@teatwig Please, could you post some image of the resulting drawn rectangle? Afaik, it's not possible to really draw a sub-pixel element... but I could be wrong...

Also note that DrawRectangleRounded() was reviewed recently, you should test it with latest raylib version.

I tested it again with the latest commit from master (08b089f).

This is my program:

int main(void)
{
    const int width = 800;
    const int height = 600;

    InitWindow(width, height, "rounded rect test");

    Camera2D camera = { 0 };
    camera.zoom = height;

    Rectangle rect = { 0.1f, 0.1f, 0.8f, 0.8f };

    while (!WindowShouldClose())
    {
        BeginDrawing();
            ClearBackground(RAYWHITE);
            BeginMode2D(camera);

                DrawRectangleRounded(rect, 0.4f, 3, BLACK);

            EndMode2D();
        EndDrawing();
    }

    CloseWindow();
    return 0;
}

Unchanged raylib:

With the following patch that disables the check:

--- a/src/rshapes.c
+++ b/src/rshapes.c
@@ -884,7 +884,7 @@ void DrawRectangleLinesEx(Rectangle rec, float lineThick, Color color)
 void DrawRectangleRounded(Rectangle rec, float roundness, int segments, Color color)
 {
     // Not a rounded rectangle
-    if ((roundness <= 0.0f) || (rec.width < 1) || (rec.height < 1 ))
+    if (roundness <= 0.0f)
     {
         DrawRectangleRec(rec, color);
         return;

@teatwig should a pixel have rounded corners? please, could you explain some use-case?

I think we shouldn't think of the dimensions of Rectangle as pixels since you can use the camera to zoom in on it, thus making a rectangle that has a width of 0.1 show up as 100 pixels on screen.
For my project I decided to have world coordinates between 0 and 1 to make it easier to think of positioning as a percentage of the screen, so all shapes have a size of ~0.01.

I was mostly wondering why this check exists, and it's nice that I can adjust raylib so it works slightly better for my use case 😄