C# How to crop an image without using lots of memory?

Question

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How to crop huge image

There is a question asking how to crop an image here

Ive read it and used the answers. But they all seem to require loading the image into a Bitmap or Image.

This is causing memory issues for me. As Im trying to crop 5 images (8000 x 8000) into tiles. One at a time.

Correct me if im wrong, but thats 8000x8000x4 bytes = 244 MB. per image.

And randomly I get out of memory problem.

How can I get a 1000x1000 image from another image, with reduced memory consumption.

Answer 1

So, this is a decidedly non-trivial thing to do - basically, you'd have to re-implement the image decoder for a given image format. This is not simple.

For the "simple" Windows BMP format, there are these beasts to contend with:

• BMP File Format
• DIB Header Structure

That said, I had to give it a try over my lunch break...here's what I was able to come up with, in a nice LINQPad-ready script.

(NOTE: Windows BMP only!)

void Main()
{
    // Carve out a 100x100 chunk
    var top = 100;
    var left = 100;
    var bottom = 300;
    var right = 300;

    // For BMP only - open input
    var fs = File.OpenRead(@"c:\temp\testbmp.bmp");

    // Open output
    if(File.Exists(@"c:\temp\testbmp.cropped.bmp")) File.Delete(@"c:\temp\testbmp.cropped.bmp");
    var output = File.Open(@"c:\temp\testbmp.cropped.bmp", FileMode.CreateNew);
    var bw = new BinaryWriter(output);

    // Read out the BMP header fields
    var br = new BinaryReader(fs);
    var headerField = br.ReadInt16();
    var bmpSize = br.ReadInt32();
    var reserved1 = br.ReadInt16();
    var reserved2 = br.ReadInt16();
    var startOfData = br.ReadInt32();   

    // Read out the BMP DIB header
    var header = new BITMAPV5Header();  
    var headerBlob = br.ReadBytes(Marshal.SizeOf(header));
    var tempMemory = Marshal.AllocHGlobal(Marshal.SizeOf(header));
    Marshal.Copy(headerBlob, 0, tempMemory, headerBlob.Length);
    header = (BITMAPV5Header)Marshal.PtrToStructure(tempMemory, typeof(BITMAPV5Header));
    Marshal.FreeHGlobal(tempMemory);

    // This file is a 24bpp rgb bmp, 
    var format = PixelFormats.Bgr24;
    var bytesPerPixel = (int)(format.BitsPerPixel / 8);
    Console.WriteLine("Bytes/pixel:{0}", bytesPerPixel);

    // And now I know its dimensions
    var imageWidth = header.ImageWidth;
    var imageHeight = header.ImageHeight;
    Console.WriteLine("Input image is:{0}x{1}", imageWidth, imageHeight);

    var fromX = left;
    var toX = right;
    var fromY = imageHeight - top;
    var toY = imageHeight - bottom;

    // How "long" a horizontal line is
    var strideInBytes = imageWidth * bytesPerPixel;
    Console.WriteLine("Stride size is:0x{0:x}", strideInBytes);

    // new size
    var newWidth = Math.Abs(toX - fromX);
    var newHeight = Math.Abs(toY - fromY);
    Console.WriteLine("New slice dimensions:{0}x{1}", newWidth, newHeight);

    // Write out headers to output file 
    {
        // header = "BM" = "Windows Bitmap"
        bw.Write(Encoding.ASCII.GetBytes("BM"));    
        var newSize = 14 + Marshal.SizeOf(header) + (newWidth * newHeight * bytesPerPixel);
        Console.WriteLine("New File size: 0x{0:x} bytes", newSize);
        bw.Write((uint)newSize);    
        // 2 reserved shorts
        bw.Write((ushort)0);    
        bw.Write((ushort)0);            
        // offset to "data"
        bw.Write(header.HeaderSize + 14);

        // Tweak size in header to cropped size
        header.ImageWidth = newWidth;
        header.ImageHeight = newHeight;

        // Write updated DIB header to output
        tempMemory = Marshal.AllocHGlobal(Marshal.SizeOf(header));
        Marshal.StructureToPtr(header, tempMemory, true);
        byte[] asBytes = new byte[Marshal.SizeOf(header)];
        Marshal.Copy(tempMemory, asBytes, 0, asBytes.Length);
        Marshal.FreeHGlobal(tempMemory);
        bw.Write(asBytes);
        asBytes.Dump();
    }

    // Jump to where the pixel data is located (on input side)
    Console.WriteLine("seeking to position: 0x{0:x}", startOfData);
    fs.Seek(startOfData, SeekOrigin.Begin);

    var sY = Math.Min(fromY, toY);
    var eY = Math.Max(fromY, toY);
    for(int currY = sY; currY < eY; currY++)
    {
        long offset =  startOfData + ((currY * strideInBytes) + (fromX * bytesPerPixel));
        fs.Seek(offset, SeekOrigin.Begin);      

        // Blast in each horizontal line of our chunk
        var lineBuffer = new byte[newWidth * bytesPerPixel];
        int bytesRead = fs.Read(lineBuffer, 0, lineBuffer.Length);
        output.Write(lineBuffer, 0, lineBuffer.Length);
    }

    fs.Close();
    output.Close();
}

[StructLayout(LayoutKind.Sequential, Pack=0)]
public struct BITMAPV5Header 
{
    public uint HeaderSize;
    public int ImageWidth;
    public int ImageHeight;
    public ushort Planes;
    public ushort BitCount;

    [MarshalAs(UnmanagedType.ByValArray, SizeConst=36)]
    public byte[] DontCare;
}