HOWTO: Crop Image Margins Automatically

Recently we have been asked how to implement auto-crop operation. I believe that code better than words in most cases, so here is simple sample. It is written in C# and has unsafe blocks, so you should have "Project Properties->Build->Allow unsafe code" option turned on.

Usage:

• Load RGB image (the sample doesn't support transparency or CMYK/indexed/grayscale images);

• Choose color which to treat as "empty space";

• Press "Find Margins" button.

The code inside is very straightforward. It iterates through the input picture along each side searching for any pixel which is not equal to the specified "margin color".

The result should looks like on the screenshot below. Feel free to post your comments (or maybe, bugreports? ;)).

Edited by moderator Monday, May 28, 2012 8:23:31 PM(UTC)  | Reason: Not specified

File Attachment(s):

AutoCropSample.zip (12kb) downloaded 679 time(s).

Alex Kon attached the following image(s):

You can use the same code with minor changes. Instead of comparison with "emptyColor" you should check pixel's alpha channel in MarginsTracer class. For example, method which inspects image scanline should be changed in such way:

private static bool rowIsEmpty(BitmapData raster, int rowIndex, RgbColor emptyColor)
{
    int w = raster.Width;
    int j = 0;

    unsafe
    {
        byte* pAlpha= (byte*)raster.Scan0.ToPointer() + rowIndex * raster.Stride + 3 /*skip 3 color channels to get alpha*/;
        for (; j < w; j++)
        {
            if (*pAlpha> 0)
                break;

            pAlpha+= 4; //Shift to the next pixel in 32bppArgb.
        }
    }
    return (j >= w);
}

columnIsEmpty() method should be modified in the same way. Also, do not forget to check that PixelFormat of source image is Format32bppArgb. Otherwise you can get memory access violation errors.

Best regards,

Alex Kon

Edited by moderator Friday, July 8, 2011 6:04:50 PM(UTC)  | Reason: Not specified

Unfortunately we do not have the VB.NET project.

I've tried to google your problem and I found this topic:

http://stackoverflow.com...-sharp-project-to-vb-net

You can use some "C# to VB.NET" code converter.

Best regards,

Vitaly Kustov

Aurigma Technical Support

It turns out pointers are not supported in VB.Net so there is no easy way to convert it.

I was able to compile the C# as a Class Library and reference it in VB. The only problem is I tried making the same changes to columnIsEmpty() as Alex made for rowIsEmpty() but it isn't working. Could you post the correct code for columnIsEmpty() to scan the alpha channel instead of checking for a color match?

Thanks,

Eddie

Sorry for the long delayed. I had a holiday.

Thanks a lot for your code snippet.

Best regards,

Vitaly Kustov

Aurigma Technical Support