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LibSoftGPU: Use approximation for maximum depth slope

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OpenGL allows GPUs to approximate a triangle's maximum depth slope
which prevents a number computationally expensive instructions. On my
machine, this gives me +6% FPS in Quake III.

We are able to reuse `render_bounds` here since it is the containing
rect of the (X, Y) window coordinates of the triangle, thus its width
and height are the maximum delta X and delta Y, respectively.
This commit is contained in:
Jelle Raaijmakers 2022-09-08 10:10:01 +02:00 committed by Tim Flynn
parent 08204efe1f
commit b42feb76a0
Notes: sideshowbarker 2024-07-17 07:21:04 +09:00 
Author: https://github.com/gmta
Commit: https://github.com/SerenityOS/serenity/commit/b42feb76a0
Pull-request: https://github.com/SerenityOS/serenity/pull/15165
1 changed files with 7 additions and 27 deletions
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34
Userland/Libraries/LibSoftGPU/Device.cpp
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@ -743,33 +743,13 @@ void Device::rasterize_triangle(Triangle& triangle)
// Calculate depth offset to apply
float depth_offset = 0.f;
if (m_options.depth_offset_enabled) {
// Edge value deltas
auto edge_value_step_x = FloatVector3 {
static_cast<float>(v1.y() - v2.y()),
static_cast<float>(v2.y() - v0.y()),
static_cast<float>(v0.y() - v1.y()),
};
auto edge_value_step_y = FloatVector3 {
static_cast<float>(v2.x() - v1.x()),
static_cast<float>(v0.x() - v2.x()),
static_cast<float>(v1.x() - v0.x()),
};
// Barycentric deltas
auto barycentric_step_x = edge_value_step_x * one_over_area;
auto barycentric_step_y = edge_value_step_y * one_over_area;
// Depth delta vector and slope (magnitude)
auto depth_coordinates = FloatVector3 {
vertex0.window_coordinates.z(),
vertex1.window_coordinates.z(),
vertex2.window_coordinates.z(),
};
auto depth_step = FloatVector2 {
depth_coordinates.dot(barycentric_step_x),
depth_coordinates.dot(barycentric_step_y),
};
auto depth_max_slope = depth_step.length();
// OpenGL 2.0 § 3.5.5 allows us to approximate the maximum slope
auto delta_z = max(
max(
abs(vertex0.window_coordinates.z() - vertex1.window_coordinates.z()),
abs(vertex1.window_coordinates.z() - vertex2.window_coordinates.z())),
abs(vertex2.window_coordinates.z() - vertex0.window_coordinates.z()));
auto depth_max_slope = max(delta_z / render_bounds.width(), delta_z / render_bounds.height());
// Calculate total depth offset
depth_offset = depth_max_slope * m_options.depth_offset_factor + NumericLimits<float>::epsilon() * m_options.depth_offset_constant;