tonekurve

Tonekurve, or tone curve, is a graphical tool used in digital image processing to map input brightness values to output brightness values. It expresses how the tonal values in an image are remapped to achieve adjustments in brightness, contrast, and color. The usual display shows input luminance on the horizontal axis and output luminance on the vertical axis; the identity mapping is a diagonal line from bottom-left to top-right.

In editing software, the curve is manipulated by placing and dragging control points along the curve. Moving a point changes the output value for the corresponding input level. Curves can be applied to all color channels together to adjust luminance, or to individual channels (red, green, blue) to correct color balance or create color grading effects. A straight line indicates no change, while bends introduce non-linear remapping that affects shadows, midtones, and highlights.

Common uses include increasing or reducing contrast, lifting shadows to reveal detail, and shaping the midtones. An S-shaped curve is frequently used to increase overall contrast, while different per-channel curves can correct color casts or create stylistic looks. The curve’s effect depends on the surrounding tonal data and can be refined by adjusting the curve’s curvature in specific regions.

Relation to other concepts: the tone curve is related to gamma encoding and nonlinear display responses, and is a core component of RAW processing and color management. Practical considerations include avoiding clipping of detail in the extremes, and being mindful that aggressive curves can amplify noise and introduce color artifacts.