c# image resize with new bitmap vs draw image

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what is the difference?

Bitmap newImage = new Bitmap(_currentBitmap, newWidth, newHeight);
return newImag;

AND

Bitmap animage= new Bitmap( newWidth, newHeight);
using (Graphics gr = Graphics.FromImage(animage))
{
gr.SmoothingMode = SmoothingMode.HighQuality;
gr.InterpolationMode = InterpolationMode.HighQualityBicubic;
gr.PixelOffsetMode = PixelOffsetMode.HighQuality;
gr.CompositingQuality = CompositingQuality.HighQuality;
gr.DrawImage(_currentBitmap, new Rectangle(0, 0, newWidth, newHeight));
}
return animage;

Posted 4-Sep-13 16:11pm

scorpzonex

Updated 9-Apr-20 19:47pm

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3 solutions

Solution 3

I thought I would add to this. If you are looking to scale a bitmap, try this:

var bmp = new Bitmap(newWidth, newHeight);
using var g = Graphics.FromImage(bmp);
g.SmoothingMode = SmoothingMode.None;
g.InterpolationMode = InterpolationMode.NearestNeighbor;
g.PixelOffsetMode = PixelOffsetMode.HighQuality;
g.CompositingQuality = CompositingQuality.AssumeLinear;
g.DrawImage(Image, new Rectangle(0, 0, newWidth, newHeight));
return bmp;

Make sure to set the interpolation mode to Nearest neighbor, remove smoothing, and high quality pixel offset. Assume it is a linear scale. For best results, make the new width/height an integer multiple of the original.

Posted 9-Apr-20 19:48pm

drlockett

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Ron Beyer · Accepted Answer · 2013-09-04T16:47:00

Solution 1

Almost nothing, here is the source code of the System.Drawing.Bitmap(Image, int, int) in the .NET framework:

C#

public Bitmap(Image original, int width, int height) : this(width, height) {
            Graphics g = null;
            try {
                g = Graphics.FromImage(this);
                g.Clear(Color.Transparent);
                g.DrawImage(original, 0, 0, width, height);
            }
            finally {
                if (g != null) {
                    g.Dispose();
                }
            }
        }

Posted 4-Sep-13 16:47pm

Ron Beyer

Comments

Sergey Alexandrovich Kryukov 4-Sep-13 23:25pm

Good info, my 5, but there is essential difference: 1) scaling quality, 2) the code you show is not guaranteed. Please see my answer.
—SA

Ron Beyer 5-Sep-13 0:03am

Yup, the interface won't change (may depreciate) but the implementation is not guaranteed to remain the same. Other than setting some GDI flags its almost the same process, but setting the flags does give predictable quality.

Sergey Alexandrovich Kryukov · Accepted Answer · 2013-09-04T17:23:00

Look at the documentation: http://msdn.microsoft.com/en-us/library/334ey5b7.aspx[^].

As you can see, the documentation on this constructor does not specify how the image is scaled, which what interpolation quality. In your second code sample, you can specify it. You can play with the second code sample and figure out how exactly scaling is done (as you can compare final results pixel by pixel). However, you cannot rely on those results: as the fact this detail is not described in documentation, the result may depend on some unknown factors, such as version of the .NET framework. I would always use the second variant of the code, as soon as image quality is important.

—SA