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How do I blend two colors using SrcAlpha and OneMinusSrcAlpha when I have HDR range (rgb and a values can be > 1)?

The typical formula for non-HDR is (src * srcAlpha) + (dst * (1.0 - srcAlpha)) but this generates a negative value if srcAlpha is > 1.

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    \$\begingroup\$ How did alpha end up greater than 1? Only the RGB channels should exceed the 0-1 range in HDR, not the 'opaqueness' or 'transparency' of a scene. \$\endgroup\$ – Chuck Walbourn Sep 20 '17 at 20:29
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Clamp your srcAlpha between 0 and 1.

Your pixels can't be more opaque than 100% or more transparent than 0% opacity.

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