Hi,

I have an app (written in .NET 2.0) which updates a picturebox
according to some user input (a slider control).  when the user makes a
change i loop through all of the pixels, do a calculation and update
the picture.  i currently use the GDI SetPixel method on each pixel of
the Pictureboxes image.  This is proving far to slow, about 1.5 seconds
on average.  This app needs to display the update as fast as possible.
Has anyone got any ideas as to how to speed this up?  i was thinking
about trying to access the picturebox's memory space directly, pseudo
code:

For Each pixel in <PictureBox.Image's bytearray>
    <PictureBox.Image's bytearray> (pixel) = <new pixel value from
calculation>
Next

This way you're only updating some byte values in memory rather than
calling GDI method's.

Am i barking up the wrong tree? Has anyone got any better solutions?
Any help would be much appreciated.

Alex

On 24 Jul 2006 10:08:16 -0700, keba***@gmail.com wrote:

Show quoteHide quoteIt would be helpful to know exactly what you are changing (i.e. Contrast, Saturation)?

Gene

Hi gene,

The calculation is a bespoke alteration that matches the 3rd party
software we're integrating with.  in a nutshell for each RGB of every
pixel i have to to a look up based on the starting byte value and the
correction value.  this look up gives a new byte value.  so there are
three lookups for each pixel that returns a new RGB value.  what i need
to know is the fasted possible way of setting this new pixel value to
the picturebox.

Hope that helps,
Alex

gene kelley wrote:
Show quoteHide quote

On 25 Jul 2006 02:38:58 -0700, keba***@gmail.com wrote:

Show quoteHide quoteYou may want to investigate the ColorMatrix Class.  If you are using VB2005, go to the help menu,
search GDI+ Image.  Download the example.  The example shows a couple of uses of the ColorMatrix
like convert to grayscale which would be fairly slow using SetPixel method.

Back in VB6, would have used Get/Set DibBits routines in place of Get/Set Pixels, but I have not
tried using any of those routines to day in .NET.

Gene

Hi

I haven't had to do any of this in .Net yet but you may want to look at
using a Bit Blit technique (which is what you're really describing when
you're talking about doing it in memory) - it's commonly referred to as
BitBlt and involves an API call.

Hope that helps
Martin
gene kelley wrote:
Show quoteHide quote

BitBlt is simply a method of copying data form a source to a destination.  It has no use with
regards to "changing" the color data in the source bitmap.

In .Net, the BitBlt API is wrapped  in the Graphics.CopyFromScreen method.


Gene

Hi

I'm aware of that but the original post asked about whether it was
possible to do the work in memory as opposed to changing each pixel 1
at a time - if you're doing something like that then I understand that
it is (or can be) quicker to do in memory and then bitblt the results
back to the picture box data.


gene kelley wrote:
Show quoteHide quote

keba***@gmail.com wrote:
Show quoteHide quoteElsethread what you have written suggests that the only way to do what
you want to do is indeed to perform a calculation on each pixel's color
value in turn. So given that, I present this:

(from the docs for Bitmap.LockBits)
Private Sub LockUnlockBitsExample(ByVal e As PaintEventArgs)

     ' Create a new bitmap.
     Dim bmp As New Bitmap("c:\fakePhoto.jpg")

     ' Lock the bitmap's bits.
     Dim rect As New Rectangle(0, 0, bmp.Width, bmp.Height)
     Dim bmpData As System.Drawing.Imaging.BitmapData = bmp.LockBits(rect, _
         Drawing.Imaging.ImageLockMode.ReadWrite, bmp.PixelFormat)

     ' Get the address of the first line.
     Dim ptr As IntPtr = bmpData.Scan0

     ' Declare an array to hold the bytes of the bitmap.
     ' This code is specific to a bitmap with 24 bits per pixels.
     Dim bytes As Integer = bmp.Width * bmp.Height * 3
     Dim rgbValues(bytes - 1) As Byte

     ' Copy the RGB values into the array.
     System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes)

     ' Set every red value to 255.
     For counter As Integer = 0 To rgbValues.Length - 1 Step 3
         rgbValues(counter) = 255
     Next

     ' Copy the RGB values back to the bitmap
     System.Runtime.InteropServices.Marshal.Copy(rgbValues, 0, ptr, bytes)

     ' Unlock the bits.
     bmp.UnlockBits(bmpData)

     ' Draw the modified image.
     e.Graphics.DrawImage(bmp, 0, 150)

End Sub


Things to note:
- the PixelFormat of the bitmap must be one with a definite
bits-per-pixel: you can't use indexed bitmaps like this
- the number of bits needed for each pixel depends on the PixelFormat of
the bitmap; as a consequence, the size of the Byte array needed also
depends on that
- the format of the data in the Byte array you get *also* depends on the
BPP of the PixelFormat. eg if it is 32 bpp, then you will get <alpha
byte> <red byte> <green byte> <blue byte> repeated; if it is 24bpp with
no alpha, you will get <red byte> <green byte> <blue byte> repeated; and
all the others.

Finally I must point out that you shouldn't get too attached to this
technique - it's only appropriate when you (as in this case) *have* to
examine each and every pixel individually. Things other people have
mentioned, like ColorMatrix, are more appropriate for general image
manipulation.

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