logcontrast

Logcontrast, or log-contrast, is a measure of luminance contrast defined in logarithmic luminance space. For a pair of luminance values I1 and I2, the log contrast is C_log = log10(I1) − log10(I2) = log10(I1/I2). In a broader sense for a scene, a common global form is C_log = log10(Imax) − log10(Imin) = log10(Imax/Imin). In image analysis, local versions compute log contrast by comparing a pixel to its surrounding neighborhood, yielding spatially varying contrast maps.

The logarithmic formulation aligns with aspects of human visual perception, where brightness discrimination is roughly proportional to log luminance. This makes log-contrast more robust to uniform lighting changes: a multiplicative change in illumination increases all luminances by the same factor, while log luminance changes additively, preserving relative contrast values in many contexts.

Variants and scope: Log-contrast can be computed globally across the entire image or locally. Global log-contrast emphasizes overall dynamic range, while local log-contrast highlights fine-grained edges and textures. Log-contrast measures are distinct from linear or Weber and Michelson contrasts and are often used alongside them depending on the modeling or processing goals.

Applications and usage: Log-contrast is used in tone mapping for high dynamic range images, image enhancement, radiometric calibration, and psychophysical experiments to quantify perceived contrast. It is also employed in computer vision to reduce sensitivity to absolute luminance and focus on relative luminance ratios. When implementing on digital data, care is needed for zero-valued pixels, typically by adding a small offset or using log1p to avoid undefined results.

Limitations: While useful for perceptual modeling, log-contrast can overweight very low luminance differences and may introduce bias in images with extensive shadows. The choice of base and offset can influence the resulting values.