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Draw with arbitrary shaped pen

asked 9 days ago by The Lazy Coder

I need to draw lines onto a bitmap using a custom aperture. In the example below, there are two lines. The left line is horizontal. The right line is diagonal - down and to the right: Example image In the example image, the aperture and the path is shown in red. The resulting geometry is shown in black. Both of the above examples use the same aperture, but wipe it along a different path.

I'd like to be able to draw with an arbitrary shaped pen, even though the above example only shows rectangles.

I made an attempt at using the System.Drawing.2D namespace but it doesn't seem to do what I need it to do. Using custom end-caps on pens seemed promising, but the end caps rotate with the direction of the line. Also, getting the line-width right seems challenging.failed end-caps example

I've considered drawing the aperture over and over again, centered at different points along the path, but that doesn't seem performant. It is also quite difficult to minimize the number of aperture draws.

The best idea I've got is to try to draw the 'line' as a filled shape. At first, I thought a convex hull algorithm would be the answer - simply take the vertices of the aperture at the beginning of the draw and at the end of the draw and put them through a convex hull algorithm to find the 'outer' vertices. This works for my 1st example above, - but a star-shaped aperture demonstrates that this solution is incomplete. It only works when the aperture is convex itself. enter image description here

Simply putting all of the vertices through a convex hull algorithm would result in the areas highlighted in blue being filled in, but I need to only fill in the black areas.

Your answer

2 Answers

answered 9 days ago by Steve Tauber

This is an ugly, inelegant solution, but you could take the vector that represents the stroke direction and length and redraw your shape many times along that vector. (You'd have to choose an interval at which to draw it that didn't leave gaps.) This would only produce correct results if you want aliased output; it would screw up any anti-aliasing done by GDI+, because pixels with midtones would potentially be drawn over, making them darker than they should be.

answered 9 days ago by The Lazy Coder

The best idea I've got involves some redundant drawing, but I think it will work.

Treat the aperture polygon as a list of line segments. For each line segment in the list, draw a parallelogram using the line segment of the aperture at the start of the move, and using the same line segment of the aperture at the end of the move Parallelogram Draw

The above image shows this for three of the ten line segments that make up the star. For some pen shapes, there is potentially a lot of overlap between segments, but this method should result in all of the correct pixels being filled in. Notice that most of the green area is already filled in by the blue area and the red area, with just a small section in the lower-right star that's green-only. Redundantly filling in these pixel is wasteful, but will result in the correct image, I think.

Other Thoughts:

There may be a way to do this without redundantly filling in the same area, and it probably involves ray casting. Notice how the intersection of the red and green line in the lower-right would fall on a vertex that is not part of the original aperture. You would need to draw a ray from the right-most vertex of the right polygon along the direction of the move and find where that intersects the green line in order to draw the final shape correctly.

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