internal/ui: bug fix: a big-offscreen size could be 0

Closes #2589

internal/ui/image.go

@@ -346,6 +346,10 @@ func (i *bigOffscreenImage) drawTriangles(srcs [graphics.ShaderImageCount]*Image
 		i.region = r
 	}
 
+	if i.region.Width == 0 || i.region.Height == 0 {
+		return
+	}
+
 	if i.image == nil {
 		i.image = NewImage(int(i.region.Width)*bigOffscreenScale, int(i.region.Height)*bigOffscreenScale, i.imageType)
 	}

vector/util_test.go

@@ -0,0 +1,37 @@
+// Copyright 2023 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package vector_test
+
+import (
+	"image/color"
+	"testing"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	t "github.com/hajimehoshi/ebiten/v2/internal/testing"
+	"github.com/hajimehoshi/ebiten/v2/vector"
+)
+
+func TestMain(m *testing.M) {
+	t.MainWithRunLoop(m)
+}
+
+// Issue #2589
+func TestLine0(t *testing.T) {
+	dst := ebiten.NewImage(16, 16)
+	vector.StrokeLine(dst, 0, 0, 0, 0, 2, color.White)
+	if got, want := dst.At(0, 0), (color.RGBA{}); got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+}