Reset Scale in a GDI+ Transformation Matrix

gdi+netscalingtransformwinforms

I'm writing a function to draw UI handles (rotate, resize and so on) in my client area in a Windows Forms application. The function is called when an object is selected.

The caller sends me a Graphics object properly rotated, scaled and translated to fit into the larger scheme of things (the selected object has its own rotation/translation/scale and the UI handle has a relative translation and rotation w.r.t the selected object). Now, I want my UI handles to be the same size regardless of the scale of the parent (selected object).

How do I eliminate/reset the scale factor in the transformation matrix? How do I reset to a scale of 1 while retaining the precious translation and rotation values?

Best Answer

Anti-Grain Geometry uses a basic method for determining the scaling of a transformation (implementation found in agg_trans_affine.cpp). It does so by:

Calculating the rotation of the transformation
Duplicating the transformation and applying the opposite rotation
Transforming two known points and calculating the scale from the result

Translated to C#, this looks like:

Matrix transform = (Matrix)graphics.Transform.Clone();

PointF[] rotationPoints = new PointF[] { new PointF(0, 0), new PointF(1, 0) };
transform.TransformPoints(rotationPoints);

double rotationRadians = Math.Atan2(rotationPoints[1].Y - rotationPoints[0].Y, rotationPoints[1].X - rotationPoints[0].X);
transform.Rotate((float)(-rotationRadians * (180.0 / Math.PI)));

PointF[] scalePoints = new PointF[] { new PointF(0, 0), new PointF(1, 1) };
transform.TransformPoints(scalePoints);

float xScale = scalePoints[1].X - scalePoints[0].X;
float yScale = scalePoints[1].Y - scalePoints[0].Y;

The AGG code also contains a warning that there are degenerate cases where this will not work correctly, but it may be useful for your situation.

Related Solutions

C# – A generic error occurred in GDI+, JPEG Image to MemoryStream

OK I seem to have found the cause just by sheer luck and its nothing wrong with that particular method, it's further back up the call stack.

Earlier I resize the image and as part of that method I return the resized object as follows. I have inserted two calls to the above method and a direct save to a file.

// At this point the new bitmap has no MimeType
// Need to output to memory stream
using (var m = new MemoryStream())
{
       dst.Save(m, format);

       var img = Image.FromStream(m);

       //TEST
       img.Save("C:\\test.jpg");
       var bytes = PhotoEditor.ConvertImageToByteArray(img);


       return img;
 }

It appears that the memory stream that the object was created on has to be open at the time the object is saved. I am not sure why this is. Is anyone able to enlighten me and how I can get around this.

I only return from a stream because after using the resize code similar to this the destination file has an unknown mime type (img.RawFormat.Guid) and Id like the Mime type to be correct on all image objects as it makes it hard write generic handling code otherwise.

EDIT

This didn't come up in my initial search but here's the answer from Jon Skeet

Don’t scale the child movieClip

Try this:

inside_mc.addEventListener(Event.ENTER_FRAME, function(){
    inside_mc.scaleX = 1/inside_mc.parent.scaleX;
    inside_mc.scaleY = 1/inside_mc.parent.scaleY;
});

Best Answer

Related Solutions

C# – A generic error occurred in GDI+, JPEG Image to MemoryStream

Don’t scale the child movieClip

Related Topic