Adding upstream version 4.22.0.upstream/4.22.0

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.\" This man page was generated by the Netpbm tool 'makeman' from HTML source.
+.\" Do not hand-hack it!  If you have bug fixes or improvements, please find
+.\" the corresponding HTML page on the Netpbm website, generate a patch
+.\" against that, and send it to the Netpbm maintainer.
+.TH "Image Processing By Interp and Extrapolation" 5 "" "netpbm documentation"
+
+Created: 17 April 2003
+.SH NAME
+extendedopacity - theory of netpbm interpolation and extrapolation
+.SH DESCRIPTION
+.PP
+This page is a copy of http://www.sgi.com/misc/grafica/interp/ on
+April 17, 2003, with some slight formatting changes, included in the
+Netpbm documentation for convenience.  Since at least June 11, 2005,
+the source page has been missing.
+
+.SH Image Processing By Interpolation and Extrapolation
+\fIPaul Haeberli and Douglas Voorhies\fP
+
+.SS Introduction
+.PP
+Interpolation and extrapolation between two images offers a general,
+unifying approach to many common point and area image
+processing operations.  Brightness, contrast, saturation, tint, and
+sharpness can all be controlled with one formula, separately or
+simultaneously.  In several cases, there are also performance benefits.
+.PP
+Linear interpolation is often used to blend two images.
+Blend fractions (alpha) and (1 - alpha) are used in a weighted average
+of each component of each pixel:
+
+.nf
+      out = (1 - alpha)*in0 + alpha*in1
+
+.fi
+.PP
+Typically alpha is a number in the range 0.0 to 1.0.  This is
+commonly used to linearly interpolate two images.
+What is less often considered is that alpha may range beyond the
+interval 0.0 to 1.0.
+Values above one subtract a portion of in0 while scaling in1.  Values
+below 0.0 have the opposite effect.
+.PP
+Extrapolation is particularly useful if a degenerate version of the
+image is used as the image to get "away from."  Extrapolating away from
+a black-and-white image increases saturation.  Extrapolating away from a
+blurred image increases sharpness.  The interpolation/extrapolation
+formula offers one-parameter control, making display of a series of
+images, each differing in brightness, contrast, sharpness, color, or
+saturation, particularly easy to compute, and inviting hardware acceleration.
+.PP
+In the following examples, a single alpha value is used per image.
+However other processing is possible, for example where alpha is a function
+of X and Y, or where a brush footprint controls alpha near the cursor.
+
+.SS Changing Brightness
+.PP
+To control image brightness, we use pure black as the degenerate (zero
+alpha) image.  Interpolation darkens the image, and extrapolation
+brightens it.  In both cases, brighter pixels are affected more.
+
+.B brightness
+.IMG -C blend1.gif
+
+.SS Changing Contrast
+.PP
+Contrast can be controlled using a constant gray image with the average image
+luminance.  Interpolation reduces contrast and extrapolation boosts it.
+Negative alpha generates inverted images with varying contrast.  In
+all cases, the average image luminance is constant.
+
+.B contrast
+.IMG -C blend3.gif
+.PP
+If middle gray or the average pixel color is used instead, contrast is
+again altered, but with middle gray or the average color left unaffected.
+Shades and colors far away from the chosen value are most affected.
+
+.SS Changing Saturation
+.PP
+To alter saturation, pixel components must move towards or away from the
+pixel's luminance value. By using a black-and-white image as the
+degenerate version, saturation can be decreased using interpolation, and
+increased using extrapolation.  This avoids computationally more
+expensive conversions to and from HSV space.  Repeated update in
+an interactive application is especially fast, since the luminance
+of each pixel need not be recomputed.  Negative alpha preserves luminance
+but inverts the hue of the input image.
+
+.B saturation
+.IMG -C blend4.gif
+
+.SS Sharpening an Image
+.PP
+Any convolution, such as sharpening or blurring, can be adjusted by
+this approach.
+If a blurred image is used as the degenerate image,
+interpolation attenuates high frequencies to varying degrees, and
+extrapolation boosts them, sharpening the image by unsharp masking.
+Varying alpha acts as a kernel scale factor, so a series of
+convolutions differing only in scale can be done easily, independent of
+the size of the kernel.  Since blurring, unlike sharpening, is often a
+separable operation, sharpening by extrapolation may be far more
+efficient for large kernels.
+
+.B sharpening
+.IMG -C blend6.gif
+.PP
+Note that global contrast control, local contrast control, and
+sharpening form a continuum.
+Global contrast pushes pixel components
+towards or away from the average image luminance.  Local contrast is
+similar, but uses local area luminance.  Unsharp masking is the extreme
+case, using only the color of nearby pixels.
+
+.SS Combined Processing
+.PP
+An unusual property of this interpolation/extrapolation approach is that
+all of these image parameters may be altered simultaneously.  Here
+sharpness, tint, and saturation are all altered.
+
+.B combined
+.IMG -C blend7.gif
+
+.SS Conclusion
+.PP
+Image applications frequently need to produce multiple degrees of
+manipulation interactively.
+Image applications frequently need to interactively manipulate
+an image by continuously changing a single parameter.
+The best hardware mechanisms employ a
+single "inner loop" to achieve a wide variety of effects.  Interpolation
+and extrapolation of images can be a unifying approach, providing a single
+function that can do many common image processing operations.
+.PP
+Since a degenerate image is sometimes easier to calculate, extrapolation
+may offer a more efficient method to achieve effects such as sharpening
+or saturation.  Blending is a linear operation, and so it must be
+performed in linear, not gamma-warped space.  Component range must also be
+monitored, since clamping, especially of the degenerate image, causes
+inaccuracy.
+.PP
+These image manipulation techniques can be used in paint programs to
+easily implement brushes that saturate, sharpen, lighten, darken,
+or modify contrast and color.  The only major change needed is to work with
+alpha values outside the range 0.0 to 1.0.
+.PP
+It is surprising and unfortunate how many graphics software packages
+needlessly limit interpolant values to the range 0.0 to 1.0.  Application
+developers should allow users to extrapolate parameters when practical.
+.SS References
+.PP
+For a slightly extended version of this article, see:
+P. Haeberli and D. Voorhies. \fIImage Processing by Linear 
+Interpolation and Extrapolation\fP. 
+IRIS Universe Magazine No. 28, Silicon Graphics, Aug, 1994.
+.PP
+.B 
+.IMG -C gobot.gif
+.SH DOCUMENT SOURCE
+This manual page was generated by the Netpbm tool 'makeman' from HTML
+source.  The master documentation is at
+.IP
+.B http://netpbm.sourceforge.net/doc/extendedopacity.html
+.PP
\ No newline at end of file