在C＃中快速使用Bitmaps

Question

我需要访问Bitmap的每个像素，使用它们，然后将它们保存到Bitmap。

使用Bitmap.GetPixel()和Bitmap.SetPixel()，我的程序运行缓慢。

如何快速将Bitmap转换为byte[]并返回？

我需要byte[] length = (4 * width * height)，其中包含每个像素的RGBA数据。

Answer 1

你可以通过几种不同的方式做到这一点。您可以使用unsafe直接访问数据，也可以使用编组来回复制数据。不安全的代码更快，但封送不需要不安全的代码。这是performance comparison我做了一段时间。

以下是使用lockbits的完整示例：

/*Note unsafe keyword*/
public unsafe Image ThresholdUA(float thresh)
{
    Bitmap b = new Bitmap(_image);//note this has several overloads, including a path to an image

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

    byte bitsPerPixel = GetBitsPerPixel(bData.PixelFormat);

    /*This time we convert the IntPtr to a ptr*/
    byte* scan0 = (byte*)bData.Scan0.ToPointer();

    for (int i = 0; i < bData.Height; ++i)
    {
        for (int j = 0; j < bData.Width; ++j)
        {
            byte* data = scan0 + i * bData.Stride + j * bitsPerPixel / 8;

            //data is a pointer to the first byte of the 3-byte color data
            //data[0] = blueComponent;
            //data[1] = greenComponent;
            //data[2] = redComponent;
        }
    }

    b.UnlockBits(bData);

    return b;
}

这是同样的事情，但是编组：

/*No unsafe keyword!*/
public 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);

    /* GetBitsPerPixel just does a switch on the PixelFormat and returns the number */
    byte bitsPerPixel = GetBitsPerPixel(bData.PixelFormat);

    /*the size of the image in bytes */
    int size = bData.Stride * bData.Height;

    /*Allocate buffer for image*/
    byte[] data = new byte[size];

    /*This overload copies data of /size/ into /data/ from location specified (/Scan0/)*/
    System.Runtime.InteropServices.Marshal.Copy(bData.Scan0, data, 0, size);

    for (int i = 0; i < size; i += bitsPerPixel / 8 )
    {
        double magnitude = 1/3d*(data[i] +data[i + 1] +data[i + 2]);

        //data[i] is the first of 3 bytes of color

    }

    /* This override copies the data back into the location specified */
    System.Runtime.InteropServices.Marshal.Copy(data, 0, bData.Scan0, data.Length);

    b.UnlockBits(bData);

    return b;
}

Answer 2

您可以使用Bitmap.LockBits方法。此外，如果要使用并行任务执行，可以使用System.Threading.Tasks命名空间中的Parallel类。以下链接有一些示例和解释。

• http://csharpexamples.com/fast-image-processing-c/
• http://msdn.microsoft.com/en-us/library/dd460713%28v=vs.110%29.aspx
• http://msdn.microsoft.com/tr-tr/library/system.drawing.imaging.bitmapdata%28v=vs.110%29.aspx

Answer 3

你想要LockBits。然后，您可以从它为您提供的BitmapData对象中提取所需的字节。

Answer 4

如果您使用的是C＃8.0，建议您使用新的Span<T>以提高效率。

这是一个粗略的实现

public unsafe class FastBitmap : IDisposable
{
    private Bitmap _bmp;
    private ImageLockMode _lockmode;
    private int _pixelLength;

    private Rectangle _rect;
    private BitmapData _data;
    private byte* _bufferPtr;

    public int Width { get => _bmp.Width; }
    public int Height { get => _bmp.Height; }
    public PixelFormat PixelFormat { get => _bmp.PixelFormat; }

    public FastBitmap(Bitmap bmp, ImageLockMode lockMode)
    {
        _bmp = bmp;
        _lockmode = lockMode;

        _pixelLength = Image.GetPixelFormatSize(bmp.PixelFormat) / 8;
        _rect = new Rectangle(0, 0, Width, Height);
        _data = bmp.LockBits(_rect, lockMode, PixelFormat);
        _bufferPtr = (byte*)_data.Scan0.ToPointer();
    }

    public Span<byte> this[int x, int y]
    {
        get
        {
            var pixel = _bufferPtr + y * _data.Stride * x * _pixelLength;
            return new Span<byte>(pixel, _pixelLength);
        }
        set
        {
            value.CopyTo(this[x, y]);
        }
    }

    public void Dispose()
    {
        _bmp.UnlockBits(_data);
    }
}

Answer 5

在@notJim回答的基础上（以及http://www.bobpowell.net/lockingbits.htm的帮助），我开发了以下内容，这使得我的生活变得更轻松，因为我最终得到了一个数组数组，允许我跳转到一个像素它是x和y坐标。当然，x坐标需要通过每个像素的字节数来校正，但这是一个简单的扩展。

Dim bitmapData As Imaging.BitmapData = myBitmap.LockBits(New Rectangle(0, 0, myBitmap.Width, myBitmap.Height), Imaging.ImageLockMode.ReadOnly, myBitmap.PixelFormat)

Dim size As Integer = Math.Abs(bitmapData.Stride) * bitmapData.Height
Dim data(size - 1) As Byte

Marshal.Copy(bitmapData.Scan0, data, 0, size)

Dim pixelArray(myBitmap.Height)() As Byte

'we have to load all the opacity pixels into an array for later scanning by column
'the data comes in rows
For y = myBitmap.Height - 1 To 0 Step -1
    Dim rowArray(bitmapData.Stride) As Byte
    Array.Copy(data, y * bitmapData.Stride, rowArray, 0, bitmapData.Stride)
    'For x = myBitmap.Width - 1 To 0 Step -1
    '   Dim i = (y * bitmapData.Stride) + (x * 4)
    '   Dim B = data(i)
    '   Dim G = data(i + 1)
    '   Dim R = data(i + 2)
    '   Dim A = data(i + 3)
    'Next
    pixelArray(y) = rowArray
Next

Answer 6

另一种方式更快，更方便。如果你看一下Bitmap构造函数，你会发现一个将IntPtr作为最后一个参数。 IntPtr用于保存像素数据。那么你如何使用它？

Dim imageWidth As Integer = 1920
Dim imageHeight As Integer = 1080

Dim fmt As PixelFormat = PixelFormat.Format32bppRgb
Dim pixelFormatSize As Integer = Image.GetPixelFormatSize(fmt)

Dim stride As Integer = imageWidth * pixelFormatSize
Dim padding = 32 - (stride Mod 32)
If padding < 32 Then stride += padding

Dim pixels((stride \ 32) * imageHeight) As Integer
Dim handle As GCHandle = GCHandle.Alloc(pixels, GCHandleType.Pinned)
Dim addr As IntPtr = Marshal.UnsafeAddrOfPinnedArrayElement(pixels, 0)

Dim bitmap As New Bitmap(imageWidth, imageHeight, stride \ 8, fmt, addr)

您现在拥有的是一个简单的Integer数组和一个引用相同内存的Bitmap。您对Integer数组所做的任何更改都将直接影响Bitmap。让我们通过简单的亮度变换来尝试这个。

Public Sub Brightness(ByRef pixels() As Integer, ByVal scale As Single)
    Dim r, g, b As Integer
    Dim mult As Integer = CInt(1024.0f * scale)
    Dim pixel As Integer

    For i As Integer = 0 To pixels.Length - 1
        pixel = pixels(i)
        r = pixel And 255
        g = (pixel >> 8) And 255
        b = (pixel >> 16) And 255

        'brightness calculation
        'shift right by 10 <=> divide by 1024
        r = (r * mult) >> 10
        g = (g * mult) >> 10
        b = (b * mult) >> 10

        'clamp to between 0 and 255
        If r < 0 Then r = 0
        If g < 0 Then g = 0
        If b < 0 Then b = 0
        r = (r And 255)
        g = (g And 255)
        b = (b And 255)

        pixels(i) = r Or (g << 8) Or (b << 16) Or &HFF000000
    Next
End Sub

您可能会注意到我使用了一个小技巧来避免在循环中进行浮点运算。这样可以提高性能。 当你完成后，你需要清理一点......

addr = IntPtr.Zero
If handle.IsAllocated Then
    handle.Free()
    handle = Nothing
End If
bitmap.Dispose()
bitmap = Nothing
pixels = Nothing

我在这里忽略了alpha组件，但你也可以自由使用它。我用这种方式将很多位图编辑工具拼凑在一起。它比Bitmap.LockBits（）快得多，也更可靠，最重要的是，它需要零内存复制才能开始编辑你的位图。

Answer 7

试试这个C＃解决方案。

创建一个winforms app进行测试。

添加一个Button和一个PictureBox，以及一个click事件和一个表单结束事件。

为表单使用以下代码：

public partial class Form1 : Form
{
    uint[] _Pixels { get; set; }

    Bitmap _Bitmap { get; set; }

    GCHandle _Handle { get; set; }

    IntPtr _Addr { get; set; }


    public Form1()
    {
        InitializeComponent();

        int imageWidth = 100; //1920;

        int imageHeight = 100; // 1080;

        PixelFormat fmt = PixelFormat.Format32bppRgb;

        int pixelFormatSize = Image.GetPixelFormatSize(fmt);

        int stride = imageWidth * pixelFormatSize;

        int padding = 32 - (stride % 32);

        if (padding < 32)
        {
            stride += padding;
        }

        _Pixels = new uint[(stride / 32) * imageHeight + 1];

         _Handle = GCHandle.Alloc(_Pixels, GCHandleType.Pinned);

        _Addr = Marshal.UnsafeAddrOfPinnedArrayElement(_Pixels, 0);

        _Bitmap = new Bitmap(imageWidth, imageHeight, stride / 8, fmt, _Addr);

        pictureBox1.Image = _Bitmap;

    }

    private void button1_Click(object sender, EventArgs e)
    {
        for (int i = 0; i < _Pixels.Length; i++)
        {
            _Pixels[i] = ((uint)(255 | (255 << 8) | (255 << 16) | 0xff000000));

        }

    }

    private void Form1_FormClosing(object sender, FormClosingEventArgs e)
    {
        _Addr = IntPtr.Zero;

        if (_Handle.IsAllocated)
        {
            _Handle.Free();

        }

        _Bitmap.Dispose();

        _Bitmap = null;

        _Pixels = null;

    }

}

现在，您对数组所做的任何编辑都会自动更新位图。

您需要调用图片框上的刷新方法才能看到这些更改。