As far as functionality, it's pretty useful once I get it set up, but it is confusing to use. It defiantly needs some work.
As far as UI wise I would add radial lines going out of each point so user can predict better where the lines will intersect. And also have each vanishing point a certain color so user can differentiate which lines come from where. When I did that, it felt much easier to understand.
Also having the points generate at the center of the canvas window or frame instead of off to the top right would also help.
Plus instead of having the users manually set and configure each point, how about having presets for:
- Straight Horizontal
- Straight Vertical
- Vertical and Straight
- Single Point Perspective
- Two Point Perspective
- Three Point Perspective
Keep in mind, it would be a nice option to have the left and right points no always be symmetrical to eachother, but stay on the same plane as the central dot. It will help create much more dynamic two point or three point perspective grids.
Here is a mock up of the tool box design.
For those who want to customize each grid further, they should click on the Wrench icon and have a window for that. Same way you would make gamut masks for the color wheels. This should make easy to use, but also a powerful tool.
Like @achadwick mention in the pull request, the dots should also be able to do elliptical lines lines. It will help creating 5 point perspective grids for the fisheye effects much easier. It could also be an alternative fast way to draw arches as well.
Lastly having it respecting pressure is a must. The perspective grids should only effect where the cursor is and not how the brush interacts with the canvas.
Overall, it looks like a promising start. Great job @gwojcik I can't wait to see this land in master once it's ready.