Move "+" without erasing something already drawn

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I use Line function to draw line in a picture box, but here is a problem.
I want to use some kind of mark that moves on the line I have drawn previously.
I am not sure if I should use a letter, "+" sign, because it will erase the line that has been drawn. How to move some kind of mark without erasing something that has been drawn already.

Please help me.

[Michael Nattfalk]

You can do this using the XOR function for drawing. The XOR works like this, it changes all 0's to 1's, and all 1's to 0's (binary).

If you have a 2 colored bitmap with a cross, where the cross is colored white and the surrounding area is black, then you XOR this bitmap at the current position of your mouse. In this same event you store the X and Y coordinate. At the next time you enter the drawing event you XOR the same bitmap at the previously stored coorindate to remove the cross. Then you XOR it again at the current position and store the coordinates.
So when you moving your mouse around on the bitmap you will have the cross following you without destroying what you have drawn.
You can do the XOR draw using the BitBlit API.

Hope this helps you a bit.

Regards,
Michael Nattfalk

[show]

Thanks for help.
But as far as I know, Bitblt is not supported by VB.
I guess I have to use DLL or something.
How can I do that with XOR? Somebody?

[Aaron Young]

You don't need to use the API, you can set the DrawMode to vbInvert.

Here's an Example I put together which demonstrates the use of vbInvert to get a Rubber Band Effect for Cropping an Image:

On a Form with 2 Pictureboxes, Load a Picture into Picture1..

private oX as Integer
private oY as Integer
private lX as Integer
private lY as Integer

private Sub Form_Load()
Picture1.ScaleMode = vbPixels
Picture2.ScaleMode = vbPixels
Picture2.AutoRedraw = true
End Sub

private Sub Picture1_MouseDown(Button as Integer, Shift as Integer, X as Single, Y as Single)
If Button = vbLeftButton then
With Picture1
.DrawMode = vbInvert
oX = X
oY = Y
lX = oX
lY = oY
End With
End If
End Sub

private Sub Picture1_MouseMove(Button as Integer, Shift as Integer, X as Single, Y as Single)
If Button = vbLeftButton then
Picture1.Line (oX, oY)-step(lX - oX, lY - oY), , B
Picture1.Line (oX, oY)-step(X - oX, Y - oY), , B
lX = X
lY = Y
End If
End Sub

private Sub Picture1_MouseUp(Button as Integer, Shift as Integer, X as Single, Y as Single)
Dim iTmp as Integer
Dim iW as Integer
Dim iH as Integer
If Button = vbLeftButton then
Picture1.Line (oX, oY)-step(lX - oX, lY - oY), , B
Picture2.Cls
If X < oX then
iTmp = oX
oX = X
X = iTmp
End If
If Y < oY then
iTmp = oY
oY = Y
Y = iTmp
End If
If X = oX then X = 1
If Y = oY then Y = 1
iW = X - oX
iH = Y - oY
Picture2.Width = ScaleX(iW, vbPixels, vbTwips)
Picture2.Height = ScaleY(iH, vbPixels, vbTwips)
Picture2.PaintPicture Picture1.Image, 0, 0, iW, iH, oX, oY, iW, iH
SavePicture Picture2.Image, InputBox("Save Cropped Image as..", "Save as..", "C:\Cropped.bmp")
End If
End Sub




Aaron Young
Analyst Programmer
adyoung@win.bright.net
aarony@redwingsoftware.com

[Michael Nattfalk]

The BitBlt API is located in the GDI32.DLL.

The declare looks like this:
Declare Function BitBlt Lib "gdi32" (byval hDestDC as Long, byval x as Long, _
byval y as Long, byval nWidth as Long, byval nHeight as Long, byval hSrcDC as _
Long, byval xSrc as Long, byval ySrc as Long, byval dwRop as Long) as Long



Where:
hDestDC .. is the handle of the destinations Device Context.
x .. is the left position on the x axis
y .. is the top position on the y axis
nWidth .. is the destination width on the BitBlt'ed image
nHeight .. is the destination height on the BitBlt'ed image
hSrcDC .. is the handle of the source's Device Context
xSrc .. is how much from the source image to copy, on the x axis
ySrc .. is how much from the source image to copy, on the y axis
dwRop .. is the Raster Operations. Here you define that you want to use
the XOR function.

The Device Context can be retrieved using the GetDC/ReleaseDC API in USER.DLL, with the PictureBox's hWnd as parameter.

ROP's (Raster Operations)
-------------------------
Const SRCCOPY = &HCC0020 ' (DWORD) dest = source
Const SRCPAINT = &HEE0086 ' (DWORD) dest = source OR dest
Const SRCAND = &H8800C6 ' (DWORD) dest = source AND dest
Const SRCINVERT = &H660046 ' (DWORD) dest = source XOR dest
Const SRCERASE = &H440328 ' (DWORD) dest = source AND (NOT dest)



These are only the most common ROP's.

Hope this helps you a bit more. There are lot's of examples out there on the net. Just search and I bet you'll find some.

Regards,
Michael Nattfalk