If "Add to mask" is not checked the new alpha values will be the product of the element's alpha and the key. This has the effect of reducing the keyed area, further restricting the area to be blended.
Checking "Add to mask" changes the process to add the element's alpha values with the key values. This has the effect of increasing the key area, thereby expanding the area to be blended. During the summing process any sum that exceeds fully opaque is clipped at fully opaque, avoiding strange artefacts from arithmetic overflow. You can also check "Limiter" to prevent values from exceeding the element's original alpha values. This can be used by the final alpha filter when more than one filter that can affect alpha is in use to prevent loss of anti-aliasing on edges.
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