Line width does not follow specification?

[byorgey]

nand` in #haskell reports that lines look much different with SVG backend than with cairo. Need to look into this more carefully.

[haasn]

Here's what the following file renders as for me: http://ompldr.org/vZnd6bA

import Diagrams.Prelude
import Diagrams.Backend.SVG.CmdLine

main = defaultMain (unitCircle # lw 2)

Running with -o output.svg -w 500. What does this example render like using cairo, for comparison?

[fryguybob]

Is this just a matter of diagrams-svg's lw being relative to output pixels while Cairo is in points? In @nandykins example the diagrams lw argument is 2 and the result svg stroke-wdith is also 2.

[byorgey]

No, Cairo is not in points. Line width should be measured with respect to the coordinate system of the diagram being rendered; it should NOT depend on the requested output size. The idea is that once you have made a diagram, you can request to render it at size 50x50, 100x100, 900x700, or whatever, and all those outputs will just look like zoomed/scaled copies of one another. (In fact, the way this is handled internally is by calling freeze before applying one final transformation to make the diagram fit the requested size -- so it may be that if we fix #19 this issue will be magically fixed as well.)

So in (unitCircle # lw 2) the "2" means two units in the coordinate system of the diagram itself -- and since the diagram just consists of a single radius-1 circle, that is quite a fat line indeed. It should essentially render as a filled circle of radius 2, though the line width is not taken into account when computing the envelope, so the outer part gets cut off and in practice you just get a completely black image (which is indeed what I get with the cairo backend).

The tricky part is that the line width is measured with respect to the coordinate system of the final diagram which is being rendered. Put another way, scaling operations do not affect line width (except after an application of freeze). For example, (unitCircle # lw 2) # scale 100 is a radius-100 circle drawn with a line of width 2, not a line of width 200.

All of this probably needs to be written down more clearly in a separate document for backend implementors...

[byorgey]

See diagrams/diagrams-doc@32727f0#L0R23193 for relevant IRC discussion.