04/19/23 Visualization Laboratory, Texas A&M University

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ENDS 375

Foundations of Visualization

9/20/05 Notes

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Image Statistics

Useful input into computational algorithms

–measures of image quality

–basis for automated decisions about images

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Image Statistics

Arithmetic Meanmean = sum(Pxy)/(x*y)

Variance

variance = (sum(Pxy*Pxy)/(x*y)-mean*mean)

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Image Statistics

Standard Deviation

stdev = square root (variance) Histogram

– two axis plot of pixel values vs number of pixels

– basis for deciding - contrast range, overall brightness, thresholding, ...

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Point Operations on Images Numeric Transformation

– (often not reversible) Transfer Functions Often implemented using look-up tables

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Specific Operations

Unity

Invert

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Specific Operations

Contrast Adjustment

Higher

Lower

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Specific Operations

Threshold

Gamma

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Color Modification

LessRed

MoreYellow

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Arithmetic Operations

Two or more images

Cxy = Axy < operation > Bxy

– Addition

– Subtraction

– Averaging, etc ...

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Logical Operations

and, or

nand, nor

xor, xnor

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Image Averaging

Add corresponding pixels from multiple images then divide by the number of images

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Alpha Blending

Cxy = Axy*Mxy

+ Bxy*(max -Mxy )

“Blends” two images

Need a “matte” imageBasis for image compositing

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Compositing

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Readings

Course notes section 1-7 Course notes section 1-8