Stimuli design. Original stimulus. Psychophysics result. CIWaM prediction. Colour induction effects are modelled by a low-level multiresolution wavelet framework. Xavier Otazu, Maria Vanrell, Alejandro Párraga.
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Colour induction effects are modelled by a low-levelmultiresolution wavelet framework
Xavier Otazu, Maria Vanrell, Alejandro Párraga
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.
A new multiresolution wavelet model for chromatic induction (CIWaM) is presented here. It is based on a previously published  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.
Three basic assumptions
Perceptual CIWaM model
The weighting function C’s,o,i(r,n) (Extended CSF) is based on the psychophysically-measured CSF .
it can be shown as a continuous set of modified CSF’s.
It depends on just two variables
the center-surround contrast energy ratio r
the visual frequency n
It can be represented as a two-dimensional function.
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
wj are the wavelet planes of I
cn,i the final residual planes I
C’s,o,i(r,n) is a weighting function to deal with induction processes
Assumptions 1 and 2 are naturally implemented by a multiresolution wavelet transform  .
Assumption 3 is implemented by the introduction of the C’s,o,i(r,n).
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.
Colours of inductors and reference rings in the lsY colour space
Examples of stimulus
Psychophysics vs CIWaM
 X.Otazu, M. Vanrell and A. Párraga, Vision Research, 48, 733-751 (2008).
 K.T. Mullen. J. Physiology, 359, 381-400 (1985).
 D.I.A. MacLeod and R.M. Boynton, J. Optical Society of America, 69(8), 1183-1187 (1979).
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.
ECVP’08 – Utrecht