Direct2D/DirectX rasterization rule for lines

Question

I've read the online docs about DirectX rasterization rules but I still can't understand why doesn't this code produce anything visible on the screen?

target->SetAntialiasMode(D2D1_ANTIALIAS_MODE_ALIASED);
target->DrawLine(D2D1::Point2F(0.f, 0.f), D2D1::Point2F(10.f, 0.f), redBrush, 1.f);

Of course, if I change y from 0.0f to 1.0f for both line points I get a visible horizontal line at the top-left side of the Window client area but I would like to understand what principles are involved here. I couldn't figure them out from available documentation.

Answer 1

You should draw your line "in the middle of the pixel":

target->DrawLine(D2D1::Point2F(0.0f, 0.5f), D2D1::Point2F(10.f, 0.5f), redBrush, 1.f);

Otherwise, if you don't use antialiasing, your line can be drawn on either side 0/0...10/0 line. In your case it gets drawn outside of window canvas.

Note that pixel on screen is actually a rectangle with coordinates D2D1::Point2F(0.0f, 0.0f) to D2D1::Point2F(1.0f, 1.0f).

And D2D1::Point2F(0.5f, 0.5f) is the center of that rectangle. If you draw 1x1 rectangle around that coordinate it will cover the pixel exactly.