Continuing from Biomedical Image Processing – I
Once an image is stored in digital format, it can be described by a number of different parameters. Some of the relevant parameters are briefly discussed here. The traditional convention for an image coordinate system is depicted in Figure 4.
Figure 4. General convention for image coordinate systems.
While biomedical images are generally viewed in 2D, it is sometimes helpful to view gray-scale (monochrome) images in perspective, with the third axis being brightness. This is illustrated with “pseudoimage” data in Figure 5.
Figure 5. Pseudoimage data viewed in 2D and perspective modes. Data values (either a “0” or a “2”) are placed within a 31-row by 24-column matrix.
Important image parameters
Images can be described by a large number of different parameters. Some of these are listed here.
- Pixels (a.k.a. picture elements, pels, image elements) – Individual rectangular
elements that comprise an image. The term voxel describes a pixel’s 3D analog.
- Gray levels – An 8-bit, gray-scale image with 1024 × 1024 pixels requires a
megabyte of storage.
- Color depth – Usually reported as powers of 2, this can range from 2 colors up to 32-
bit color (a.k.a. True Color). Colors are often defined using RGB (red-green-blue) or
HSB (hue-saturation-brightness) combinations (see Figure 6).
- Aspect ratio – The scaling ratio between the x and y axes.
- Contrast – The relationship between the brightest and dimmest pixel in the image.
- Histogram – A binned representation of the gray levels, colors, or brightness levels
in the image.
- Because of the optimization properties of 2D Fourier transforms, it is often
advantageous to select image sizes whose number of rows and columns are both
powers of 2 (e.g., 512 x 512 array with 128 gray levels – comparable to a
monochrome TV image)
- In motion videos, images are displayed at a rate of 30 frames per second
Primary colors: red, green, blue
Secondary colors: yellow = red + green, cyan = green +
blue, magenta = blue + red.
White = red + green + blue
Black = no light.
HUE: actual color
Measured in angular degrees around the cone starting and ending at red = 0 or 360 (so yellow = 60, green = 120, etc.).
SATURATION: purity of the color
Measured in percent from the center of the cone (0) to the surface (100). At 0% saturation, hue is meaningless
BRIGHTNESS: measured in percent from black (0) to white (100). At 0% brightness, both hue and saturation are meaningless.
Figure 6. RGB and HSB color descriptions.
The next Biomedical Image Processing lesson will discuss image analysis, classification, and component labeling. Happy Holidays!