Fix GPU collision missing sprites flipped with negative scale

CHANGELOG.md

@@ -26,6 +26,7 @@ Arcade [PyPi Release History](https://pypi.org/project/arcade/#history) page.
 - Fixed `SpriteList.pop()` with a negative index other than `-1` (such as `pop(-2)`) drawing the wrong sprites: the removed sprite stayed on screen and another sprite disappeared.
 - Fixed `SpriteList.rescale()` moving the list's center while rescaling, so sprites after the first were scaled around the wrong point.
 - Fixed `SpriteList.preload_textures()` raising `AttributeError` on a lazy sprite list that hadn't been drawn yet. It now preloads into the atlas the list will use.
+- Fixed GPU collision checks (`CollisionMethod.GPU`, also used automatically for sprite lists over 1500 sprites without a spatial hash) missing sprites flipped with a negative scale, whose negative width or height made them look smaller than they are.
 
 ### New Features
 - Added `HitBox.get_adjusted_bounds()`, which returns the cached `(left, right, bottom, top)` bounds of the adjusted hit box points.

arcade/resources/system/shaders/collision/col_tex_trans_gs.glsl

@@ -19,7 +19,8 @@ void main() {
 
     // Get the maximum x and y size
     // max() works per component
-    vec2 size = max(size[0], check_size);
+    // Sizes are negative for sprites flipped with a negative scale
+    vec2 size = max(abs(size[0]), abs(check_size));
 
     // Destroy the sprite if too far away
     if (dist < max(size.x, size.y) * 1.42) {

arcade/resources/system/shaders/collision/col_trans_gs.glsl

@@ -19,7 +19,8 @@ void main() {
 
     // Get the maximum x and y size
     // max() works per component
-    vec2 size = max(size[0], check_size);
+    // Sizes are negative for sprites flipped with a negative scale
+    vec2 size = max(abs(size[0]), abs(check_size));
 
     // Destroy the sprite if too far away
     if (dist < max(size.x, size.y) * 1.42) {

tests/unit/sprite/test_sprite_collision.py

@@ -499,6 +499,41 @@ def test_check_for_collision_between_lists_type_errors(window):
         arcade.check_for_collision_between_lists(arcade.SpriteList(), "moo")
 
 
+@pytest.mark.parametrize(
+    "wall_scale, bullet_scale",
+    [
+        ((1, 1), (1, 1)),
+        ((-1, 1), (1, 1)),
+        ((1, -1), (1, 1)),
+        ((-1, -1), (1, 1)),
+        ((1, 1), (-1, -1)),
+        ((-1, -1), (-1, -1)),
+    ],
+)
+def test_gpu_collision_flipped_sprites(window, wall_scale, bullet_scale):
+    """The GPU path finds sprites flipped with a negative scale"""
+    if window.ctx._gl_api == "webgl":
+        pytest.skip("GPU collision isn't supported on WebGL")
+
+    # A long wall and a small bullet near its end, so only the wall's own
+    # size can bring them close enough
+    wall = arcade.SpriteSolidColor(200, 20)
+    wall.scale = wall_scale
+    walls = arcade.SpriteList()
+    walls.append(wall)
+    bullet = arcade.SpriteSolidColor(4, 4, center_x=80)
+    bullet.scale = bullet_scale
+
+    assert walls.get_nearby_sprites_gpu(bullet.position, bullet.size) == [wall]
+    gpu = arcade.CollisionMethod.GPU
+    assert arcade.check_for_collision_with_list(bullet, walls, method=gpu) == [wall]
+
+    # And still not when it's far away
+    bullet.position = 400, 0
+    assert walls.get_nearby_sprites_gpu(bullet.position, bullet.size) == []
+    assert arcade.check_for_collision_with_list(bullet, walls, method=gpu) == []
+
+
 def test_check_for_collision_with_list(window):
     # TODO: Check that the right collision function is called internally
     a = arcade.SpriteSolidColor(50, 50, color=arcade.csscolor.RED)