are thus re-used n times.

• No Scaling
  Pixel values are mapped directly into color levels.

Mapping Scaled Values to Colors

Colors are specified by the intensities, i.e. voltages, applied to the red, blue, and green color guns in the monitor. The assignment of actual colors to color levels is accomplished through the use of pseudo-color Color Maps. In such maps, any color can be assigned to any color level, and all image pixels whose values are binned into a particular color level have the color assigned to that level. Color maps are designed to highlight differences among the pixel values. Depending on the levels which best distinguish the detail one wishes to study, the shifts from blue to red and red to yellow can be placed at higher or lower image pixel values and closer together or farther apart. The changes in color can be made gradual or sharp.

SAOtng implements color maps by specifying colors at a few color levels, and linearly interpolating to assign colors at intermediate levels. For example, the basic grey-scale color map specifies no intensity for any color at the lowest color level, full intensity for all colors at the highest color level, with the red, green and blue intensities varying linearly for intermediate levels. SAOtng offers a number of popular pseudo-color maps as well as a basic grey-scale map. The selected color map is displayed graphically in a "one-dimensional" image below the main display window. The x-axis corresponds to color level and the display intensity corresponds to the color assigned to that level.

Contrast and Bias

Once a color map has been selected, it can be further manipulated by the adjustment of contrast and bias.

Contrast refers to the rate of change of color with color level. At low contrast, color changes gradually over many color levels, while at high contrast it can change rapidly within a few levels. Contrast is adjusted by dragging the right mouse button vertically in the display window.

Bias refers to any offset added to the color levels before the color map is applied. Changing the bias corresponds to translating the color map with respect to the color levels without changing the shape of the map. At low bias, low color levels (i.e., low pixel values) will have non-zero intensities, while at high bias only high pixel values will have non-zero intensity. Bias is adjusted by dragging the right mouse button horizontally in the display window.

Last Updated June 14, 1997