Colour value differences between generated reference images and those in the repository

Hi Thomas.

I’ve been looking in to why the reference images in the
repository often
are different from the images that are generated when I run
regard.  The
main difference seems to be when non-opaque colours are
composited.

As an example, I took a look at the second row fourth column
sub-test of
samples/tests/spec/rendering/paintOpacity.svg, specifically
one of the
pixels that is the 0.5 opacity gold painted on the opaque
#eee
background square.

The reference image in the repository is
http://svn.apache.org/repos/asf/xmlgraphics/batik/trunk
/test-references/samples/tests/spec/rendering/paintOpacity.p
ng
and the image generated on my machine is
http://m
cc.id.au/temp/2007/paintOpacity.png.  The PNGs
themselves are
identical except for the IDAT chunk.

In the repository reference image, the pixel value is
rgba(224, 222, 118, 255), while in my image it is
rgba(246, 226, 119, 255).

Taking the alpha compositing rules from
http://www.w3.org/TR/SVG11/masking.html#SimpleAlphaBl
ending, this is my
calculation of the pixel value:

  // Premultiplied #eee background

  Ca = 1
  Cr = 238
  Cg = 238
  Cb = 238

  // Premultiplied 0.5 opacity gold rgb(255,215,0)

  Ea = trunc(trunc(0.5 * 255) / 255) = 0.49804
  Er = trunc(Ea * 255)               = 127
  Eg = trunc(Ea * 215)               = 107
  Eb = trunc(Ea * 0)                 = 0

  // Premultiplied final pixel value

  Ca' = trunc(1 - (1 - Ea) * (1 - Ca))
      = trunc(1 - 0.50196  * 0)
      = 1

  Cr' = trunc((1 - Ea) * Cr  + Er)
      = trunc(0.50196  * 238 + 127)
      = 246

  Cg' = trunc((1 - Ea) * Cg  + Eg)
      = trunc(0.50196  * 238 + 107)
      = 226

  Cb' = trunc((1 - Ea) * Cb  + Eb)
      = trunc(0.50196  * 238 + 0)
      = 119

and un-premultiplied this is the same as rgb(246, 226, 119,
255), which
matches the value in my image.  So my question is: are the
reference
images in the repository wrong?  I’m not an expert on
colour, so I don’t
know if there are other factors (colour spaces that the
compositing
could occur in, for example) that would allow the values in
the
repository reference image.  Any ideas?

Thanks,

Cameron

-- 
Cameron McCormack, http://mcc.id.au/
	xmpp:heycamjabber.org  ▪  ICQ 26955922  ▪  MSN cammcc.id.au

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Cameron McCormack:
>   // Premultiplied 0.5 opacity gold rgb(255,215,0)
> 
>   Ea = trunc(trunc(0.5 * 255) / 255) = 0.49804

One too many trunc()s in there, should be:

    Ea = trunc(0.5 * 255) / 255        = 0.49804

-- 
Cameron McCormack, http://mcc.id.au/
	xmpp:heycamjabber.org  ▪  ICQ 26955922  ▪  MSN cammcc.id.au

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Cameron McCormack:
> > As an example, I took a look at the second row
fourth column sub-test of
> > samples/tests/spec/rendering/paintOpacity.svg,
specifically one of the
> > pixels that is the 0.5 opacity gold painted on the
opaque #eee
> > background square.
> 
> > In the repository reference image, the pixel value
is
> > rgba(224, 222, 118, 255), while in my image it is
> > rgba(246, 226, 119, 255).

Thomas DeWeese:
>    Hmm, on my machine, using gimp, the reference image

> shows the pixel value as rgba(246, 225, 118, 255),
> and your generated one has rgba(247, 227, 119, 255). 
> I can easily imagine reasons why the two would vary by

> a code value (or two).
> 
>    How are you checking the reference/output Image's
pixel values?

I am using gimp, too (2.4.0-rc3).  I turned the colour
management mode
to “No color management” in the preferences window just
to be sure it
wasn’t doing anything, but get the same values.

-- 
Cameron McCormack, http://mcc.id.au/
	xmpp:heycamjabber.org  ▪  ICQ 26955922  ▪  MSN cammcc.id.au

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