Stranica 25 [25]
ta CHARISM A IR 7
greement n" 228330
file (see Chapter 3), as it contains the exact information reguired to reverse any processing
carried out on the image, including the gamma correction applied. Note that although
camera profiles can also theoretically be approximated for JPEG images straight from the
camera, this process has traditionally been difficult, as it involves an image that has already
been "profiled" once to a particular colour space but may also include additional
hue/saturation settings which distort the raw data.
The luminance of the linear image produced using this embedded ICC profile can then be
calibrated according to a grey reference scale. For visible-reflected images, existing
methods used for colour calibration based on use of a Macbeth target (for a description of
colour calibration targets see Chapter 2) and an existing nip2? tool," can be employed for
this purpose. The luminance is set by analysing the Macbeth greyscale and carrying outa
linear regression to find a scale factor on the linear image which minimises the average error.
The procedure then finds the average brightness of all Macbeth chart patches and generates
a matrix. Leastmean-square optimisation against patches with known values produces a
3x3 matrix from camera RGB to XYZ. Provided that the linearization is carried out correctly,
this should give safer and more reproducible results than manual adjustment of the tone
curve. Figure 1-14 shows an example of an image which has been corrected for both
Camera-applied gamma and the spectral density of the light source using this method. The
muted appearance is typical of a corrected linear image, as all in-camera aesthetic
enhancements have been removed.
Figure 1-14. Visible-reflected image of a detail from a wall painting fragment from the British Museum
(Winged youth from the Tomb of the Nasonii, 1883,0505.5) (a) as shot and (b) after compensation for
the distribution of the incoming radiation and the non-linear response of the camera using the
embedded ICC profile and calibration according to a grey reference scale.
! Nip2 is the graphical user-interface of VIPS (http://www.vips.ecs.soton.ac.uk/index.php?title=VIPS), a free
image processing system which will be used in the development of the post-processing software
addressing the correction of images (see Chapter 3).
Version No. 1.0 17 Date : 14/10/2013