Move origin to left-top and draw from top left to bottom right

[pieterclaerhout]

From what I understand, the coordinate system is designed in such a way that the origin is in the lower left corner of canvas and that you draw from left to right and bottom to top, correct?

Is there a way to change it so that the 0,0 point is in the upper left corner (like most design programs do) and draw from left to right and top to bottom?

[kpym]

You can apply ReflectY.

[pieterclaerhout]

That doesn't do the trick unfortunately. It does make it draw from top to bottom, but it keeps the 0, 0 point in the lower left corner (rendering everything outside the viewport).

I've then tried added Translate(0, -c.H) after the ReflectY call. This did move the 0,0 point to the upper left corner, but I then figured out that all the text is mirrored along the Y-axis…

Any other ideas?

[kpym]

There is SetCoordSystem that is supposed to do this (the code is equivalent to what you do, symmetry by the middle of the canvas), but if you say that the text is inverted (and probably the images too in this case, I haven't checked) then may be this should be considered as a bug?

[kpym]

It looks like that the coordinate transforms here in canvas are over simplistic. In PGF/TikZ, for example, there are two kind of transforms : coordinate transforms (that do not affect the line widths, the shapes, the text orientation...) and the canvas transforms (that transform everything). And there is actually a third option : when we apply a coordinate transform to some node (containing text or image) and we use transform shape the global coordinate changes are applied to the node, but this do not change for example the line widths.
All this to conclude that having only one transformation matrix, without additional flags to what this matrix should apply, is probably not enough.

[pieterclaerhout]

OK, good to know.

I'll keep on using the extra calculations I've setup to get everything on the right location…

[tdewolff]

There are two transformations though, much like PGF/TikZ. A coordinate transform and a path/image/text transform. You can set the former using ctx.SetCoordView(matrix) or ctx.SetCoordSystem(canvas.CartesianIV) in this case. This will not transform the actual path/stroke/text/image. The latter can be set using ctx.SetView(matrix), this will not transform the coordinates (but perhaps it should?).

Please let me know if setting ctx.SetCoordSystem(canvas.CartesianIV) works for you!

[kpym]

@tdewolff Sorry, I didn't see that Context has separate view and coordView. Very good news! Thanks.

[tdewolff]

No worries! You think the view should also apply to coordinates as well, and coordView only to coordinates? I'm unaware of how other implementation have handled this.

[kpym]

I know only PGF/TikZ well : there transform canvas applies to the canvas after everything is drawn, so it transform really everything, and this looks probably as yours view. And when you apply a "standard" transform it applies before drawing, so only to coordinates (but not to objects put at some coordinates by default, except if you say transform shape), and this is probably like yours coordViews. Sorry if I'm not clear.

[tdewolff]

I've incorporated implementing view to work for coordinates as well in the roadmap from where it will be tracked: #74

[carbocation]

Looping back to this, in case I am misunderstanding the docs and the discussion, the pieterclaerhout solution of

ctx.ReflectY()
ctx.Translate(0, -c.H)

is still the only solution that works for me. Other suggested approaches such as

ctx.SetCoordSystem(canvas.CartesianIII)
ctx.ReflectY()

do not cause the drawing to render within the viewport.

[tdewolff]

I think in the second approach you should not call ReflectY. In any case I think coordinate systems should be implemented differently to make it easier to use top-left as the origin, or perhaps use non-Cartesian coordinate systems. I will come back to this!

[tdewolff]

I've added a change that adds better support for CartesianIV (i.e. origin is top-left). It differs from CartesianI (i.e. origin is bottom-left) in that it anchors differently to images (anchors top-left vs bottom-left) and that text has always been anchored to the baseline (TextLine) or to top-left (TextBox) and remains so. This means that switching to CartesianIV may change the position of text and images.

You can change to CartesianIV using:

ctx.SetCoordSystem(canvas.CartesianIV)