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Image Representation

Image Representation. Digital Image Sources. Digital Cameras Scanned Film & Photographs Digitized TV Signals Computer Graphics Radar & Sonar Medical Imaging Devices (X-Ray, CT) The Internet . Images - 2D array of values = pixels. Pixel = “Picture Element”

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Image Representation

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  1. Image Representation

  2. Digital Image Sources • Digital Cameras • Scanned Film & Photographs • Digitized TV Signals • Computer Graphics • Radar & Sonar • Medical Imaging Devices (X-Ray, CT) • The Internet 

  3. Images - 2D array of values = pixels • Pixel = “Picture Element” • Image [x,y] = pixel value (number)

  4. Pixels and Pixel Values • Pixel – an element of the 2-D image array • Pixel Value = brightness • - black = 0 • - gray = 128 • - white = 255 • - many shades over the 0-255 range

  5. Digitizing Images Images are digitized using a two step process: • sampling the continuous tone image • quantizing pixels

  6. Sampling

  7. Quantization pixel’s samples are averaged

  8. Quantization Example

  9. Image Resolution 68 x 104 136 x 208 272 x 416 less detail more detail less storage more storage

  10. Digital Cameras • Low-Res 640x480 (TV grade) • Medium-Res 1536x1024 • Hi-Res 2048x1536 • Very Hi-Res 2560x1920

  11. Dynamic Range • The number of quantized pixel values: 256 levels 16 levels 4 levels 2 levels

  12. Images - 2D array of values • Binary Images (pixel values = 0,1) • Grayscale Images (pixel values = 0-255) • Color Images • Each pixel has three color components • For example, (red, green, blue) or RGB • Each color component is 0-255

  13. Color Images 3 Images Overlayed Blue Green Red

  14. 1 3 21 31 21 4 2 1 2 Histograms: What’s in the image? • What is a histogram? • Simple numeric example • Histogram 1 2 3 4

  15. Color Image Histograms • Histogram for each color

  16. RGB Additive Color Model RED GREEN BLUE bright values => high amounts of that color dark values => low amounts of that color

  17. CMYK Subtractive Color Model Bright => use less of that ink color Dark => use lots of that ink color CYAN MAGENTA YELLOW BLACK

  18. HSB Visual Color Model HSB: how artists perceive color properties

  19. o 0 HSB Visual Color Model HSB: how artists perceive color properties o 360 Hue Select Hue

  20. o 0 HSB Visual Color Model HSB: how artists perceive color properties o 360 Saturation Hue Select Hue - then click in box for saturation, brightness

  21. o 0 HSB Visual Color Model HSB: how artists perceive color properties o 360 Saturation Brightness Hue Select Hue - then click in box for saturation, brightness

  22. Storing Digital Images • Digital images are converted to files for storage and transfer • The file type is a special format for ordering and storing the bytes that make up the image • File types or formats are not necessarily compatible • You must often match the file type with the application

  23. Storing Digital Images • GIF (Graphic Interchange Format) • indexed color (up to 256 colors) • compressed • used in Web applications • JPEG (Joint Photographic Experts Group) • lossy compression with variable controls • also used in Web applications

  24. Storing Digital Images • PNG (Portable Network Graphics) • designed for online viewing (e.g., Web) • patent-free replacement for GIF • lossless compression • BMP • MS Windows image format

  25. How Many Bytes to Store an Image? • Suppose we a have an image that is 500x500 pixels in size • That’s a total of 250,000 pixels • Binary image (1 bit/pixel) = 31,250 bytes • Grayscale image (8 bits/pixel) = 250,000 bytes • Color image (24 bits/pixel) = 750,000 bytes

  26. Indexed Color • “Indexed Color” can be used to reduce the size of a color image file = 27 bytes = 18 bytes 0 1 2

  27. Indexed Color Images • are derived from full color images • are smaller or more compact in storage • are composed of pixels selected from a limited palette of colors or shades

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