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LibSoftGPU: Reuse edge function for front/back culling

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We sat on a throne of lies: our `edge_function()` returned positive
values for _clockwise_ vertex rotation instead of _counter-clockwise_,
which was hidden by the fact that we were forcing everything into CW
rotation by an erroneous area comparison (`> 0` instead of `< 0`).

This simplifies our culling code significantly.
This commit is contained in:
Jelle Raaijmakers 2022-04-17 20:44:34 +02:00 • committed by Andreas Kling
parent 26a463506e
commit f680d82086
1 changed files with 10 additions and 19 deletions
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29
Userland/Libraries/LibSoftGPU/Device.cpp
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@ -45,14 +45,16 @@ using AK::SIMD::to_f32x4;
using AK::SIMD::to_u32x4;
using AK::SIMD::u32x4;
// Both of these edge functions return positive values for counter-clockwise rotation of vertices.
// Note that they return the area of a parallelogram with sides {a, b} and {b, c}, so _double_ the area of the triangle {a, b, c}.
constexpr static float edge_function(FloatVector2 const& a, FloatVector2 const& b, FloatVector2 const& c)
{
return (c.x() - a.x()) * (b.y() - a.y()) - (c.y() - a.y()) * (b.x() - a.x());
return (c.y() - a.y()) * (b.x() - a.x()) - (c.x() - a.x()) * (b.y() - a.y());
}
constexpr static f32x4 edge_function4(FloatVector2 const& a, FloatVector2 const& b, Vector2<f32x4> const& c)
{
return (c.x() - a.x()) * (b.y() - a.y()) - (c.y() - a.y()) * (b.x() - a.x());
return (c.y() - a.y()) * (b.x() - a.x()) - (c.x() - a.x()) * (b.y() - a.y());
}
template<typename T, typename U>
@ -184,12 +186,11 @@ void Device::rasterize_triangle(Triangle const& triangle)
if (m_options.enable_alpha_test && m_options.alpha_test_func == GPU::AlphaTestFunction::Never)
return;
// Vertices
GPU::Vertex const& vertex0 = triangle.vertices[0];
GPU::Vertex const& vertex1 = triangle.vertices[1];
GPU::Vertex const& vertex2 = triangle.vertices[2];
// Calculate area of the triangle for later tests
// Determine the area of a parallelogram with sides (v0,v1) and (v1,v2) to calculate barycentrics
FloatVector2 const v0 = vertex0.window_coordinates.xy();
FloatVector2 const v1 = vertex1.window_coordinates.xy();
FloatVector2 const v2 = vertex2.window_coordinates.xy();
@ -872,10 +873,6 @@ void Device::draw_primitives(GPU::PrimitiveType primitive_type, FloatMatrix4x4 c
// https://learnopengl.com/Getting-started/Coordinate-Systems
// "Note that if only a part of a primitive e.g. a triangle is outside the clipping volume OpenGL
// will reconstruct the triangle as one or more triangles to fit inside the clipping range. "
//
// ALL VERTICES ARE DEFINED IN A CLOCKWISE ORDER
// Okay, let's do some face culling first
m_clipped_vertices.clear_with_capacity();
m_clipped_vertices.append(triangle.vertices[0]);
@ -924,19 +921,12 @@ void Device::draw_primitives(GPU::PrimitiveType primitive_type, FloatMatrix4x4 c
}
for (auto& triangle : m_processed_triangles) {
// Let's calculate the (signed) area of the triangle
// https://cp-algorithms.com/geometry/oriented-triangle-area.html
float dxAB = triangle.vertices[0].window_coordinates.x() - triangle.vertices[1].window_coordinates.x(); // A.x - B.x
float dxBC = triangle.vertices[1].window_coordinates.x() - triangle.vertices[2].window_coordinates.x(); // B.X - C.x
float dyAB = triangle.vertices[0].window_coordinates.y() - triangle.vertices[1].window_coordinates.y();
float dyBC = triangle.vertices[1].window_coordinates.y() - triangle.vertices[2].window_coordinates.y();
float area = (dxAB * dyBC) - (dxBC * dyAB);
if (area == 0.0f)
auto area = edge_function(triangle.vertices[0].window_coordinates.xy(), triangle.vertices[1].window_coordinates.xy(), triangle.vertices[2].window_coordinates.xy());
if (area == 0.f)
continue;
if (m_options.enable_culling) {
bool is_front = (m_options.front_face == GPU::WindingOrder::CounterClockwise ? area > 0 : area < 0);
bool is_front = (m_options.front_face == GPU::WindingOrder::CounterClockwise ? area > 0.f : area < 0.f);
if (!is_front && m_options.cull_back)
continue;
@ -945,7 +935,8 @@ void Device::draw_primitives(GPU::PrimitiveType primitive_type, FloatMatrix4x4 c
continue;
}
if (area > 0)
// Force counter-clockwise ordering of vertices
if (area < 0.f)
swap(triangle.vertices[0], triangle.vertices[1]);
if (texture_coordinate_generation_enabled) {