GIMP Multiply vs Darken Only Blend Modes Explained

When blending layers in GIMP, both the Multiply and Darken Only modes darken the resulting image, but they do so using fundamentally different mathematical algorithms. Multiply scales the color values of the overlapping layers together, creating a compound darkening effect that generates entirely new, darker color values. In contrast, Darken Only performs a channel-by-channel comparison, simply selecting whichever pixel channel value is already darker.

Color Representation

In digital image processing and GIMP's internal calculations, RGB channel values are normalized to a floating-point range from \(0.0\) (pure black) to \(1.0\) (pure white). Alternatively, in standard 8-bit color space, these values range from \(0\) to \(255\).

The Multiply Blend Mode

The Multiply mode calculates the product of the corresponding color channels from the base layer (\(A\)) and the active/blend layer (\(B\)).

  • Normalized Formula:
    \(f(A, B) = A \times B\)
  • 8-bit Integer Formula:
    \(f(A, B) = \frac{A \times B}{255}\)

Because multiplying two fractions between \(0\) and \(1\) always yields a smaller fraction, the result of the Multiply mode is always darker than both original layers, unless one of the pixels is pure white (\(1.0\)), which leaves the other layer unchanged. Multiplying by pure black (\(0.0\)) always results in pure black (\(0.0\)).

The Darken Only Blend Mode

The Darken Only mode does not multiply or scale values. Instead, it evaluates each individual color channel (Red, Green, and Blue) of the two layers and outputs the minimum value between the two.

  • Formula:
    \(f(A, B) = \min(A, B)\)

This evaluation is calculated independently for each channel:

  • \(R_{\text{result}} = \min(R_A, R_B)\)
  • \(G_{\text{result}} = \min(G_A, G_B)\)
  • \(B_{\text{result}} = \min(B_A, B_B)\)

Because it selects the minimum value rather than performing arithmetic scaling, Darken Only never produces a value darker than the darkest existing channel between the two inputs.

Practical Mathematical Comparison

Consider two overlapping pixels where both the base layer and blend layer have a 50% gray value (\(0.5\) in normalized space, or \(128\) in 8-bit space):

  • Multiply:
    \(0.5 \times 0.5 = 0.25\) (or \(\frac{128 \times 128}{255} \approx 64\)).
    The output drops to 25% gray, producing a composite significantly darker than either source layer.
  • Darken Only:
    \(\min(0.5, 0.5) = 0.5\) (or \(\min(128, 128) = 128\)).
    The output remains at 50% gray because neither pixel is darker than the other.

Summary of Differences

  • Operation Type: Multiply is an arithmetic scaling function; Darken Only is a conditional minimum function.
  • Color Output: Multiply calculates entirely new, lower tonal values by multiplying fractions; Darken Only retains existing values directly from whichever layer has the lower channel intensity.
  • Compound Darkening: Multiply compounds darkness so identical overlapping tones become darker; Darken Only leaves identical overlapping tones completely unchanged.