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Comment on SVG 19991203

From: Steve Lay <S.W.Lay@ucles-red.cam.ac.uk>
Date: Tue, 1 Feb 2000 07:40:08 +0000
Message-Id: <l03130300b4bc31b6e41e@[195.102.198.235]>
To: www-svg@w3.org

Firstly, may I congratulate the working group on a much needed specification!

My comment/question is to do with the definition of a pixel that appears in
chapter 7.  In particular, I'm concerned about the alignment of pixels to
the co-ordinate system.  The specification makes no specific mention of
this issue but the example at the beginning of 7.3 appears to favour an
abstract model in which co-ordinates align between pixels (like QuickDraw).

<svg width="300px" height="100px">
<desc>Example InitialCoords - SVG's initial coordinate system</desc>
<g style="fill:none; stroke:black; stroke-width:3">
<line x1="0" y1="1.5" x2="300" y2="1.5" />
<line x1="1.5" y1="0" x2="1.5" y2="100" />
</g>

This example will clearly encourage people who wish to draw lines one-pixel
thick along the top boundary of the viewport to use commands such as:

<line x1="0" y1="0.5" x2="300" y2="0.5" />

I have no objection to this but I do feel that some notes in this section
on turning real valued co-ordinates into co-ordinates suitable for an
integer based graphics package are important.  People used to using
integer-based graphics packages may not spot the use of "1.5" (or even the
stroke width) in your example and the diagram appears to fudge the whole
issue by simply drawing red squares with the following co-ordinates and
labelling them as indicated:

(x=0, y=0, w=3, h=3) is labelled (0,0)
(x=297, y=0, w=3, h=3) is labelled (300,0)
(x=0, y=97, w=3, h=3) is labelled (0,100)

Which weren't even the points that were drawn.  I had to go away and look
up the default line-cap attribute in order to work out what was going on.
I think this diagram encourages people to draw boundaries as if pixels are
centered on co-ordinates which is clearly not what is intended (given the
origin of the viewport at 0,0 and not -0.5,-0.5).

Keep up the good work.

Steve Lay


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Received on Tuesday, 1 February 2000 02:40:20 GMT

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