Clear and scissor issues: #1435

arcade/gl/framebuffer.py

@@ -198,9 +198,10 @@ def _get_scissor(self) -> Optional[Tuple[int, int, int, int]]:
     def _set_scissor(self, value):
         self._scissor = value
 
-        if self._scissor is not None:
-            # If the framebuffer is bound we need to set the scissor box.
-            # Otherwise it will be set on use()
+        if self._scissor is None:
+            if self._ctx.active_framebuffer == self:
+                gl.glScissor(*self._viewport)
+        else:
             if self._ctx.active_framebuffer == self:
                 gl.glScissor(*self._scissor)
 
@@ -351,11 +352,12 @@ def clear(
         """
         Clears the framebuffer::
 
-            # Clear framebuffer using the color red in normalized form
-            fbo.clear(color=(1.0, 0.0, 0.0, 1.0), normalized=True)
             # Clear the framebuffer using arcade's colors (not normalized)
             fb.clear(color=arcade.color.WHITE)
 
+            # Clear framebuffer using the color red in normalized form
+            fbo.clear(color=(1.0, 0.0, 0.0, 1.0), normalized=True)
+
         If the background color is an ``RGB`` value instead of ``RGBA```
         we assume alpha value 255.
 
@@ -365,8 +367,12 @@ def clear(
         :param Tuple[int, int, int, int] viewport: The viewport range to clear
         """
         with self.activate():
+            scissor_values = self._scissor
+
             if viewport:
-                gl.glScissor(*viewport)
+                self.scissor = viewport
+            else:
+                self.scissor = None
 
             if normalized:
                 # If the colors are already normalized we can pass them right in
@@ -388,11 +394,7 @@ def clear(
             else:
                 gl.glClear(gl.GL_COLOR_BUFFER_BIT)
 
-            if viewport:
-                if self._scissor is None:
-                    gl.glScissor(*self._viewport)
-                else:
-                    gl.glScissor(*self._scissor)
+            self.scissor = scissor_values
 
     def read(
         self, *, viewport=None, components=3, attachment=0, dtype="f1"

tests/unit2/test_opengl_framebuffer.py

@@ -63,6 +63,31 @@ def test_clear(ctx):
     assert ctx.active_framebuffer == ctx.screen
 
 
+def test_clear_viewport(ctx):
+    fb = create(ctx, 4, 4, components=1)
+    fb.clear(color=(64, 64, 64, 64))
+    assert fb.read(components=1) == b'\x40' * 16
+
+    # Clear only the center pixels and verify that the rest is unchanged
+    fb.clear()
+    fb.clear(color=(255, 255, 255, 255), viewport=(1, 1, 2, 2))
+    expected = (
+        b'\x00\x00\x00\x00'
+        b'\x00\xff\xff\x00'
+        b'\x00\xff\xff\x00'
+        b'\x00\x00\x00\x00'
+    )
+    assert bytes(fb.read(components=1)) == expected
+
+
+def test_clear_with_scissor(ctx):
+    fb = create(ctx, 4, 4, components=1)
+    fb.clear()
+    fb.scissor = 1, 1, 2, 2
+    fb.clear(color=(255, 255, 255, 255))
+    assert bytes(fb.read(components=1)) == b'\xff' * 16
+
+
 def test_multi_attachment(ctx):
     """Create framebuffers with multiple layers"""
     for i in range(ctx.info.MAX_COLOR_ATTACHMENTS):