Crop image without pixel colour change

Paul Turner

i have been using the following code to crop images:

public static System.Drawing.Image cropImage(System.Drawing.Image img, Rectangle cropArea)
{
Bitmap bmpImage = new Bitmap(img);
Bitmap bmpCrop = bmpImage.Clone(cropArea, bmpImage.PixelFormat);
return (System.Drawing.Image)(bmpCrop);
}

which works fine except that the pixels RGB colour values change when you compare the cropped image to the original. i need them to stay exactly the same. it seems that by default the new image created has a smaller colour palette, and i have found ways to get better results, but not a way which just gives the same colour palette and the same RGB values for the pixels in the new cropped image which you create. it seems crazy to me that you can't do this.

one way around this would be to avoid creating a new image but instead to actually crop the original image but again i cant find anything on this.

is there anyone really clever that can help me here?

6 Replies

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Javeed M Shaikh15y ago
Other options:

http://imageresizing.net/

http://getlara.com/north-america/canada/alberta/edmonton/post/2008/10/13/png-jpg-gif-image-resize-with-net-with-transparency

http://glennjones.net/2005/10/high-quality-dynamically-resized-images-with-dot-net/

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Javeed M Shaikh15y ago
Hi Paul,

Let me look into other options for this, also did you try using the PixelFormat option while creating the new Bitmap?

not sure if this helps.

http://www.gutgames.com/post/Getting-an-HTML-Based-Color-Palette-from-an-Image-in-C.aspx
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Paul Turner15y ago
i dont think this will work either as it also creates a new image for the cropped area. from what i have read, i think the problem occurs when a new image is created because you cannot specify that the new image has the same color palette as the original. asp.net doesn't allow you to copy the color palette from the original image to the new cropped image, and by default a smaller palette is used with less colours so some pixel colours get changed in order to fit the new palette.

the two ways i can see to solve the problems would be;

1. copy the colour palette from the original image to the new one,

or

2. crop the original image - do not create a new cropped image from it,

i cant see how to do either of the above though!
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Paul Turner15y ago
Hi Javeed

thanks for your quick response. unfortunately this doesn't seem to do any different, the pixels still change colour. i had better show you the other part of the code which i use to call this method in case you think the problem lies here:

System.Drawing.Image i = System.Drawing.Image.FromFile(path);
i = SupersizePrint.DB.cropImage(i, CropRectangle);

ImageCodecInfo myImageCodecInfo;
System.Drawing.Imaging.Encoder myEncoder;
EncoderParameter myEncoderParameter;
EncoderParameters myEncoderParameters;

myImageCodecInfo = GetEncoderInfo("image/jpeg");
myEncoder = System.Drawing.Imaging.Encoder.Quality;
myEncoderParameters = new EncoderParameters(1);

// Save the bitmap as a JPEG file with quality level 100.
myEncoderParameter = new EncoderParameter(myEncoder, 100L);
myEncoderParameters.Param[0] = myEncoderParameter;
i.Save(path, myImageCodecInfo, myEncoderParameters);
i.Dispose();
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Prabhu Raja15y ago
Also try this Article : Click Here

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Javeed M Shaikh15y ago
Hi Paul,

can you try this:

public static System.Drawing.Image cropImage(System.Drawing.Image img, Rectangle cropArea)
{

Bitmap target = new Bitmap(cropArea.Width, cropArea.Height);

using (Graphics g = Graphics.FromImage(target))
{
g.InterpolationMode = InterpolationMode.HighQualityBicubic;
g.DrawImage(img, new Rectangle(0, 0, target.Width, target.Height), cropArea, GraphicsUnit.Pixel);

}

return img;
}

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