How Can I Efficiently Manipulate Bitmaps in C#?

Fast bitmap processing in C#

When working with large bitmaps, accessing and performing operations on a pixel-by-pixel basis may impact performance. C#'s built-in Bitmap.GetPixel() and Bitmap.SetPixel() methods are convenient but slow. This article explores alternative ways to quickly convert a bitmap to a byte array and back to a bitmap, allowing for efficient pixel operations.

Convert bitmap to byte array

• LockBits method: This method allows direct access to the bitmap's internal data buffer. By using unsafe code, you can access the raw pixel data as pointers, allowing for fast operations. However, this approach requires careful memory management.
• Marshaling: Marshaling involves copying data between managed and unmanaged memory. Using the Marshal.Copy() method you can copy pixel data from a bitmap buffer into a byte array. Although marshaling does not require unsafe code, it may be slightly slower than the LockBits method.

Convert byte array to bitmap

• LockBits method: After modifying the pixel data in the byte array, it can be copied back to the bitmap through the LockBits method. This is similar to converting a bitmap into a byte array.
• Marshaling: You can use the Marshal.Copy() method to copy modified pixel data from a byte array back to a bitmap buffer.

Performance comparison

• LockBits: faster, requires unsafe code
• Marshaling: Slower, but no need for unsafe code

LockBits method example

using System;
using System.Drawing;
using System.Runtime.InteropServices;

public unsafe class FastBitmap
{
    public static Image ThresholdUA(float thresh)
    {
        Bitmap b = new Bitmap(_image);

        BitmapData bData = b.LockBits(new Rectangle(0, 0, _image.Width, _image.Height), ImageLockMode.ReadWrite, b.PixelFormat);

        byte bitsPerPixel = GetBitsPerPixel(bData.PixelFormat);

        byte* scan0 = (byte*)bData.Scan0.ToPointer();

        for (int i = 0; i < ... }

Example of marshaling method

using System;
using System.Drawing;
using System.Runtime.InteropServices;

public class FastBitmap
{
    public static Image ThresholdMA(float thresh)
    {
        Bitmap b = new Bitmap(_image);

        BitmapData bData = b.LockBits(new Rectangle(0, 0, _image.Width, _image.Height), ImageLockMode.ReadWrite, b.PixelFormat);

        byte bitsPerPixel = GetBitsPerPixel(bData.PixelFormat);

        int size = bData.Stride * bData.Height;

        byte[] data = new byte[size];

        Marshal.Copy(bData.Scan0, data, 0, size);

        for (int i = 0; i < ... }

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