colour induction effects are modelled by a low-level multiresolution wavelet framework xavier otazu,...
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Colour induction effects are modelled by a low-levelmultiresolution wavelet framework
Xavier Otazu, Maria Vanrell, Alejandro PárragaComputer Vision Center, 08193 Cerdanyola, Spain. [email protected], [email protected], [email protected]
AbstractA new multiresolution wavelet model for chromatic induction (CIWaM) is presented here. It is based on a previously published [1] model, the BIWaM (or Brightness Induction Wavelet Model) which accounts for several brightness induction effects. The new model is just a simple extension to include chromatic induction processes and, likewise the old model, it is based on three simple assumptions related to spatial frequency, spatial orientation and contrast surround energy, all implemented on a multiresolution framework.
Conclusions Bibliography
Three basic assumptions Perceptual CIWaM model Extended CSF
CIWaM is defined using just the same three assumptions defined in the BIWaM and simplisticly extending it to a MacLeod-Boynton colour space. CIWaM can predict with acceptable accuracy the psychophysical results obtained in experiments designed to shown chromatic induction effects.
[1] X.Otazu, M. Vanrell and A. Párraga, Vision Research, 48, 733-751 (2008).[2] K.T. Mullen. J. Physiology, 359, 381-400 (1985).[3] D.I.A. MacLeod and R.M. Boynton, J. Optical Society of America, 69(8), 1183-1187 (1979).
ECVP’08 – Utrecht
CIWaM is based on the same basic assumptions of BIWAM:
• Assumption 1: Assimilation in a colour channel is only performed when both central and surround stimuli have similar spatial frequency within a frequency range of about one octave.
• Assumption 2: Assimilation in a colour channel is strongest when the central stimulus and the surround stimulus have similar spatial orientation.
• Assumption 3: When the contrast energy in a colour channel of the surround features increases, assimilation increases (i.e. contrast in a colour axis decreases) and vice-versa.
Now we add its generalization by applying it to every i-th colour channel, we use lsY MacLeod-Boynton colour space [3]
An stimulus image I can be represented by its multiresolution wavelet transform as
Given I, the perceptual image predicted by CIWaM can be defined as
where j are the wavelet planes of Icn,i the final residual planes IC’s,o,i(r,) is a weighting function to deal with induction processes
Assumptions 1 and 2 are naturally implemented by a multiresolution wavelet transform [1] .
Assumption 3 is implemented by the introduction of the C’s,o,i(r,).
The weighting function C’s,o,i(r,) (Extended CSF) is based on the psychophysically-measured CSF [2].
it can be shown as a continuous set of modified CSF’s.
It depends on just two variablesthe center-surround contrast energy ratio rthe visual frequency
It can be represented as a two-dimensional function.
Experimental design
Stimuli design
Original stimulus Psychophysics result CIWaM prediction
Experimental setup
Experiment Configurations(rings per disk)
Condition(lsY colours of test, first and
second inductors)
# 1(Striped
background)
51117
4 sets of 3 colours
# 2(Uniform
background)
51117
4 sets of 2 colours(equal 1st and 2nd inductors)
n
sin
dhvojosiosi C
1,
,,,,,,
percep ),( crI
Examples of stimulus
Colours of inductors and reference rings in the lsY colour space
Experiment 1 Experiment 2
Psychophysics vs CIWaMParticular result
n
sin
dhvoiosi
1,
,,,, cI
Experimental procedure: Two experiments were conducted on a gamma-corrected 21” CRT monitor (Viewsonic pf227f) viewed binocularly from 146 cm inside a dark room. The monitor was run by a digital video processor (Cambridge Research Systems Bits++)
capable of displaying 14-bit colour depths at a 75Hz (non-interlaced) rate. The full monitor screen subtended some 15.5 × 11.5 deg to the observer. The task was to match the appearance of the test ring on the right to that of the reference ring on the left by navigating the lsY colour space using a gamepad controller. There were three main observers (one naïve) who repeated the experiments three times and six other observers (five naïve) who did the experiments only once. Only the main observer’s results are shown.
The CIWaM can be interpreted as a straightforward extension of BIWaM to the MacLeod-Boynton colour contrast channels, therefore unifying both chromatic assimilation and chromatic contrast effects in a single mathematical formulation. The performance of CIWaM was compared to that of normal observers in two psychophysical experiments, obtaining an acceptable agreement between the experimental results and CIWaM predictions.