How to: Use Interpolation Mode to Control Image Quality During Scaling

 

The interpolation mode of a Graphics object influences the way GDI+ scales (stretches and shrinks) images. The InterpolationMode enumeration defines several interpolation modes, some of which are shown in the following list:

To stretch an image, each pixel in the original image must be mapped to a group of pixels in the larger image. To shrink an image, groups of pixels in the original image must be mapped to single pixels in the smaller image. The effectiveness of the algorithms that perform these mappings determines the quality of a scaled image. Algorithms that produce higher-quality scaled images tend to require more processing time. In the preceding list, NearestNeighbor is the lowest-quality mode and HighQualityBicubic is the highest-quality mode.

To set the interpolation mode, assign one of the members of the InterpolationMode enumeration to the InterpolationMode property of a Graphics object.

The following example draws an image and then shrinks the image with three different interpolation modes.

The following illustration shows the original image and the three smaller images.

Image with Varied Interpolation Settings

        Dim image As New Bitmap("GrapeBunch.bmp")
        Dim width As Integer = image.Width
        Dim height As Integer = image.Height

        ' Draw the image with no shrinking or stretching. Pass in the destination
        ' rectangle (2nd argument), the upper-left corner (3rd and 4th arguments),
        ' width (5th argument),  and height (6th argument) of the source 
        ' rectangle.
        e.Graphics.DrawImage( _
            image, _
            New Rectangle(10, 10, width, height), _
            0, _
            0, _
            width, _
            height, _
            GraphicsUnit.Pixel, _
            Nothing)

        ' Shrink the image using low-quality interpolation. 
        e.Graphics.InterpolationMode = InterpolationMode.NearestNeighbor

        ' Pass in the destination rectangle, and the upper-left corner, width, 
        ' and height of the source rectangle as above.
        e.Graphics.DrawImage( _
        image, _
        New Rectangle(10, 250, CInt(0.6 * width), CInt(0.6 * height)), _
        0, _
        0, _
        width, _
        height, _
        GraphicsUnit.Pixel)

        ' Shrink the image using medium-quality interpolation.
        e.Graphics.InterpolationMode = InterpolationMode.HighQualityBilinear

        ' Pass in the destination rectangle, and the upper-left corner, width, 
        ' and height of the source rectangle as above.
        e.Graphics.DrawImage( _
        image, _
        New Rectangle(150, 250, CInt(0.6 * width), _
        CInt(0.6 * height)), _
        0, _
        0, _
        width, _
        height, _
        GraphicsUnit.Pixel)

        ' Shrink the image using high-quality interpolation.
        e.Graphics.InterpolationMode = InterpolationMode.HighQualityBicubic

        ' Pass in the destination rectangle, and the upper-left corner, width, 
        ' and height of the source rectangle as above.
        e.Graphics.DrawImage( _
            image, _
            New Rectangle(290, 250, CInt(0.6 * width), CInt(0.6 * height)), _
            0, _
            0, _
            width, _
            height, _
            GraphicsUnit.Pixel)

The preceding example is designed for use with Windows Forms, and it requires PaintEventArgs e, which is a parameter of the Paint event handler.

Images, Bitmaps, and Metafiles
Working with Images, Bitmaps, Icons, and Metafiles

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