Support and allow negative animation speed

neothesia-core/src/render/waterfall/pipeline/shader.wgsl

@@ -39,11 +39,17 @@ struct VertexOutput {
 @vertex
 fn vs_main(vertex: Vertex, note: NoteInstance) -> VertexOutput {
     let speed = time_uniform.speed / view_uniform.scale;
-    let size = vec2<f32>(note.size.x, note.size.y * speed);
+    let size = vec2<f32>(note.size.x, note.size.y * abs(speed));
 
     let keyboard_start_y = view_uniform.size.y - view_uniform.size.y / 5.0;
     let y = keyboard_start_y - size.y / 2.0;
-    let pos = vec2<f32>(note.n_position.x, y) - vec2<f32>(0.0, size.y / 2.0);
+    var pos = vec2<f32>(note.n_position.x, y) - vec2<f32>(0.0, size.y / 2.0);
+
+    // If notes are falling from down to top, we need to adjust the position,
+    // as their start is on top of the quad rather than down
+    if speed < 0.0 {
+        pos.y += size.y;
+    }
 
     let offset = vec2<f32>(0.0, -(note.n_position.y - time_uniform.time) * speed);
 

neothesia/src/scene/playing_scene/mod.rs

@@ -262,9 +262,16 @@ fn handle_settings_input(
 
             if key == NamedKey::PageUp {
                 target.config.animation_speed += amount;
+                // 0.0 is invalid speed, let's skip it and jump to positive
+                if target.config.animation_speed == 0.0 {
+                    target.config.animation_speed += amount;
+                }
             } else {
                 target.config.animation_speed -= amount;
-                target.config.animation_speed = target.config.animation_speed.max(100.0);
+                // 0.0 is invalid speed, let's skip it and jump to negative
+                if target.config.animation_speed == 0.0 {
+                    target.config.animation_speed -= amount;
+                }
             }
 
             waterfall