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.\" RCSid "$Id"
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.TH MACBETHCAL 1 1/16/97 RADIANCE
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.SH NAME
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macbethcal - compute color compensation based on measured Macbeth chart
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.SH SYNOPSIS
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.B macbethcal
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[
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.B "\-d debug.pic"
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][
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.B "\-p xul yul xur yur xll yll xlr ylr"
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]
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.B scannedin.pic
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[
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.B calibout.cal
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]
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.br
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.B macbethcal
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.B \-c
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[
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.B "\-d debug.pic"
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]
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[
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.B measured.xyY
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[
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.B calibout.cal
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]
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]
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.SH DESCRIPTION
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.I Macbethcal
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takes a scanned image or measurement set
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of a Macbeth ColorChecker\u\s-3TM\s+3\d color
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rendition chart and computes a color mapping
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function suitable as input to
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.I pcomb(1).
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.PP
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In the first form,
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.I macbethcal
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takes a scanned image of a Macbeth chart that has been converted
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into a Radiance picture using a fixed procedure.
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When used properly as input to
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.I pcomb,
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the computed calibration file will adjust the
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brightness and color of any similarly scanned and converted image
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so as to best match the original.
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If the lighting conditions are carefully controlled (as in the case
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of a flatbed scanner), it is even possible to get reliable
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reflectance values this way, at least within 10% or so.
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The input picture is named on the command line.
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The output calibration file will be written to the standard output
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if no file name is given on the command line.
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.PP
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In the second form, the input is from a file containing measured
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values for each Macbeth color.
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This file must contain entries of the form:
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.sp
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.nf
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N x y Y
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.fi
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.sp
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Where
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.I N
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is the number of the corresponding Macbeth color.
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(See back of ColorChecker chart
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for color names and indexing, but it basically starts from the upper
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left with 1 and proceeds in English text order to the lower right,
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which is 24.)
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The values
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.I x,
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.I y
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and
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.I Y
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are the 1931 CIE (x,y) chromaticity coordinates followed by the
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luminance for that color, which can be in any units.
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If a white value is known (i.e. maximum output level), then it may
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be given as entry number 0.
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The entries may be in any order, and comments may be included
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delimited by a pound sign ('#') and continuing to the end of line.
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It is recommended that measurements be done for all 24 colors,
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but the only required entries are the 6 neutral values on the bottom
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row of the chart.
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.PP
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Computing a mapping from measured colors is usually more convenient
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when calibrating a particular output device.
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This is accomplished by printing the picture
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.I macbeth_spec.pic
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(which may be found in the standard RADIANCE library directory in
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the lib subdirectory) and measuring the output with a chroma meter
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or spectrophotometer.
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.PP
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For a scanned image, the locations of the 24 Macbeth patches
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in the input picture must be known.
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If the chart borders are not at the edges of the input picture, or
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the chart has been reversed or rotated or is uncentered or at an
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oblique angle, then it is necessary to specify the pixel locations
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of the corners of the chart with the
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.I \-p
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option.
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The corner postions (x,y pixel addresses as given by the
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.I ximage(1)
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"p" command)
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are ordered on the command line:
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upper-left, upper-right, lower-left, lower-right
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(i.e. English text ordering).
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These coordinates should be the outside corner positions of
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the following patches:
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.sp
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.nf
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upper-left = 1. dark skin
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upper-right = 6. bluish green
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lower-left = 19. white
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lower-right = 24. black
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.fi
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.sp
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If the chart has been flipped or rotated, simply give the pixel
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positions of the appropriate patch corners, wherever
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they are in the image.
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(Note: if the Radiance picture has been flipped or rotated with
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.I pflip(1)
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or
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.I protate(1),
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.I ximage
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will report the original pixel positions if the
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.I \-c
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option was not used by the reorienting program(s).
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This will be wrong, so be sure to use the
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.I \-c
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option.)
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.I Macbethcal
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can handle a chart with any orientation or perspective warping if
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the corner coordinates are given correctly.
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The debug picture output is the best way to check for consistency.
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(See the
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.I \-d
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option, below.)
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.PP
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The
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.I \-d
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option may be used to specify an additional output file, which will
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be a picture comparing the scanned image processed according to the
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computed mapping against the standard Macbeth colors.
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It is a good idea to use the debug option to check that the color
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patches are being located correctly, and to see how well
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.I macbethcal
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does at matching colors.
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The center of each patch will show the target color;
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the left side of each patch will show the original
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color, and the right side will show the corrected value.
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If the match works well, the debug picture should have a sort of
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"notch on the left" look in each patch.
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Macbeth colors that could not be matched because they were out of
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gamut on this device are indicated with diagonal lines drawn
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through the associated target colors.
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.SH METHOD
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.I Macbethcal
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computes the color mapping in two stages.
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The first stage uses the six neutral color patches at the bottom of
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the Macbeth chart to compute a piecewise linear approximation to the
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brightness mapping of each RGB primary.
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The second stage looks at all the colors that are within the
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device's gamut to compute a
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least-sqaures fit for a linear color transformation from the
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measured space into the standard Radiance RGB space (as defined by
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the three primaries in src/common/color.h).
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.PP
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Thanks to the nature of inverse mappings, this method should work
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either for converting scanned data to match the original, or for
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preconditioning pictures to be sent to specific output devices.
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In other words, the same calibration file works either for
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correcting scanned images OR precorrecting images before printing.
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.PP
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A warning is printed if some unsaturated colors are determined to
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be out of gamut, as this may indicate a poor rendition or improper
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picture alignment.
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The debug picture will show which colors were excluded by drawing
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diagonal lines through their entries.
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.SH NOTE
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It is very important that the same settings be applied when
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scanning or printing other images to be calibrated with the computed file.
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In particular, all exposure adjustments should be fixed manually,
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and no tweaking of the settings should be done along the way.
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The final result will be best if the original scanned image is not
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too far off from what it should be.
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In the case of slide and negative scanners, it is best to apply the
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recommended calibration file for the type of film used, so long as
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this calibration is fixed and not adjusted on a per-image basis.
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.SH CHART AVAILABILITY
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The Macbeth chart is available at most photographic supply stores,
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or may be ordered directly from Macbeth:
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.sp
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.nf
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Macbeth
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Munsell Color
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405 Little Britain Rd.
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New Windsor, NY 12553-6148
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tel. 1-800-622-2384 (USA)
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fax. 1-914-561-0267
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.fi
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.sp
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The chart sells for under $50 US at the time of this writing.
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.SH EXAMPLES
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To compute a calibration for a FunkyThing scanner and check the
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results:
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.IP "" .2i
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ra_tiff -r mbscan.tif mbscan.pic
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.br
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macbethcal -d debug.pic mbscan.pic FunkyThing.cal
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.br
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ximage debug.pic
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.PP
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To apply this computed calibration to another scanned image:
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.IP "" .2i
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ra_tiff -r another.tif | pcomb -f FunkyThing.cal - > another_calib.pic
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.PP
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To compute a calibration file for the BigWhiz film recorder, after
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taking measurements of a slide made from macbeth_spec.pic:
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.IP "" .2i
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macbethcal -c macbeth_spec.xyY BigWhiz.cal
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.PP
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To prepare a picture prior to output on the same film recorder:
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.IP "" .2i
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pcomb -f BigWhiz.cal standard.pic > toprint.pic
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.PP
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To use
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.I pcond(1)
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to also adjust the image for human response:
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.IP "" .2i
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pcond -f BigWhiz.cal -h standard.pic > toprint.pic
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.SH AUTHOR
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Greg Ward
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.br
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Paul Heckbert supplied code for perspective projective mapping
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.SH "SEE ALSO"
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calc(1), pcomb(1), pcond(1), pfilt(1), ximage(1)
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