why using viewport.twistangle affects viewport.viewcenter

[mohamedalmaz6]

i am trying to rotate ucs to algine with rotated rectangle in modelspace but when using viewport.twistangle it affects viewport.viewcenter although it algin with center of view if i didnot use viewport.twist angle ot the angle is zero (horizontal rectangle as example)

    Autodesk.AutoCAD.DatabaseServices.ObjectId vpid = layout1.GetViewports()[1];

    Viewport vp = ts.GetObject(vpid, OpenMode.ForWrite) as Viewport;

    var WCS2DCS =
Matrix3d.WorldToPlane(vp.ViewDirection) *
Matrix3d.Displacement(vp.ViewTarget.GetAsVector().Negate()) *
Matrix3d.Rotation(vp.TwistAngle, vp.ViewDirection, vp.ViewTarget);


    var centertrans = centerpoint.TransformBy(WCS2DCS);

    double vpRatio = vp.Width / vp.Height;
    double extWidth = width;
    double extHeight = height;
    double extRatio = extWidth / extHeight;
    double scale = extRatio < vpRatio ? vp.Height / extHeight : vp.Width / extWidth;
    vp.TwistAngle = Math.PI * 2 - angle;
    vp.ViewCenter = new Point2d(centertrans.X, centertrans.Y);
    vp.ViewDirection = Vector3d.ZAxis;
    vp.CustomScale=scale;

@_gile 

[_gile · Accepted solution]

Hi,

What about setting the Viewport.TwistAngle before computing the WCS2DCS matrix?

[mohamedalmaz6]

yes your idea has worked

thanks gile

but why it affects it 

how just computing the transformation matirx overwrite anything

[_gile · Accepted solution]

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@mohamedalmaz6 wrote:

yes your idea has worked

thanks gile

but why it affects it 

how just computing the transformation matirx overwrite anything

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If you attentively read the transformation matrix code, you can see it is built from the the viewport ViewDirection, ViewTarget and TwistAngle values. That's why you have to set these properties before building the transformation matrix and use it to transform the 'centerpoint'.