Image Processing. Processing Digital Images digital images are often processed using “digital...
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Transcript of Image Processing. Processing Digital Images digital images are often processed using “digital...
Image ProcessingImage 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
MotionMotionFilterFilter
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
Out
put I
mag
e S
cale
Input Image ScaleO
utpu
t Im
age
Sca
le
Contrast
Knob
Brightnesss
Knob
Contrast and Brightness functions
• like the knobs on your TV set
Input Image Scale
Out
put I
mag
e S
cale
Increase Increase
Slope Contrast
Input Image ScaleO
utpu
t Im
age
Sca
le
Contrast
Knob
Brightnesss
Knob
Contrast and Brightness functions
• like the knobs on your TV set
Input Image Scale
Out
put I
mag
e S
cale
Increase Increase
Slope Contrast
Input Image ScaleO
utpu
t Im
age
Sca
le
Increase 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
Input Output0 01 12 23 34 45 56 6. .. .
255 255
What would this function do?Brightness
Input Output0 01 02 13 14 35 36 4. .. .
255 255
Increasing Brightness
What would this function do?Brightness
Input Output0 81 92 103 114 125 136 14. .. .
255 255
Contrast & Brightness Demo
• Photoshop– Image->Adjust->Brightness/Contrast– Image->Adjust->Curves (more advanced)
Global Filters
• Brightness and Contrast controlHistogram 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 thresholdingHistogram stretching or equalization
• Color corrections
• Inversions
Histogram Stretching
The Algorithmfor 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 equalizationColor 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 correctionsInversions
Inversions
Inversion Demo
• Photoshop– Image->Adjust->Invert
Review
• Global Filters- Contrast & Brightness Control
- Thresholding- Histogram stretching & equalization
- Color corrections- Inversion