man/man1/macbethcal.1

.\" RCSid "$Id"
.TH MACBETHCAL 1 1/16/97 RADIANCE
.SH NAME
macbethcal - compute color compensation based on measured Macbeth chart
.SH SYNOPSIS
.B macbethcal
[
.B "\-d debug.pic"
][
.B "\-p xul yul xur yur xll yll xlr ylr"
]
.B scannedin.pic
[
.B calibout.cal
]
.br
.B macbethcal
.B \-c
[
.B "\-d debug.pic"
]
[
.B measured.xyY
[
.B calibout.cal
]
]
.SH DESCRIPTION
.I Macbethcal
takes a scanned image or measurement set
of a Macbeth ColorChecker\u\s-3TM\s+3\d color
rendition chart and computes a color mapping
function suitable as input to
.I pcomb(1).
.PP
In the first form,
.I macbethcal
takes a scanned image of a Macbeth chart that has been converted
into a Radiance picture using a fixed procedure.
When used properly as input to
.I pcomb,
the computed calibration file will adjust the
brightness and color of any similarly scanned and converted image
so as to best match the original.
If the lighting conditions are carefully controlled (as in the case
of a flatbed scanner), it is even possible to get reliable
reflectance values this way, at least within 10% or so.
The input picture is named on the command line.
The output calibration file will be written to the standard output
if no file name is given on the command line.
.PP
In the second form, the input is from a file containing measured
values for each Macbeth color.
This file must contain entries of the form:
.sp
.nf
N       x       y       Y
.fi
.sp
Where
.I N
is the number of the corresponding Macbeth color.
(See back of ColorChecker chart
for color names and indexing, but it basically starts from the upper
left with 1 and proceeds in English text order to the lower right,
which is 24.)
The values
.I x,
.I y
and
.I Y
are the 1931 CIE (x,y) chromaticity coordinates followed by the
luminance for that color, which can be in any units.
If a white value is known (i.e. maximum output level), then it may
be given as entry number 0.
The entries may be in any order, and comments may be included
delimited by a pound sign ('#') and continuing to the end of line.
It is recommended that measurements be done for all 24 colors,
but the only required entries are the 6 neutral values on the bottom
row of the chart.
.PP
Computing a mapping from measured colors is usually more convenient
when calibrating a particular output device.
This is accomplished by printing the picture
.I macbeth_spec.pic
(which may be found in the standard RADIANCE library directory in
the lib subdirectory) and measuring the output with a chroma meter
or spectrophotometer.
.PP
For a scanned image, the locations of the 24 Macbeth patches
in the input picture must be known.
If the chart borders are not at the edges of the input picture, or
the chart has been reversed or rotated or is uncentered or at an
oblique angle, then it is necessary to specify the pixel locations
of the corners of the chart with the
.I \-p
option.
The corner postions (x,y pixel addresses as given by the
.I ximage(1)
"p" command)
are ordered on the command line:
upper-left, upper-right, lower-left, lower-right
(i.e. English text ordering).
These coordinates should be the outside corner positions of
the following patches:
.sp
.nf
upper-left      = 1. dark skin
upper-right     = 6. bluish green
lower-left      = 19. white
lower-right     = 24. black
.fi
.sp
If the chart has been flipped or rotated, simply give the pixel
positions of the appropriate patch corners, wherever
they are in the image.
(Note: if the Radiance picture has been flipped or rotated with
.I pflip(1)
or
.I protate(1),
.I ximage
will report the original pixel positions if the
.I \-c
option was not used by the reorienting program(s).
This will be wrong, so be sure to use the
.I \-c
option.)
.I Macbethcal
can handle a chart with any orientation or perspective warping if
the corner coordinates are given correctly.
The debug picture output is the best way to check for consistency.
(See the
.I \-d
option, below.)
.PP
The
.I \-d
option may be used to specify an additional output file, which will
be a picture comparing the scanned image processed according to the
computed mapping against the standard Macbeth colors.
It is a good idea to use the debug option to check that the color
patches are being located correctly, and to see how well
.I macbethcal
does at matching colors.
The center of each patch will show the target color;
the left side of each patch will show the original
color, and the right side will show the corrected value.
If the match works well, the debug picture should have a sort of
"notch on the left" look in each patch.
Macbeth colors that could not be matched because they were out of
gamut on this device are indicated with diagonal lines drawn
through the associated target colors.
.SH METHOD
.I Macbethcal
computes the color mapping in two stages.
The first stage uses the six neutral color patches at the bottom of
the Macbeth chart to compute a piecewise linear approximation to the
brightness mapping of each RGB primary.
The second stage looks at all the colors that are within the
device's gamut to compute a
least-sqaures fit for a linear color transformation from the
measured space into the standard Radiance RGB space (as defined by
the three primaries in src/common/color.h).
.PP
Thanks to the nature of inverse mappings, this method should work
either for converting scanned data to match the original, or for
preconditioning pictures to be sent to specific output devices.
In other words, the same calibration file works either for
correcting scanned images OR precorrecting images before printing.
.PP
A warning is printed if some unsaturated colors are determined to
be out of gamut, as this may indicate a poor rendition or improper
picture alignment.
The debug picture will show which colors were excluded by drawing
diagonal lines through their entries.
.SH NOTE
It is very important that the same settings be applied when
scanning or printing other images to be calibrated with the computed file.
In particular, all exposure adjustments should be fixed manually,
and no tweaking of the settings should be done along the way.
The final result will be best if the original scanned image is not
too far off from what it should be.
In the case of slide and negative scanners, it is best to apply the
recommended calibration file for the type of film used, so long as
this calibration is fixed and not adjusted on a per-image basis.
.SH CHART AVAILABILITY
The Macbeth chart is available at most photographic supply stores,
or may be ordered directly from Macbeth:
.sp
.nf
Macbeth
Munsell Color
405 Little Britain Rd.
New Windsor, NY  12553-6148
tel. 1-800-622-2384 (USA)
fax. 1-914-561-0267
.fi
.sp
The chart sells for under $50 US at the time of this writing.
.SH EXAMPLES
To compute a calibration for a FunkyThing scanner and check the
results:
.IP "" .2i
ra_tiff -r mbscan.tif mbscan.pic
.br
macbethcal -d debug.pic mbscan.pic FunkyThing.cal
.br
ximage debug.pic
.PP
To apply this computed calibration to another scanned image:
.IP "" .2i
ra_tiff -r another.tif | pcomb -f FunkyThing.cal - > another_calib.pic
.PP
To compute a calibration file for the BigWhiz film recorder, after
taking measurements of a slide made from macbeth_spec.pic:
.IP "" .2i
macbethcal -c macbeth_spec.xyY BigWhiz.cal
.PP
To prepare a picture prior to output on the same film recorder:
.IP "" .2i
pcomb -f BigWhiz.cal standard.pic > toprint.pic
.PP
To use
.I pcond(1)
to also adjust the image for human response:
.IP "" .2i
pcond -f BigWhiz.cal -h standard.pic > toprint.pic
.SH AUTHOR
Greg Ward
.br
Paul Heckbert supplied code for perspective projective mapping
.SH "SEE ALSO"
calc(1), pcomb(1), pcond(1), pfilt(1), ximage(1)
Revision:	1.1
Committed:	Tue Mar 11 19:20:21 2003 UTC (22 years, 7 months ago) by greg
Branch:	MAIN
CVS Tags:	rad3R5
Log Message:	Added documentation to repository
#	User	Rev	Content
1	greg	1.1	.\" RCSid "$Id"
2			.TH MACBETHCAL 1 1/16/97 RADIANCE
3			.SH NAME
4			macbethcal - compute color compensation based on measured Macbeth chart
5			.SH SYNOPSIS
6			.B macbethcal
7			[
8			.B "\-d debug.pic"
9			][
10			.B "\-p xul yul xur yur xll yll xlr ylr"
11			]
12			.B scannedin.pic
13			[
14			.B calibout.cal
15			]
16			.br
17			.B macbethcal
18			.B \-c
19			[
20			.B "\-d debug.pic"
21			]
22			[
23			.B measured.xyY
24			[
25			.B calibout.cal
26			]
27			]
28			.SH DESCRIPTION
29			.I Macbethcal
30			takes a scanned image or measurement set
31			of a Macbeth ColorChecker\u\s-3TM\s+3\d color
32			rendition chart and computes a color mapping
33			function suitable as input to
34			.I pcomb(1).
35			.PP
36			In the first form,
37			.I macbethcal
38			takes a scanned image of a Macbeth chart that has been converted
39			into a Radiance picture using a fixed procedure.
40			When used properly as input to
41			.I pcomb,
42			the computed calibration file will adjust the
43			brightness and color of any similarly scanned and converted image
44			so as to best match the original.
45			If the lighting conditions are carefully controlled (as in the case
46			of a flatbed scanner), it is even possible to get reliable
47			reflectance values this way, at least within 10% or so.
48			The input picture is named on the command line.
49			The output calibration file will be written to the standard output
50			if no file name is given on the command line.
51			.PP
52			In the second form, the input is from a file containing measured
53			values for each Macbeth color.
54			This file must contain entries of the form:
55			.sp
56			.nf
57			N x y Y
58			.fi
59			.sp
60			Where
61			.I N
62			is the number of the corresponding Macbeth color.
63			(See back of ColorChecker chart
64			for color names and indexing, but it basically starts from the upper
65			left with 1 and proceeds in English text order to the lower right,
66			which is 24.)
67			The values
68			.I x,
69			.I y
70			and
71			.I Y
72			are the 1931 CIE (x,y) chromaticity coordinates followed by the
73			luminance for that color, which can be in any units.
74			If a white value is known (i.e. maximum output level), then it may
75			be given as entry number 0.
76			The entries may be in any order, and comments may be included
77			delimited by a pound sign ('#') and continuing to the end of line.
78			It is recommended that measurements be done for all 24 colors,
79			but the only required entries are the 6 neutral values on the bottom
80			row of the chart.
81			.PP
82			Computing a mapping from measured colors is usually more convenient
83			when calibrating a particular output device.
84			This is accomplished by printing the picture
85			.I macbeth_spec.pic
86			(which may be found in the standard RADIANCE library directory in
87			the lib subdirectory) and measuring the output with a chroma meter
88			or spectrophotometer.
89			.PP
90			For a scanned image, the locations of the 24 Macbeth patches
91			in the input picture must be known.
92			If the chart borders are not at the edges of the input picture, or
93			the chart has been reversed or rotated or is uncentered or at an
94			oblique angle, then it is necessary to specify the pixel locations
95			of the corners of the chart with the
96			.I \-p
97			option.
98			The corner postions (x,y pixel addresses as given by the
99			.I ximage(1)
100			"p" command)
101			are ordered on the command line:
102			upper-left, upper-right, lower-left, lower-right
103			(i.e. English text ordering).
104			These coordinates should be the outside corner positions of
105			the following patches:
106			.sp
107			.nf
108			upper-left = 1. dark skin
109			upper-right = 6. bluish green
110			lower-left = 19. white
111			lower-right = 24. black
112			.fi
113			.sp
114			If the chart has been flipped or rotated, simply give the pixel
115			positions of the appropriate patch corners, wherever
116			they are in the image.
117			(Note: if the Radiance picture has been flipped or rotated with
118			.I pflip(1)
119			or
120			.I protate(1),
121			.I ximage
122			will report the original pixel positions if the
123			.I \-c
124			option was not used by the reorienting program(s).
125			This will be wrong, so be sure to use the
126			.I \-c
127			option.)
128			.I Macbethcal
129			can handle a chart with any orientation or perspective warping if
130			the corner coordinates are given correctly.
131			The debug picture output is the best way to check for consistency.
132			(See the
133			.I \-d
134			option, below.)
135			.PP
136			The
137			.I \-d
138			option may be used to specify an additional output file, which will
139			be a picture comparing the scanned image processed according to the
140			computed mapping against the standard Macbeth colors.
141			It is a good idea to use the debug option to check that the color
142			patches are being located correctly, and to see how well
143			.I macbethcal
144			does at matching colors.
145			The center of each patch will show the target color;
146			the left side of each patch will show the original
147			color, and the right side will show the corrected value.
148			If the match works well, the debug picture should have a sort of
149			"notch on the left" look in each patch.
150			Macbeth colors that could not be matched because they were out of
151			gamut on this device are indicated with diagonal lines drawn
152			through the associated target colors.
153			.SH METHOD
154			.I Macbethcal
155			computes the color mapping in two stages.
156			The first stage uses the six neutral color patches at the bottom of
157			the Macbeth chart to compute a piecewise linear approximation to the
158			brightness mapping of each RGB primary.
159			The second stage looks at all the colors that are within the
160			device's gamut to compute a
161			least-sqaures fit for a linear color transformation from the
162			measured space into the standard Radiance RGB space (as defined by
163			the three primaries in src/common/color.h).
164			.PP
165			Thanks to the nature of inverse mappings, this method should work
166			either for converting scanned data to match the original, or for
167			preconditioning pictures to be sent to specific output devices.
168			In other words, the same calibration file works either for
169			correcting scanned images OR precorrecting images before printing.
170			.PP
171			A warning is printed if some unsaturated colors are determined to
172			be out of gamut, as this may indicate a poor rendition or improper
173			picture alignment.
174			The debug picture will show which colors were excluded by drawing
175			diagonal lines through their entries.
176			.SH NOTE
177			It is very important that the same settings be applied when
178			scanning or printing other images to be calibrated with the computed file.
179			In particular, all exposure adjustments should be fixed manually,
180			and no tweaking of the settings should be done along the way.
181			The final result will be best if the original scanned image is not
182			too far off from what it should be.
183			In the case of slide and negative scanners, it is best to apply the
184			recommended calibration file for the type of film used, so long as
185			this calibration is fixed and not adjusted on a per-image basis.
186			.SH CHART AVAILABILITY
187			The Macbeth chart is available at most photographic supply stores,
188			or may be ordered directly from Macbeth:
189			.sp
190			.nf
191			Macbeth
192			Munsell Color
193			405 Little Britain Rd.
194			New Windsor, NY 12553-6148
195			tel. 1-800-622-2384 (USA)
196			fax. 1-914-561-0267
197			.fi
198			.sp
199			The chart sells for under $50 US at the time of this writing.
200			.SH EXAMPLES
201			To compute a calibration for a FunkyThing scanner and check the
202			results:
203			.IP "" .2i
204			ra_tiff -r mbscan.tif mbscan.pic
205			.br
206			macbethcal -d debug.pic mbscan.pic FunkyThing.cal
207			.br
208			ximage debug.pic
209			.PP
210			To apply this computed calibration to another scanned image:
211			.IP "" .2i
212			ra_tiff -r another.tif \| pcomb -f FunkyThing.cal - > another_calib.pic
213			.PP
214			To compute a calibration file for the BigWhiz film recorder, after
215			taking measurements of a slide made from macbeth_spec.pic:
216			.IP "" .2i
217			macbethcal -c macbeth_spec.xyY BigWhiz.cal
218			.PP
219			To prepare a picture prior to output on the same film recorder:
220			.IP "" .2i
221			pcomb -f BigWhiz.cal standard.pic > toprint.pic
222			.PP
223			To use
224			.I pcond(1)
225			to also adjust the image for human response:
226			.IP "" .2i
227			pcond -f BigWhiz.cal -h standard.pic > toprint.pic
228			.SH AUTHOR
229			Greg Ward
230			.br
231			Paul Heckbert supplied code for perspective projective mapping
232			.SH "SEE ALSO"
233			calc(1), pcomb(1), pcond(1), pfilt(1), ximage(1)