render: Emulate drawing lines using quads

render/src/lib.rs

@@ -6,6 +6,7 @@ pub mod bitmap;
 pub mod blend;
 pub mod error;
 pub mod filters;
+pub mod lines;
 pub mod matrix;
 pub mod pixel_bender;
 // The `renderdoc` crate doesn't compile on apple platforms

render/src/lines.rs

@@ -0,0 +1,53 @@
+use crate::commands::CommandHandler;
+use crate::matrix::Matrix;
+use swf::{Color, Point, PointDelta, Twips};
+
+/// Draw a line using [`CommandHandler::draw_rect()`].
+///
+/// This is a universal way of drawing a line, independent of
+/// the rendering backend's implementation and rasterization rules.
+pub fn emulate_line(handler: &mut impl CommandHandler, color: Color, matrix: Matrix) {
+    let a = matrix * Point::new(Twips::ZERO, Twips::ZERO);
+    let b = matrix * Point::new(Twips::ONE, Twips::ZERO);
+    emulate_line_as_rect(handler, color, a, b);
+}
+
+/// Similar to [`emulate_line`], but emulates drawing a rectangle with lines as its sides.
+pub fn emulate_line_rect(handler: &mut impl CommandHandler, color: Color, matrix: Matrix) {
+    let a = matrix * Point::new(Twips::ZERO, Twips::ZERO);
+    let b = matrix * Point::new(Twips::ONE, Twips::ZERO);
+    let c = matrix * Point::new(Twips::ONE, Twips::ONE);
+    let d = matrix * Point::new(Twips::ZERO, Twips::ONE);
+    emulate_line_as_rect(handler, color, a, b);
+    emulate_line_as_rect(handler, color, b, c);
+    emulate_line_as_rect(handler, color, c, d);
+    emulate_line_as_rect(handler, color, d, a);
+}
+
+// Note that we cannot simply crate a rect and then transform it using
+// the transformation matrix, as the thickness and line caps should not be transformed.
+// That's why two already transformed [`Point`]s are taken as input,
+// and then a 1px wide rectangle is fit inbetween them.
+fn emulate_line_as_rect(
+    handler: &mut impl CommandHandler,
+    color: Color,
+    a: Point<Twips>,
+    b: Point<Twips>,
+) {
+    let PointDelta { dx, dy } = b - a;
+    let (dx, dy) = (dx.to_pixels(), dy.to_pixels());
+
+    let len = (dx * dx + dy * dy).sqrt() as f32;
+    let angle = dy.atan2(dx) as f32;
+
+    let rotation = Matrix::rotate(angle);
+    let translation = Matrix::translate(a.x + Twips::HALF, a.y + Twips::HALF);
+
+    // Step 1: Create a 1px thick line with the proper length.
+    let line = Matrix::create_box(len, 1.0, Twips::ZERO, -Twips::HALF);
+    // Step 2: Rotate it, so it still starts at `(0,0)` but points to `b-a`.
+    let line = rotation * line;
+    // Step 3: Translate it to `a`, so that it points to `b`.
+    let line = translation * line;
+    handler.draw_rect(color, line);
+}