图像数据读取LockBits理解

Many image processing tasks and even file type conversions, say from 32 bit-per-pixel to 8 bit-per-pixel can be speeded up by accessing the pixel data array directly, rather than relying on GetPixel and SetPixel or other methods.

You will be aware that .NET is a managed code system which most often uses managed data so it's not often that we need to gain access to bytes stored in memory anymore however, image manipulation is one of the few times when managed data access is just too slow and so we need to delve once again into the knotty problems of finding the data and manipulating it.

Before I start on the subject in hand, I'll just remind you that the methods used to access any unmanaged data will be different depending on the language in which your program is written. C# developers have the opportunity, via the unsafe keyword and use of pointers, to access data in memory directly. Visual basic programmers should access such data through the Marshal class methods which may also show a small performance loss.

Lock up your bits

The Bitmap class provides the LockBits and correspondingUnlockBits methods which enable you to fix a portion of the bitmap pixel data array in memory, access it directly and finally replace the bits in the bitmap with the modified data.LockBits returns aBitmapData class that describes the layout and position of the data in the locked array.

The BitmapData class contains the following important properties;

• Scan0 The address in memory of the fixed data array
• Stride The width, in bytes, of a single row of pixel data. This width is a multiple, or possiblysub-multiple, of the pixel dimensions of the image and may be padded out to include a few more bytes. I'll explain why shortly.
• PixelFormat The actual pixel format of the data. This is important for finding the right bytes
• Width The width of the locked image
• Height The height of the locked image

The relationship of Scan0 and Stride to the array in memory is shown in figure1.

Figure 1: The basic layout of a locked bitmap array

The Stride property, as shown in figure 1, holds the width of one row in bytes. The size of a row however may not be an exact multiple of the pixel size because for efficiency, the system ensures that the data is packed into rows that begin on a four byte boundary and are padded out to a multiple of four bytes. This means for example that a 24 bit per pixel image 17 pixels wide would have a stride of 52. The used data in each row would take up 3*17 = 51 bytes and the padding of 1 byte would expand each row to 52 bytes or 13*4 bytes. A 4BppIndexed image of 17 pixels wide would have a stride of 12. Nine of the bytes, or more properly eight and a half,  would contain data and the row would be padded out with a further 3 bytes to a 4 byte boundary.

The data carrying portion of the row, as has been suggested above, is laid out according to the pixel format. A 24 bit per pixel image containing RGB data would have a new pixel every 3 bytes, a 32 bit per pixel RGBA every four bytes. Pixel formats that contain more than one pixel per byte, such as the 4 bit per pixel Indexed and 1 bit per pixel indexed, have to be processed carefully so that the pixel required is not confused with it's neigbour pixels in the same byte.

Finding the right byte.

Because the stride is the width of a row, to index any given row or Y coordinate you can multiply the stride by the Y coordinate to get the beginning of a particular row. Finding the correct pixel within the row is possibly more difficult and depends on knowing the layout of the pixel formats. The following examples show how to access a particular pixel for a given pixel format.

• Format32BppArgb Given X and Y coordinates,  the address of the first element in the pixel isScan0+(y * stride)+(x*4). This Points to the blue byte. The following three bytes contain the green, red and alpha bytes.

• Format24BppRgb Given X and Y coordinates, the address of the first element in the pixel isScan0+(y*Stride)+(x*3). This points to the blue byte which is followed by the green and the red.

• Format8BppIndexed Given the X and Y coordinates the address of the byte isScan0+(y*Stride)+x. This byte is the index into the image palette.

• Format4BppIndexed Given X and Y coordinates the byte containing the pixel data is calculated asScan0+(y*Stride)+(x/2). The corresponding byte contains two pixels, the upper nibble is the leftmost and the lower nibble is the rightmost of two pixels. The four bits of the upper and lower nibble are used to select the colour from the 16 colour palette.

• Format1BppIndexed Given the X and Y coordinates, the byte containing the pixel is calculated byScan0+(y*Stride)+(x/8). The byte contains 8 bits, each bit is one pixel with the leftmost pixel in bit 8 and the rightmost pixel in bit 0. The bits select from the two entry colour palette.

Iterating through the pixels

For pixel formats with one or more bytes per pixel, the formula is simple and can be accomplished by looping through all Y and X values in order. The code in the following listings sets the blue component of a 32 bit per pixel image to 255. In both cases bm is a bitmap previously created.

      BitmapData bmd=bm.LockBits(new Rectangle(0, 0, 10, 10), System.Drawing.Imaging.ImageLockMode.ReadOnly, bm.PixelFormat);

      int PixelSize=4;

      for(int y=0; y<bmd.Height; y++){
        byte* row=(byte *)bmd.Scan0+(y*bmd.Stride);
        for(int x=0; x<bmd.Width; x++){
          row[x*PixelSize]=255;
        }
      }

Sub-byte pixels.

The Format4BppIndexed and Format1BppIndexed pixel formats mentioned earlier both have more than one pixel stored in a byte. In such cases, it's up to you to ensure that changing the data for one pixel does not effect the other pixel or pixels held in that byte.

The method for indexing a 1 bit per pixel image is shown in the GDI+ FAQ article"Converting and RGB Image to 1 bit-per-pixel monochrome ". It relies on using bitwise logical operations And and Or to reset or set specific bits in the byte. After using the formula shown above for 1 bit per pixel images, the lower 3 bits of the X coordinate is used to select the bit required. The listings below show this process in C# and VB. In both examples bmd is a bitmap data extracted from a 1 bit per pixel image.

C# code uses pointers and requires compiling with unsafe code 

 byte* p=(byte*)bmd.Scan0.ToPointer();

      int index=y*bmd.Stride+(x>>3);

      byte mask=(byte)(0x80>>(x&0x7));

      if(pixel) p[index]|=mask;

      else      p[index]&=(byte)(mask^0xff); 

Note, it's quite valid to use the Marshal class from C# code. I used pointers because it offers the best performance.

Accessing individual pixels in a 4 bit per pixel image is handled in a similar manner. The upper and lower nibble of the byte must be dealt with separately and changing the contents of the odd X pixels should not effect the even X pixels. The code below shows how to perform this in C# .

 int offset = (y * bmd.Stride) + (x >> 1);

      byte currentByte = ((byte *)bmd.Scan0)[offset];

      if((x&1) == 1){
        currentByte &= 0xF0;
        currentByte |= (byte)(colorIndex & 0x0F);
      }

      else{
        currentByte &= 0x0F;
        currentByte |= (byte)(colorIndex << 4);
      }
     
      ((byte *)bmd.Scan0)[offset]=currentByte;

Using LockBits and UnlockBits

The LockBits method takes a rectangle which may be the same size or smaller than the image being processed, a PixelFormat which is usually the same as that of the image being processed and a ImageLockMode value that specifies whether the data is read-only, write-only, read-write or a user allocated buffer. This last option cannot be used from C# or VB because the method overload for LockBits that specifies a user buffer is not included in the GDI+ managed wrapper.

It is very important that when all operations are complete the BitmapData is put back into the bitmap with the UnlockBits method. The snippet of code below illustrates this.

Summary

That just about covers the aspects of accessing the most popular and most difficult pixel formats directly. Using these techniques instead of the GetPixel and SetPixel methods provided by Bitmap will show a marked performance boost to your image processing and image format conversion routines.

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