[Webkit-unassigned] [Bug 90777] New: SVG FEBlend filter multiply mode could overflow

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=90777

           Summary: SVG FEBlend filter multiply mode could overflow
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: SVG
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: rgabor at webkit.org
                CC: zimmermann at kde.org


The FeBlend filter calculates with this formula in multiply mode:
 return (((255 - alphaA) * colorB + (255 - alphaB + colorB) * colorA) / 255)

The problem is that the return type is unsigned char (0-255) but the calculated value could be more.
With the following values:
colorA = 255
colorB = 255
alphaA = 0
alphaB = 0

(((255 - 0) * 255 + (255 - 0 + 255) * 255) / 255) = 765

But because of the overflow the function returns with 253.
I think we should clamp it to 255 like this:

return std::min(((255 - alphaA) * colorB + (255 - alphaB + colorB) * colorA) / 255, 255)

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