This article has been
excerpted from book "Graphics Programming with GDI+".
In the previous section we discussed how to set the pixel format of a bitmap by
reading pixels one by one. You can also set the pixel format by using the
BitmapData class and its members.
The BitmapData object specifies the attributes of a bitmap, including size,
pixel format, starting address of the pixel data in memory, and length of each
scan line (stride). These properties are described in Table 8.3. All of the
properties have both get and set types.
Now let's set the color of pixels in a bitmap by using LockBits and UnlockBits.
This approach is faster than using the SetPixel method. Listing 8.2 uses
LockBits and UnlockBits to set a bitmap pixel format. First we create an Image
object from a file, followed by a Bitmap object from the Image object. Then we
call LockBits, which returns as BitmapData object. Next we call PixelFormat to
set the pixel format. You can use any of the PixelFormat enumeration values.
Finally we call UnlockBits to unlock the locked bits. Notice that the lockedRect
rectangle in the LockBits method is the size of the bitmap.
LISTING: 8.2: Using LockBits and UnlockBits to set the grayscale of a bitmap
private void
Form1_Paint(object sender,
PaintEventArgs e)
{
Image img =
Image.FromFile("roses.jpg");
Bitmap curImage =
new Rectangle(0,
0, curImage.Width, curImage.Height);
Rectangle lockedRect =
new Rectangle(0,
0, curImage.Width, curImage.Height);
BitmapData bmpData =
curImage.LockBits(lockedRect, ImageLockMode.ReadWrite,
PixelFormat.Format24bppRgb);
// Set the format of BitmapData pixels
bmpData.PixelFormat = PixelFormat.Max;
// Unlock the locked bits
curImage.UnlockBits(bmpData);
// Draw image with new pixel format
e.Graphics.DrawImage(curImage, 0, 0,
curImage.Width, curImage.Height);
}
TABLE 8.3: BitmapData properties
| Property | Description |
| Height | Represents the
pixel height. |
| PixelFormat | Represents the
format of the pixels using the PixelFormat enumeration. |
| Scan0 | Represents the
address of the first pixel data in the bitmap. |
| Stride | Represents stride
(also called scan width). |
| Width | Represents the
pixel width. |
Figure 8.1 shows the output from listing 8.2. The entire bitmap is grayscale.
If a bitmap is huge and we want to change the format of only few pixels,
LockBits and UnlockBits really help. Using these methods, we can lock and render
only the part of a bitmap we want to work on instead of rendering the entire
bitmap. Suppose we want to change the pixel format of only the section of the
bitmap starting at point (50, 50) and ending at point (200, 200). We simply
change the rectangle passed to LockBits.
Listing 8.3 locks only that portion of the image specified by a rectangle.
FIGURE 8.1: Using BitmapData to set grayscale
LISTING 8.3: Changing the pixel format of a partial bitmap
private void
Form1_Paint(object sender,
PaintEventArgs e)
{
Image
img = Image.FromFile("Roses.jpg");
Bitmap curImage =
new Bitmap(img,
new Size(img.Width,
img.Height));
// Call LockBits, which returns a BitmapData object
Rectangle lockedRect =
new Rectangle(50,
50, 200, 200);
/* Rectangle lockedRect =
new Rectangle (0, 0 curImage.width, curImage.Height);
*/
BitmapData bmpData =
curImage.LockBits(lockedRect,
ImageLockMode.ReadWrite,
PixelFormat.Format24bppRgb);
// Set the format of BitmapData pixels
bmpData.PixelFormat = PixelFormat.Max;
// Unlock the locked bits
curImage.UnlockBits(bmpData);
// Draw image with new pixel format
e.Graphics.DrawImage(curImage, 0, 0,
curImage.Width,
curImage.Height);
}
Figure 8.2 shows the output from Listing 8.3. You may not see any difference
between this illustration and Figure 8.1, but if you run the sample code
yourself, you will notice that the color of only a small rectangle in the image
is changed.
FIGURE 8.2: Changing the pixel format of a partial bitmap
Conclusion
Hope the article would have helped you in
understanding BitmapData to Change Pixel Format in GDI+. Read other articles on GDI+ on the website.
| This book teaches
.NET developers how to work with GDI+ as they develop applications
that include graphics, or that interact with monitors or printers.
It begins by explaining the difference between GDI and GDI+, and
covering the basic concepts of graphics programming in Windows. |