how to draw N lines per inch?

my code below is supposed to display N lines per inch. instead i get a little more than N lines per inch. the distance between lines is somewhat smaller. in addition, changing the screen resolution makes the distance between lines change too. does anyone know how to handle that?

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;

namespace MyApp
{
    class MyControl : Control
    {
        private readonly ContainerVisual container = new ContainerVisual();
        private readonly DrawingVisual drawing = new DrawingVisual();

        private void RenderDrawing()
        {
            var s = PresentationSource.FromVisual(this);
            var dpiX = 96 * s.CompositionTarget.TransformToDevice.M11;
            var dpiY = 96 * s.CompositionTarget.TransformToDevice.M22;

            double N = 1;

            using (var c = drawing.RenderOpen())
            {
                var p = new Pen(new SolidColorBrush(Colors.Black), 1);

                for (int i = 0; i < 10; i++)
                {
                    var x = i * dpiX / N;
                    c.DrawLine(p, new Point(x, 0), new Point(x, 100));
                }
            }
        }

        protected overr开发者_如何学JAVAide Size ArrangeOverride(Size s)
        {
            RenderDrawing();
            return s;
        }

        protected override Visual GetVisualChild(int index)
        {
            return container;
        }

        protected override Size MeasureOverride(Size s)
        {
            return new Size();
        }

        protected override int VisualChildrenCount
        {
            get { return 1; }
        }

        public MyControl()
        {
            container.Children.Add(drawing);
            AddVisualChild(container);
        }
    }
}

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this article seems to discuss the same problem: WPF DPI issues there is no solution to this problem other than asking the user to set the correct DPI settings that correspond to the physical DPI of the screen. a workaround that i found that makes life a little easier is to use WPF application level scaling as described here: http://www.odewit.net/ArticleContent.aspx?id=WpfDpiScaling&lang=en&format=html

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Not sure if this would accomplish what you are looking for, but it should help if the issue has to do with rounding. As I said in comments, your code looks mostly correct, I think it is a rounding issue with the dpi computation. Since you are desiring to render based on 96 dpi, compute the coordinates based on 96dpi and then convert the points to your device. I wrote this more for clarity, you could use a single array of points and just remember that i is the start point and i+1 is the end point and then you would only have to make a single call to transform the points.

    private void RenderDrawing()
    {
        var s = PresentationSource.FromVisual(this);
        var dpiX = 96;

        int numberOfLines = 10;
        double N = 1;
        double spacing = dpiX / N;

        var startPoints = new Point[numberOfLines]();
        var endPoints = new Point[numberOfLines]();
        for (int i = 0; i < numberOfLines; i++)
        {
            var x = i * spacing;
            startPoints[i] = new Point(x, 0);
            endPoints[i] = new Point(x, 100);                 
        }
        s.CompositionTarget.TransformToDevice.Transform(startPoints);
        s.CompositionTarget.TransformToDevice.Transform(endPoints);

        using (var c = drawing.RenderOpen())
        {
            using (var p = new Pen(new SolidColorBrush(Colors.Black), 1))
            {
                 for(int i=0; i < numberOfLines; i++)
                 {
                    c.DrawLine(p, startPoints[i], endPoints[i]);
                 }
            }
        }
    }