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Learn about global and local filters in image processing, including blurring, sharpening, edge detection, and noise filters. Understand the algorithms and techniques used in Photoshop for various image enhancements.
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Lecture # 19 Image Processing II
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: REVIEW • Brightness and Contrast control • Histogram thresholding • Histogram stretching or equalization • Color corrections • Inversions
Local Filters • Blurring • Sharpening • Unsharp Masking • Edge and line detection • Noise filters
Blurring Algorithm For (every pixel in column x) { For (every pixel in row y of column x) { average image[x,y] with it’s neighbors; } }
1 1 1 1 1 1 1 1 1 Blurring • Average each pixel with its neighbors by: multiplying each pixel and its neighbors by 1: = - add multiplied pixels and divide by 9 - store result in a new array • The new array contains a blurred image.
Gaussian Blur Multiply neighbors by less than middle
Blur Demo • Photoshop • Filter->Blur->Blur • Filter->Blur->Gaussian Blur
Local Filters • Blurring • Sharpening • Unsharp Masking • Edge and line detection • Noise filters
Sharpening • Subtract neighboring pixels by: multiplying each pixel and its neighbors by: = - add multiplied pixels - store result in a new array • The new array contains a sharpened image. 0 -1 0 -1 +5 -1 -1 0 0
Unsharp Masking • Unsharp masking also sharpens an image • The Algorithm: 1. call the original (unblurred) image A 2. blur the image (call it B) 3. subtract B from the unblurred image (let C = A-B) 4. Multiply C by some number, k (k > 1) 5. Sharpened Image = A + kC
Sharpen/Unsharp mask • Photoshop • Filter->Sharpen->Sharpen • Filter->Sharpen->Unsharp mask
Local Filters • Blurring • Sharpening • Unsharp Masking • Edge and line detection • Noise filters
+1 -1 0 -1 +1 0 -1 +1 0 Edge and Line Detection • Multiply each pixel and its neighbors by the following corresponding patterns (numbers) - add results - clip to 255 - result = “Edge” image -1 -1 -1 0 0 0 +1 +1 +1 Gets Gets Vertical Edges Horizontal Edges
Edge Detection • edge detection is often used to recognized objects of interest in the image
Edge Detection • Photoshop • Filter->Stylize->Find Edges
Local Filters • Blurring • Sharpening • Unsharp Masking • Edge and line detection • Noise filters
Noise Filters • A median filter takes away “salt & pepper” noise (speckles)
Noise Filter • Photoshop • Filter->Noise->Median Filter
0 -1 0 -1 +5 -1 1 1 1 1 1 1 1 +1 -1 0 -1 0 0 1 -1 +1 0 1 -1 +1 0 Kernel Filtering Edges Blur Sharpen -1 -1 -1 Apply same algorithm at every pixel using different kernel values 0 0 0 +1 +1 +1
Image Editing • Selection Tools • Painting Tools • Cut & Paste • Cloning • Layers and Blending
Selection Tools Tool Bar • Lasso Tool - manually draw outline (free hand) - manually outline (line segments) - magnetic lasso (semi-automated) • Magic Wand DEMOS
Image Editing • Selection Tools • Painting Tools • Cut & Paste • Cloning • Layers and Blending
Painting Tools • Airbrush Tool • Paint Bucket Tool • Paintbrush Tool • Pencil Tool DEMOS
Image Editing • Selection Tools • Painting Tools • Cut & Paste • Cloning • Layers and Blending
Cut & Paste • Word Processors - cut & paste strings of characters (1D arrays) • Image Editing - cut & paste pixels (2D arrays) - replace old pixels with new pixels
Image Editing • Selection Tools • Painting Tools • Cut & Paste • Cloning • Layers and Blending
Cloning • Copy pixels from one part of an image - to another part of an image ... Interactively DEMO
Image Editing • Selection Tools • Painting Tools • Cut & Paste • Cloning • Layers and Blending
Layers and Blending Can create arbitrary number of layers for - animation - special effects in movies - morphing Layer n Layer 2 Layer 1
Blending • The idea: Blended image = .3 x + .7 x is a weighted combination (sum) of two or more other images.
Example Blend .3 x +.7 x = Bearastronaut
Masking • The idea: Create another image where the value of pixels is the weighting term for a blend operation:
Summary • Digital Images and pixels • Digitize a picture by • Sampling • Quantization • Color Models • RGB, CMYK, and HSB • Storage Formats • .gif, .jpg, .png, .bmp
Summary • Global Filters • Contrast & Brightness Control • Thresholding • Histogram stretching & equalization • Color corrections • Inversion
Summary • Local filters • Blurring • Sharpening • Unsharp Masking • Edge and line detection • Noise filters
Summary • Image Editing • Selection Tools • Painting Tools • Cut & Paste • Cloning • Layers and Blending