Game Development Reference
In-Depth Information
Global methods (or spatially uniform methods)
The same transformation is applied to each pixel in this type of methods.
Linear plating In this case, the maximum value (minimum resp.) of luminance in
the scene is sent to the highest (the smallest resp.) value that can be displayed on
the screen. We can immediately see that with this solution, the same scene lit by
the moon or the sun will seem similar.
Method of Tumblin and Rushmeier (1993). This model tries to conserve the
brightness (defined as the logarithm of luminance) and uses the model given by
Rushmeier et al. (1995);
Ward Model (Ward, 1994). This model establishes a linear relation between real
luminance and the luminance emitted by the screen:
L e
=
mL s
Ward obtains the following formula by using the Blackwell formula for luminance
threshold:
1 . 219
2 . 5
L 0 . 4
a ( e )
+
m
=
+
L 0 . 4
a ( s )
1 . 219
=
Ward assumes that L a ( e )
L emax / 2 and L a ( s ) is the average luminance of the scene (or
an area of the scene).
Adjustment by histogram (Larson et al., 1997). The idea here is to do an adjustment
of the luminance histogram of the image on the basis of psycho-visual functions
making it possible to follow the contrast and details and eliminate the non-visible
details.
The histogram is obtained by calculating a local image (where one pixel corresponds
to one degree of vision), then by applying a contrast sensitivity function to this image,
by taking the luminance logarithm (to obtain brightness), by dividing the interval
of brilliance values into n parts and by calculating the brightness of each interval.
The equalisation can be done in three different ways. The simple method consists
of calculating the cumulative distribution function on the basis of the histogram and
calculating the brightness to be displayed on the basis of this function. The other two
solutions consider a limit or the sensitivity of the eye.
Local methods (or spatially non-uniform methods)
In this case, the tone mapping operators apply differential transformations to different
parts of the image.
Chiu et al. (1993) were the first in the field of computer graphics to point out that
the solution to the problem of image dynamics cannot be obtained using a global
operator. They suggested a local solution with a manual adjustment based on the
results of experiments;