Fixing transforming a plane by a mat4

Author: Interrupt

src/framework/math.zig

@@ -287,6 +287,7 @@ pub const Vec4 = extern struct {
     pub const zero = Vec4.new(0.0, 0.0, 0.0, 1.0);
 };
 
+// Mat4: 4x4 matrix stored in row-major
 pub const Mat4 = extern struct {
     m: [4][4]f32,
 

src/framework/spatial/plane.zig

@@ -3,6 +3,7 @@ const math = @import("../math.zig");
 const debug = @import("../debug.zig");
 const boundingbox = @import("boundingbox.zig");
 const assert = std.debug.assert;
+const testing = std.testing;
 
 const Vec3 = math.Vec3;
 const Vec4 = math.Vec4;
@@ -159,14 +160,14 @@ pub const Plane = struct {
     }
 
     pub fn mulMat4(self: *const Plane, transform: math.Mat4) Plane {
-        const on_plane = self.normal.scale(self.d).toVec4(1.0);
+        const on_plane = self.normal.scale(-self.d).toVec4(1.0);
         const normal = self.normal.toVec4(0.0);
 
-        const transformed_on_plane = on_plane.mulMat4(transform);
+        const transformed_on_plane = on_plane.mulMat4(transform).toVec3();
         const transformed_normal = normal.mulMat4(transform.invert().transpose());
 
         const new_normal = transformed_normal.toVec3().norm();
-        const new_distance = transformed_on_plane.toVec3().dot(new_normal);
+        const new_distance = -new_normal.dot(transformed_on_plane);
 
         return .{ .normal = new_normal, .d = new_distance };
     }
@@ -242,20 +243,21 @@ test "Plane.planeIntersectPoint" {
 
 test "Plane.mulMat4" {
     const plane = Plane.init(Vec3.new(0, 0, 1), Vec3.new(1, 1, 1));
+    try testing.expectEqual(-1.0, plane.d);
 
     // multiplying by the identity should result in an identical plane
     const t1 = plane.mulMat4(math.Mat4.identity);
-    assert(std.meta.eql(plane, t1));
+    try testing.expectEqual(plane, t1);
 
     // scaling should keep the normal, but increase the distance
     const t2 = plane.mulMat4(math.Mat4.scale(Vec3.new(2, 2, 2)));
     assert(std.meta.eql(plane.normal, t2.normal));
     assert(t2.d == plane.d * 2);
 
     // translating should also just change the distance
-    const t3 = plane.mulMat4(math.Mat4.translate(Vec3.new(1, 1, 1)));
-    assert(std.meta.eql(plane.normal, t3.normal));
-    assert(t3.d == 0);
+    const t3 = plane.mulMat4(math.Mat4.translate(Vec3.new(102, 6, 10)));
+    try testing.expectEqual(plane.normal, t3.normal);
+    try testing.expectEqual(-11.0, t3.d); // remember, d is stored negated
 
     // rotating should just change the normal
     const t4 = plane.mulMat4(math.Mat4.rotate(45, Vec3.new(1, 1, 1)));

src/framework/utils/quakemap.zig

@@ -601,8 +601,8 @@ pub const QuakeMap = struct {
         const v2 = try readPoint(&iter, math.Mat4.identity);
 
         // map planes are clockwise, flip them around when computing the plane to get a counter-clockwise plane
-        face.plane = Plane.initFromTriangle(v2.mulMat4(transform), v1.mulMat4(transform), v0.mulMat4(transform));
         face.untransformed_plane = Plane.initFromTriangle(v2, v1, v0);
+        face.plane = face.untransformed_plane.mulMat4(transform);
         face.uv_direction = closestAxis(face.untransformed_plane.normal);
         face.u_axis = if (face.uv_direction.x == 1) Vec3.new(0, 1, 0) else Vec3.new(1, 0, 0);
         face.v_axis = if (face.uv_direction.z == 1) Vec3.new(0, -1, 0) else Vec3.new(0, 0, -1);