WPF, ListView. RenderTargetBitmap - how to render whole ListView, not limited to screen

I have ListView, which displays some pictures. Not all pictures are visible on screen (in Window). I want to make something like 'contact sheet' - all pictures.

I got ScrollViewer from ListView, and got its full height (scrVwr.ExtentHeight). So, I call render:

    var bmp = new RenderTargetBitmap(scrVwr.Width, scrVwr.ExtentHeight, 96, 96, PixelFormats.Pbgra32);    bmp.Render(scrVwr);

After saving

    var encoder = new JpegBitmapEncoder();    encoder.Frames.Add(BitmapFrame.Create(bmp));    var fileStream = IO.File.Create(picker.FileName);    encoder.Save(fileStream);

I got this:saved JPG

(so only ListView.ActualHeight is rendered)

I want to have rendered whole ListView/ScrollViewer (all items).