Question

我有一个大的DrawingVisual添加到Canvas中，我想在单击鼠标时获取像素的颜色值。我尝试将整个Visaul渲染到RenderTargetBitmap但得到了“Out of memory”异常。然后使用我在How to render (bitmap) only part of a Visual?

找到的示例

我尝试只渲染部分Visual，但我没有得到正确的像素值。目前我的代码看起来像这样

private void OnMouseDown(object sender, System.Windows.Input.MouseEventArgs e)
{
    // Retreive the coordinates of the mouse button event.
    Point pt = e.GetPosition((UIElement)sender);

    VisualBrush vb = new VisualBrush(myDrawingVisual);
    vb.ViewboxUnits = BrushMappingMode.Absolute;

    // create rect from the position in the canvas - starting point of the DrawingVisual
    vb.Viewbox = new Rect(point.X - myDrawingVisual.ContentBounds.Left,point.Y - myDrawingVisual.ContentBounds.Top, 1, 1);
    vb.ViewportUnits = BrushMappingMode.Absolute;
    vb.Viewport = new Rect(point, new Size(1, 1));

    System.Windows.Shapes.Rectangle  r = new System.Windows.Shapes.Rectangle();
    r.Width = 1;
    r.Height = 1;
    r.Fill = vb;

    // Use RenderTargetBitmap to get the visual, in case the image has been transformed.
    var renderTargetBitmap = new RenderTargetBitmap(1,
                                                    1,
                                                    96, 96, PixelFormats.Default);
    renderTargetBitmap.Render(r);
    var pixels = new byte[4];
    renderTargetBitmap.CopyPixels(pixels, 4, 0);
}

Answer 1

你还没有布置矩形。有关详细信息，请参阅MSDN上的Layout文章。

添加以下行：

r.Measure(new Size(1, 1));
r.Arrange(new Rect(0, 0, 1, 1));

var renderTargetBitmap = ...
renderTargetBitmap.Render(r);

仅渲染Visual的一部分

1 个答案: