LibSoftGPU: Apply regular cartesian coordinate system

Currently, LibSoftGPU is still OpenGL-minded in that it uses a
coordinate system with the origin of `(0, 0)` at the lower-left of
textures, buffers and window coordinates. Because we are blitting to a
`Gfx::Bitmap` that has the origin at the top-left, we need to flip the
Y-coordinates somewhere in the rasterization logic.

We used to do this during conversion of NDC-coordinates to window
coordinates. This resulted in some incorrect behavior when
rasterization did not pass through the vertex transformation logic,
e.g. when calling `glDrawPixels`.

This changes the coordinate system to OpenGL's throughout, only to blit
the final color buffer upside down to the target bitmap. This fixes
drawing to the depth buffer directly resulting in upside down images.

3 files changed, 21 insertions, 31 deletions

diff --git a/Userland/Libraries/LibSoftGPU/Buffer/Typed2DBuffer.h b/Userland/Libraries/LibSoftGPU/Buffer/Typed2DBuffer.h
index b056f4257e..eefde5e940 100644
--- a/Userland/Libraries/LibSoftGPU/Buffer/Typed2DBuffer.h
+++ b/Userland/Libraries/LibSoftGPU/Buffer/Typed2DBuffer.h
@@ -47,11 +47,13 @@ public:
         }
     }
 
-    void blit_to_bitmap(Gfx::Bitmap& bitmap, Gfx::IntRect const& target) const requires IsSame<T, u32>
+    void blit_flipped_to_bitmap(Gfx::Bitmap& bitmap, Gfx::IntRect const& target) const requires IsSame<T, u32>
     {
         VERIFY(bitmap.format() == Gfx::BitmapFormat::BGRA8888 || bitmap.format() == Gfx::BitmapFormat::BGRx8888);
         int source_y = 0;
-        for (int y = target.top(); y <= target.bottom(); ++y) {
+
+        // NOTE: we are flipping the Y-coordinate here, which is OpenGL-specific: (0, 0) is considered the lower-left corner of the window
+        for (int y = target.bottom(); y >= target.top(); --y) {
             auto const* buffer_scanline = scanline(source_y++);
             auto* bitmap_scanline = bitmap.scanline(y);
 
diff --git a/Userland/Libraries/LibSoftGPU/Device.cpp b/Userland/Libraries/LibSoftGPU/Device.cpp
index f026927b74..59a8855b0c 100644
--- a/Userland/Libraries/LibSoftGPU/Device.cpp
+++ b/Userland/Libraries/LibSoftGPU/Device.cpp
@@ -94,16 +94,6 @@ static Vector4<f32x4> to_vec4(u32x4 bgra)
     };
 }
 
-Gfx::IntRect Device::window_coordinates_to_target_coordinates(Gfx::IntRect const& window_rect)
-{
-    return {
-        window_rect.x(),
-        m_frame_buffer->rect().height() - window_rect.height() - window_rect.y(),
-        window_rect.width(),
-        window_rect.height(),
-    };
-}
-
 void Device::setup_blend_factors()
 {
     m_alpha_blend_factors = {};
@@ -215,7 +205,7 @@ void Device::rasterize_triangle(const Triangle& triangle)
 
     auto render_bounds = m_frame_buffer->rect();
     if (m_options.scissor_enabled)
-        render_bounds.intersect(window_coordinates_to_target_coordinates(m_options.scissor_box));
+        render_bounds.intersect(m_options.scissor_box);
 
     // Obey top-left rule:
     // This sets up "zero" for later pixel coverage tests.
@@ -759,7 +749,7 @@ void Device::draw_primitives(PrimitiveType primitive_type, FloatMatrix4x4 const&
     }
 
     // Now let's transform each triangle and send that to the GPU
-    auto const viewport = window_coordinates_to_target_coordinates(m_options.viewport);
+    auto const viewport = m_options.viewport;
     auto const viewport_half_width = viewport.width() / 2.0f;
     auto const viewport_half_height = viewport.height() / 2.0f;
     auto const viewport_center_x = viewport.x() + viewport_half_width;
@@ -939,10 +929,10 @@ void Device::draw_primitives(PrimitiveType primitive_type, FloatMatrix4x4 const&
                 one_over_w,
             };
 
-            // To window coordinates - note that we flip the Y coordinate into target space
+            // To window coordinates
             vec.window_coordinates = {
                 viewport_center_x + ndc_coordinates.x() * viewport_half_width,
-                viewport_center_y - ndc_coordinates.y() * viewport_half_height,
+                viewport_center_y + ndc_coordinates.y() * viewport_half_height,
                 depth_halfway + ndc_coordinates.z() * depth_half_range,
                 ndc_coordinates.w(),
             };
@@ -979,7 +969,7 @@ void Device::draw_primitives(PrimitiveType primitive_type, FloatMatrix4x4 const&
             continue;
 
         if (m_options.enable_culling) {
-            bool is_front = (m_options.front_face == WindingOrder::CounterClockwise ? area < 0 : area > 0);
+            bool is_front = (m_options.front_face == WindingOrder::CounterClockwise ? area > 0 : area < 0);
 
             if (!is_front && m_options.cull_back)
                 continue;
@@ -1123,18 +1113,18 @@ void Device::clear_color(FloatVector4 const& color)
 {
     auto const fill_color = to_bgra32(color);
 
-    auto fill_rect = m_frame_buffer->rect();
+    auto clear_rect = m_frame_buffer->rect();
     if (m_options.scissor_enabled)
-        fill_rect.intersect(window_coordinates_to_target_coordinates(m_options.scissor_box));
+        clear_rect.intersect(m_options.scissor_box);
 
-    m_frame_buffer->color_buffer()->fill(fill_color, fill_rect);
+    m_frame_buffer->color_buffer()->fill(fill_color, clear_rect);
 }
 
 void Device::clear_depth(DepthType depth)
 {
     auto clear_rect = m_frame_buffer->rect();
     if (m_options.scissor_enabled)
-        clear_rect.intersect(window_coordinates_to_target_coordinates(m_options.scissor_box));
+        clear_rect.intersect(m_options.scissor_box);
 
     m_frame_buffer->depth_buffer()->fill(depth, clear_rect);
 }
@@ -1143,7 +1133,7 @@ void Device::clear_stencil(StencilType value)
 {
     auto clear_rect = m_frame_buffer->rect();
     if (m_options.scissor_enabled)
-        clear_rect.intersect(window_coordinates_to_target_coordinates(m_options.scissor_box));
+        clear_rect.intersect(m_options.scissor_box);
 
     m_frame_buffer->stencil_buffer()->fill(value, clear_rect);
 }
@@ -1156,7 +1146,7 @@ void Device::blit_to_color_buffer_at_raster_position(Gfx::Bitmap const& source)
     INCREASE_STATISTICS_COUNTER(g_num_pixels, source.width() * source.height());
     INCREASE_STATISTICS_COUNTER(g_num_pixels_shaded, source.width() * source.height());
 
-    auto const blit_rect = raster_rect_in_target_coordinates(source.size());
+    auto const blit_rect = get_rasterization_rect_of_size({ source.width(), source.height() });
     m_frame_buffer->color_buffer()->blit_from_bitmap(source, blit_rect);
 }
 
@@ -1165,14 +1155,14 @@ void Device::blit_to_depth_buffer_at_raster_position(Vector<DepthType> const& de
     if (!m_raster_position.valid)
         return;
 
-    auto const raster_rect = raster_rect_in_target_coordinates({ width, height });
+    auto const raster_rect = get_rasterization_rect_of_size({ width, height });
     auto const y1 = raster_rect.y();
     auto const y2 = y1 + height;
     auto const x1 = raster_rect.x();
     auto const x2 = x1 + width;
 
     auto index = 0;
-    for (auto y = y2 - 1; y >= y1; --y) {
+    for (auto y = y1; y < y2; ++y) {
         auto depth_line = m_frame_buffer->depth_buffer()->scanline(y);
         for (auto x = x1; x < x2; ++x)
             depth_line[x] = depth_values[index++];
@@ -1181,7 +1171,7 @@ void Device::blit_to_depth_buffer_at_raster_position(Vector<DepthType> const& de
 
 void Device::blit_color_buffer_to(Gfx::Bitmap& target)
 {
-    m_frame_buffer->color_buffer()->blit_to_bitmap(target, m_frame_buffer->rect());
+    m_frame_buffer->color_buffer()->blit_flipped_to_bitmap(target, m_frame_buffer->rect());
 
     if constexpr (ENABLE_STATISTICS_OVERLAY)
         draw_statistics_overlay(target);
@@ -1343,15 +1333,14 @@ void Device::set_raster_position(FloatVector4 const& position, FloatMatrix4x4 co
     m_raster_position.eye_coordinate_distance = eye_coordinates.length();
 }
 
-Gfx::IntRect Device::raster_rect_in_target_coordinates(Gfx::IntSize size)
+Gfx::IntRect Device::get_rasterization_rect_of_size(Gfx::IntSize size)
 {
-    auto const raster_rect = Gfx::IntRect {
+    return {
         static_cast<int>(m_raster_position.window_coordinates.x()),
         static_cast<int>(m_raster_position.window_coordinates.y()),
         size.width(),
         size.height(),
     };
-    return window_coordinates_to_target_coordinates(raster_rect);
 }
 
 }
diff --git a/Userland/Libraries/LibSoftGPU/Device.h b/Userland/Libraries/LibSoftGPU/Device.h
index f2e74b510c..ad1aa0b8c9 100644
--- a/Userland/Libraries/LibSoftGPU/Device.h
+++ b/Userland/Libraries/LibSoftGPU/Device.h
@@ -143,8 +143,7 @@ public:
 
 private:
     void draw_statistics_overlay(Gfx::Bitmap&);
-    Gfx::IntRect raster_rect_in_target_coordinates(Gfx::IntSize size);
-    Gfx::IntRect window_coordinates_to_target_coordinates(Gfx::IntRect const&);
+    Gfx::IntRect get_rasterization_rect_of_size(Gfx::IntSize size);
 
     void rasterize_triangle(const Triangle& triangle);
     void setup_blend_factors();