Updates to the Drawing Module

I'd promise to get some wrote by the end of the week, so here's my attempt on covering updates I have made to the Drawing Module over pretty much the last half of the year.

In general there's been a lot of incremental small fixes that have accumulated over such a long time, which is a stark contrast when I look at the last blog post on this and might seem significant.

One of the big changes was adding initial support for drawing the page background given the width and height and recently spending time switching the origin of the coordinate system to the bottom left with the y-axis pointing upwards - see this post. QGraphicsView orients with the y-axis pointing downwards which is also used in SVG. It's a trivial fix, where the y coordinate is inverted for the position of every Drawing Views. This is needed to ensure consistency with CAD packages and other formats.

The next step for improving the functionality is adding support for templates and displaying this on the background.

These leads onto talking about further work on improving the dimensions. The most obvious detail is the improved presentation of dimensions. There are now arrow heads, which later will be allowed to be changed to a few standard ISO options and the use of a font called osifont, which I was recommended for a starting point for datum labels. The license needs checking as it appears to be GPL 3 - I'm not entirely sure if this is compatible for distribution directly with the FreeCAD release but in the worst case an be installed separately by the user or made a dependency on the packaging front and be used if available by default.

The datum values can be used calculated as project or as true values - if an orthographic projection is used then projected values are used by default. The leader lines are also calculated to the end points of the lines avoiding any overlapping. The dimensions also use caching and now don't need to recalculate when these are moved on the canvas by the user. 

There is now an orthographic container, which contains ortho-views -orthographic projections such as front, left, top etc. These benefit of having a container is that the child orthoviews are much smarter and allow various properties such as scale or the type of projection (first or third angle) to be set from the container which are then cascaded down to the child views. 

QGraphicsView is quite flexible and allows some nice interactivity too. A little feature I implemented was having alignment between views: and can be interactively moved along one axis. You can seen in the screenshot from above the are all aligned correctly. This originally took a while to figure out because the projection used in the current release of FreeCAD is not consistent - the simple trick was ensuring that the centroid of the part is used as the origin of the projection plane. 

Another little feature I added was support for angles - although there are a few cases where an exception is thrown. The final thing I currently can think of working on is improving the print support which is slowly getting there:

Anyway that's an overview and hopefully give some incentive to give the drawing branch another try!