+ClippingBox RenderItem::CalculateTransformSpaceAABB(const Matrix& transformMatrix, const Vector3& position, const Vector3& size)
+{
+ // Calculate extent vector of the AABB:
+ const float halfActorX = size.x * 0.5f;
+ const float halfActorY = size.y * 0.5f;
+
+ // To transform the actor bounds to the transformed space, We do a fast, 2D version of a matrix multiply optimized for 2D quads.
+ // This reduces float multiplications from 64 (16 * 4) to 12 (4 * 3).
+ // We create an array of 4 corners and directly initialize the first 3 with the matrix multiplication result of the respective corner.
+ // This causes the construction of the vector arrays contents in-place for optimization.
+ // We place the coords into the array in clockwise order, so we know opposite corners are always i + 2 from corner i.
+ // We skip the 4th corner here as we can calculate that from the other 3, bypassing matrix multiplication.
+ // Note: The below transform methods use a fast (2D) matrix multiply (only 4 multiplications are done).
+ Vector2 corners[4]{Transform2D(transformMatrix, -halfActorX + position.x, -halfActorY + position.y),
+ Transform2D(transformMatrix, halfActorX + position.x, -halfActorY + position.y),
+ Transform2D(transformMatrix, halfActorX + position.x, halfActorY + position.y)};
+
+ // As we are dealing with a rectangle, we can do a fast calculation to get the 4th corner from knowing the other 3 (even if rotated).
+ corners[3] = Vector2(corners[0] + (corners[2] - corners[1]));
+
+ // Calculate the AABB:
+ // We use knowledge that opposite corners will be the max/min of each other. Doing this reduces the normal 12 branching comparisons to 3.
+ // The standard equivalent min/max code of the below would be:
+ // Vector2 AABBmax( std::max( corners[0].x, std::max( corners[1].x, std::max( corners[3].x, corners[2].x ) ) ),
+ // std::max( corners[0].y, std::max( corners[1].y, std::max( corners[3].y, corners[2].y ) ) ) );
+ // Vector2 AABBmin( std::min( corners[0].x, std::min( corners[1].x, std::min( corners[3].x, corners[2].x ) ) ),
+ // std::min( corners[0].y, std::min( corners[1].y, std::min( corners[3].y, corners[2].y ) ) ) );
+ unsigned int smallestX = 0u;
+ // Loop 3 times to find the index of the smallest X value.
+ // Note: We deliberately do NOT unroll the code here as this hampers the compilers output.
+ for(unsigned int i = 1u; i < 4u; ++i)
+ {
+ if(corners[i].x < corners[smallestX].x)
+ {
+ smallestX = i;
+ }
+ }
+
+ // As we are dealing with a rectangle, we can assume opposite corners are the largest.
+ // So without doing min/max branching, we can fetch the min/max values of all the remaining X/Y coords from this one index.
+ Vector4 aabb(corners[smallestX].x, corners[(smallestX + 3u) % 4].y, corners[(smallestX + 2u) % 4].x, corners[(smallestX + 1u) % 4].y);
+
+ // Round outwards from center
+ int x = static_cast<int>(floor(aabb.x));
+ int y = static_cast<int>(floor(aabb.y));
+ int z = static_cast<int>(ceilf(aabb.z));
+ int w = static_cast<int>(ceilf(aabb.w));
+
+ return ClippingBox(x, y, z - x, fabsf(w - y));
+}