Recover color from a blue or green cast
Underwater images lose channel balance and contrast as light travels through water. The correction measures the image channels, stretches their recorded tonal ranges, compensates red loss, and trims the dominant blue or green cast.
Correction amount blends the complete result with the source. Red recovery controls only the weakened red channel. Contrast recovery stretches the channel endpoints after ignoring the darkest and brightest 1%, which reduces the influence of isolated clipped pixels.
Red channel compensation
Water attenuates wavelengths by different amounts, and red commonly weakens before blue and green in available-light dive photos. The algorithm compares the mean red value with the two stronger channels and derives a capped gain. It also adds more compensation where a pixel’s dominant water channel exceeds red.
The cap matters. Unrestricted gray-world correction can turn a deep blue scene orange because it assumes every channel should have the same average. Red recovery stops at a bounded gain, and Correction amount provides a second limit.
Contrast loss and scatter
Suspended particles scatter light between the subject and camera, compressing the visible range and adding haze. Contrast recovery expands each channel between measured low and high percentiles. It cannot estimate a physical depth map, so the same global correction applies to near and distant subjects.
Use a lower value when distant water starts to clip or when backscatter becomes more visible. Raising global contrast can expose particles that were hidden in a narrow blue-gray range.
Missing color remains missing
A single compressed frame cannot reconstruct wavelengths that never reached the sensor. If the red channel is nearly black, high Red recovery reveals red-channel noise and block artifacts. A raw file, closer subject distance, artificial light, or a physical red filter at capture time preserves more usable signal.
Temperature trim is for the final bias after channel compensation. It should not carry the whole correction; a large warm trim shifts every pixel, including water that should remain blue.
Example: a shallow reef frame
Start with Auto, 82% correction, 100% red recovery, and 35% contrast recovery. Inspect coral highlights and neutral diving gear. If the scene turns magenta, reduce Red recovery. If it remains cyan but reds look plausible, raise Correction amount. Use Temperature trim only after the channel balance is close.