Bug 105776 – feComponentTransfer linear gives wrong values

• Status: RESOLVED
• Resolution: INVALID
• Component: SVG

[Bob Wyttenbach]

The SVG feComponentTransfer linear function doesn't work as expected. A slope of -1 and intercept of 1 applied to a grayscale image should invert the image: black -> white, white-> black, 25% gray -> 75% gray, 50% gray unchanged, and so on.

http://www.w3.org/TR/filter-effects/#feComponentTransferElement says "C' = slope * C + intercept" where "C is the initial component (e.g., ‘feFuncR’), C' is the remapped component; both in the closed interval [0,1]." Thus slope -1 intercept 1 should applied to C=0.5 should give 0.5.

The following filter

<filter id="linear">
  <feComponentTransfer>
    <feFuncR type="linear" slope="-1" intercept="1" />
    <feFuncG type="linear" slope="-1" intercept="1" />
    <feFuncB type="linear" slope="-1" intercept="1" />
  </feComponentTransfer>
</filter>

maps black to white and white to black, but intermediate values are off, e.g. 50% gray maps to 90% gray and 75% gray maps to 98% gray. See http://jsfiddle.net/Rpjs2/ for a simple example.

http://src.chromium.org/chrome/branches/WebKit/195/LayoutTests/svg/custom/feComponentTransfer-Linear.svg shows another example in which the "invert" example is significantly shifted to the lighter end of the spectrum.

Tested in Safari (WebKit 536.26.17) and Chrome (WebKit 537.11) with same results.

[Robert Longson]

It gives the right results. It's operating in the linearRGB colour space and you're expecting it to operate in sRGB. You can use color-interpolation-filters to change this per http://www.w3.org/TR/SVG/painting.html#ColorInterpolationFiltersProperty

I'm afraid this bug is invalid.

[Dirk Schulze]

(In reply to comment #1)
> It gives the right results. It's operating in the linearRGB colour space and you're expecting it to operate in sRGB. You can use color-interpolation-filters to change this per http://www.w3.org/TR/SVG/painting.html#ColorInterpolationFiltersProperty
> 
> I'm afraid this bug is invalid.

Thanks for investigating Robert. Closing bug now.