Digital Image = two-dimensional array of pixels of varying color and intensity.
The image may not be stored using this structure, but we use it as a conceptual structure of an image.
Image is based on pixels
Graphic types (to make a distinction between image and graphic type) may be produced by some computational process (say drawing a line or mathematical curve on screen).
A series of slides introducing image representation http://www.engr.csufresno.edu/~twilson/csci191t/lect8/index.html
(how different colors are represented and/or produced)
the color models may be based on the graphical devise - how the color is actually produced on the screen or on human color perception.
Some color models
CIE color space (Commission Internationale de l'Eclairage) - a reference model for other color systems.
RGB (Red Green Blue) - Color is represented as a triple specifying R,G amd B intensities. Can directly be mapped to voltages of the CRT. The color is produced on a CRT as a cobination of the three color sources.
HSB (Hue, Saturation, Brightness) Hue is the angle in the color circle (from red to blue and green), saturation is the intensity of the color and brightness the amount of gray in the color. (RGB-HSB-applet)
CMYK (Cyan, Magneta, Yellow, blacK) Complement colors of R G B.
YUV: Television color codes, represented by Y = luminance (black-grey-white) UV = color difference.
Represent the areas or transparency or mask.
How many digits represent the specific color model (RGB = 3) (CMYK = 4).
The number of bits to represent the different values for the specific
channel
| Depth | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Colors | 2 | 4 | 8 | 16 | 32 | 64 | 128 | 256 |
How different channels are stored in the image data .
The image can use a standard color table (CLUT) of have one as part of the image data.
The ratio of pixel width to heigth. Can be used to correct the appearance of the image in different equipment (screen vs printer).
Reduce the space needed for an image.
Lossless compression - the original pixel data can be reconstructed Lossy compression - the original pixel data can not be reconstructed
Lossless (GIF (Ziv-Lempel-Walsh) , TIFF)
Lossy
Select - cut - copy - paste
Tresholding Color correction: Contrast - brightness - gamma
Smoothing the image - blur - sharpen. Operation may calculate the new value for the image by taking into account the values of the pixel and the neighbouring pixels.
Mixing images (foreground - background),
Displacement, rotation, mirroring, scaling, skewing, warping.
Between different formats. Most programs have a set of conversion tools.
mj@uwasa.fi 24.2.1997