LibSoftGPU: Only render complete primitives

Previously, we were expecting triangles and quads to consist of
complete sets of vertices. However, a more common behavior is to ignore
all vertices that do not make up a full primitive. For example, OpenGL
specifies for `GL_QUADS`:

  "The total number of vertices between Begin and End is 4n + k, where
   0 ≤ k ≤ 3; if k is not zero, the final k vertices are ignored."

This changes the behavior of `Device::draw_primitives()` to both return
early if no full set of vertices was provided, and to ignore any
additional vertices that are not part of a full set.

1 files changed, 6 insertions, 3 deletions

diff --git a/Userland/Libraries/LibSoftGPU/Device.cpp b/Userland/Libraries/LibSoftGPU/Device.cpp
index ef64a5526f..23d15ce1dd 100644
--- a/Userland/Libraries/LibSoftGPU/Device.cpp
+++ b/Userland/Libraries/LibSoftGPU/Device.cpp
@@ -568,7 +568,9 @@ void Device::draw_primitives(PrimitiveType primitive_type, FloatMatrix4x4 const&
     // Let's construct some triangles
     if (primitive_type == PrimitiveType::Triangles) {
         Triangle triangle;
-        for (size_t i = 0; i < vertices.size(); i += 3) {
+        if (vertices.size() < 3)
+            return;
+        for (size_t i = 0; i < vertices.size() - 2; i += 3) {
             triangle.vertices[0] = vertices.at(i);
             triangle.vertices[1] = vertices.at(i + 1);
             triangle.vertices[2] = vertices.at(i + 2);
@@ -578,8 +580,9 @@ void Device::draw_primitives(PrimitiveType primitive_type, FloatMatrix4x4 const&
     } else if (primitive_type == PrimitiveType::Quads) {
         // We need to construct two triangles to form the quad
         Triangle triangle;
-        VERIFY(vertices.size() % 4 == 0);
-        for (size_t i = 0; i < vertices.size(); i += 4) {
+        if (vertices.size() < 4)
+            return;
+        for (size_t i = 0; i < vertices.size() - 3; i += 4) {
             // Triangle 1
             triangle.vertices[0] = vertices.at(i);
             triangle.vertices[1] = vertices.at(i + 1);