Colour induction effects are modelled by a low-level multiresolution wavelet framework

1. Stimuli design Original stimulus Psychophysics result CIWaM prediction Colour induction effects are modelled by a low-levelmultiresolution wavelet framework Xavier Otazu, Maria Vanrell, Alejandro Párraga Computer Vision Center, 08193 Cerdanyola, Spain. xotazu@cvc.uab.es, maria@cvc.uab.es, aparraga@cvc.uab.es Abstract 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 [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. Three basic assumptions Perceptual CIWaM model Extended CSF The weighting function C’s,o,i(r,n) (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 variables the center-surround contrast energy ratio r the visual frequency n It can be represented as a two-dimensional function. • 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 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 [1] . Assumption 3 is implemented by the introduction of the C’s,o,i(r,n). Experimental design 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 Experiment 1 Experiment 2 Particular result Psychophysics vs CIWaM Conclusions Bibliography [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). 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