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greg |
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.\" RCSid "$Id: macbethcal.1,v 1.2 2003/12/09 15:59:06 greg Exp $" |
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greg |
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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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greg |
1.3 |
icalc(1), pcomb(1), pcond(1), pfilt(1), ximage(1) |