Perceiving edge blur: Gaussian-derivative filtering and a rectifying nonlinearity

May, Keith A. and Georgeson, Mark A. (2003). Perceiving edge blur: Gaussian-derivative filtering and a rectifying nonlinearity. IN: Applied Vision Association Christmas Meeting. 2002-12-18. (Unpublished)

Abstract

We studied the visual mechanisms that encode edge blur in images. Our previous work suggested that the visual system spatially differentiates the luminance profile twice to create the `signature' of the edge, and then evaluates the spatial scale of this signature profile by applying Gaussian derivative templates of different sizes. The scale of the best-fitting template indicates the blur of the edge. In blur-matching experiments, a staircase procedure was used to adjust the blur of a comparison edge (40% contrast, 0.3 s duration) until it appeared to match the blur of test edges at different contrasts (5% - 40%) and blurs (6 - 32 min of arc). Results showed that lower-contrast edges looked progressively sharper. We also added a linear luminance gradient to blurred test edges. When the added gradient was of opposite polarity to the edge gradient, it made the edge look progressively sharper. Both effects can be explained quantitatively by the action of a half-wave rectifying nonlinearity that sits between the first and second (linear) differentiating stages. This rectifier was introduced to account for a range of other effects on perceived blur (Barbieri-Hesse and Georgeson, 2002 Perception 31 Supplement, 54), but it readily predicts the influence of the negative ramp. The effect of contrast arises because the rectifier has a threshold: it not only suppresses negative values but also small positive values. At low contrasts, more of the gradient profile falls below threshold and its effective spatial scale shrinks in size, leading to perceived sharpening.

Divisions: Life & Health Sciences > Optometry
Additional Information: Abstract published in Human Vision - When It Works and When It Fails. Seventh Abstract published in Applied Vision Association Christmas Meeting Aston University, Birmingham, UK, 18 December 2002, Abstracts. Perception, (2003), 32 (3), p.46, 0301-0066.
Event Title: Applied Vision Association Christmas Meeting
Event Type: Other
Event Dates: 2002-12-18
Uncontrolled Keywords: edge blur,images,luminance profile,Gaussian derivative templates
Published Date: 2003
Authors: May, Keith A.
Georgeson, Mark A. ( 0000-0002-8173-9522)

Download

Item under embargo.

Export / Share Citation


Statistics

Additional statistics for this record