Perceiving edge blur: linear filtering and a rectifying nonlinearity

Keith A. May, Mark A. Georgeson

Research output: Contribution to conferenceOther

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.
Original languageEnglish
Publication statusUnpublished - 2003
EventHuman Vision--When It Works and When It Fails. 7th Applied Vision Association Christmas Meeting - Aston University, Birmingham (UK)
Duration: 8 Dec 2002 → …

Conference

ConferenceHuman Vision--When It Works and When It Fails. 7th Applied Vision Association Christmas Meeting
CityAston University, Birmingham (UK)
Period8/12/02 → …

Fingerprint

nonlinearity
effect
experiment
test
comparison

Bibliographical note

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.388, 0301-0066.

Keywords

  • edge blur
  • luminance profile
  • Gaussian derivative templates
  • sharpness

Cite this

May, K. A., & Georgeson, M. A. (2003). Perceiving edge blur: linear filtering and a rectifying nonlinearity. Human Vision--When It Works and When It Fails. 7th Applied Vision Association Christmas Meeting, Aston University, Birmingham (UK), .
May, Keith A. ; Georgeson, Mark A. / Perceiving edge blur: linear filtering and a rectifying nonlinearity. Human Vision--When It Works and When It Fails. 7th Applied Vision Association Christmas Meeting, Aston University, Birmingham (UK), .
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May, KA & Georgeson, MA 2003, 'Perceiving edge blur: linear filtering and a rectifying nonlinearity' Human Vision--When It Works and When It Fails. 7th Applied Vision Association Christmas Meeting, Aston University, Birmingham (UK), 8/12/02, .

Perceiving edge blur: linear filtering and a rectifying nonlinearity. / May, Keith A.; Georgeson, Mark A.

2003. Human Vision--When It Works and When It Fails. 7th Applied Vision Association Christmas Meeting, Aston University, Birmingham (UK), .

Research output: Contribution to conferenceOther

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T1 - Perceiving edge blur: linear filtering and a rectifying nonlinearity

AU - May, Keith A.

AU - Georgeson, Mark A.

N1 - 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.388, 0301-0066.

PY - 2003

Y1 - 2003

N2 - 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.

AB - 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.

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May KA, Georgeson MA. Perceiving edge blur: linear filtering and a rectifying nonlinearity. 2003. Human Vision--When It Works and When It Fails. 7th Applied Vision Association Christmas Meeting, Aston University, Birmingham (UK), .