Image Processing

Processing Digital Images digital images are often processed using “digital filters” digital filters are based on mathematical functions that operate on the pixels of the image MotionFilter

2 Classes of Digital Filters Global filters transform each pixel uniformly according to the function regardless of its location in the image Local filters transform a pixel depending upon its relation to surrounding ones

Global Filters  Brightness and Contrast control Histogram thresholding Histogram stretching or equalization Color corrections Inversions

Contrast and Brightness functions like the knobs on your TV set Input Image Scale Output Image Scale Input Image Scale Output Image Scale Contrast Knob Brightnesss Knob

Contrast and Brightness functions like the knobs on your TV set Input Image Scale Output Image Scale Increase Slope Contrast Input Image Scale Output Image Scale Contrast Knob Brightnesss Knob

Contrast and Brightness functions like the knobs on your TV set Input Image Scale Output Image Scale Increase Slope Contrast Input Image Scale Output Image Scale Increase Offset Brightness Contrast Knob Brightnesss Knob

Increasing Contrast

How Contrast Functions Work Index into the Input Array of brightness (index = brightness) Get the corresponding brightness value from the Output Array What does this function do? Brightness

What would this function do? Brightness

Increasing Brightness

What would this function do? Brightness

Contrast & Brightness Demo Photoshop –Image->Adjust->Brightness/Contrast –Image->Adjust->Curves (more advanced)

Global Filters Brightness and Contrast control  Histogram thresholding Histogram stretching or equalization Color corrections Inversions

Histogram Thresholding thresholding creates a binary image by converting pixels according to a threshold value

Thresholding Demo Photoshop –Image->Adjust->Threshold

Global Filters Brightness and Contrast control Histogram thresholding  Histogram stretching or equalization Color corrections Inversions

Histogram Stretching

The Algorithm for Histogram Stretching Find darkest pixel = D Find lightest pixel = L Let max possible pixel value = M Then the new value for every pixel in the image is: New pixel value = x M (Old pixel value – D) (L – D)

Histogram Equalization Can you read this?

Histogram Equalization Now can you read this?

Histogram Equalization Demo Photoshop –Image->Adjust->Equalize –Image->Adjust->Levels (more advanced)

Global Filters Brightness and Contrast control Histogram thresholding Histogram stretching or equalization  Color corrections Inversions

Color Corrections Making changes to the colors in an image - Hue shifting - Pseudo coloring Performed by manipulation of re-mapping functions

Hue-shifting Hue-shifting is used to modify the color makeup of an image

Hue-shifting Demo Photoshop –Image->Adjust->Hue/Saturation

Pseudo-coloring Pseudo-coloring can be used to add color to grayscale images Different colors are assigned to each level of gray

Pseudo-coloring Demo Photoshop –Image->Adjust->Gradient Map

Global Filters Brightness and Contrast control Histogram thresholding Histogram stretching or equalization Color corrections  Inversions

Inversions

Inversion Demo Photoshop –Image->Adjust->Invert

Review Global Filters - Contrast & Brightness Control - Thresholding - Histogram stretching & equalization - Color corrections - Inversion