Schofield Andrew J, Hesse Gillian, Rock Paul B, Georgeson Mark A
School of Psychology, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
Vision Res. 2006 Oct;46(20):3462-82. doi: 10.1016/j.visres.2006.03.014. Epub 2006 May 2.
The pattern of illumination on an undulating surface can be used to infer its 3-D form (shape-from-shading). But the recovery of shape would be invalid if the luminance changes actually arose from changes in reflectance. So how does vision distinguish variation in illumination from variation in reflectance to avoid illusory depth? When a corrugated surface is painted with an albedo texture, the variation in local mean luminance (LM) due to shading is accompanied by a similar modulation in local luminance amplitude (AM). This is not so for reflectance variation, nor for roughly textured surfaces. We used depth mapping and paired comparison methods to show that modulations of local luminance amplitude play a role in the interpretation of shape-from-shading. The shape-from-shading percept was enhanced when LM and AM co-varied (in-phase) and was disrupted when they were out of phase or (to a lesser degree) when AM was absent. The perceptual differences between cue types (in-phase vs out-of-phase) were enhanced when the two cues were present at different orientations within a single image. Our results suggest that when LM and AM co-vary (in-phase) this indicates that the source of variation is illumination (caused by undulations of the surface), rather than surface reflectance. Hence, the congruence of LM and AM is a cue that supports a shape-from-shading interpretation.
起伏表面上的光照模式可用于推断其三维形状（由阴影恢复形状）。但如果亮度变化实际上是由反射率变化引起的，那么形状恢复将是无效的。那么视觉系统如何区分光照变化和反射率变化以避免虚幻的深度感呢？当一个波纹表面被涂上反照率纹理时，由于阴影导致的局部平均亮度（LM）变化会伴随着局部亮度幅度（AM）的类似调制。对于反射率变化以及表面纹理粗糙的情况则并非如此。我们使用深度映射和配对比较方法来表明，局部亮度幅度的调制在由阴影恢复形状的解释中起作用。当LM和AM同变（同相）时，由阴影恢复形状的感知增强，而当它们异相时或（在较小程度上）当AM不存在时，这种感知会受到干扰。当两个线索在单个图像中以不同方向呈现时，线索类型（同相与异相）之间的感知差异会增强。我们的结果表明，当LM和AM同变（同相）时，这表明变化的来源是光照（由表面起伏引起），而不是表面反射率。因此，LM和AM的一致性是支持由阴影恢复形状解释的一个线索。
