GDI+ Works on %WS_Ex_Layered Window?

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Gary Beene:
In the code below, I display a rotated image (ruler) with a blue background. The dialog is given the %WS_Ex_Layered style so that the blue background will be transparent color:

--- Code: ---Dialog New Pixels, 0, "Ruler",300,300,400,50, %WS_Popup, %WS_Ex_Layered To hDlg
--- End code ---
and then ...

--- Code: ---SetLayeredWindowAttributes(hDlg, %Blue, 255, %LWA_ALPHA Or %LWA_ColorKey)
--- End code ---

When I use PlgBlt to rotate the image, transparency works fine. But when I rotate the image with GDI+, the transparency effect is lost - the blue background is visible, rather than being transparent.

Is there an interaction between GDI+ and WS_Ex_Layered windows that would explain this?

Here's the compilable code (uses Jose's includes :D). Use the mouse wheel to rotate the image, right mouse click to close the app.

--- Code: ---#Compile Exe
#Dim All
#Debug Error On
#Debug Display On
#Include "gdiplus.inc"

Global hDlg, hDC As Dword, rulerH, rulerW, rotateW, rotateH As Long, theta As Single

Function PBMain() As Long
Local StartupInput As GdiplusStartupInput, token As Dword
StartupInput.GdiplusVersion = 1 ' Initialize GDI+
GdiplusStartup(token, StartupInput, ByVal %NULL)
Dialog New Pixels, 0, "Ruler",300,300,400,50, %WS_Popup, %WS_Ex_Layered To hDlg
Dialog Show Modal hDlg Call DlgProc
GDIPlusShutDown token
End Function

CallBack Function DlgProc() As Long
Local PS As PaintStruct
Select Case Cb.Msg
Case %WM_InitDialog
rulerW = 222 : rulerH = 48 'for this example, ruler is fixed at 222x48 pixels
Dialog Set Color hDlg, %Black, %Blue
Dialog Set Size hDlg, rulerW, rulerH
SetLayeredWindowAttributes(hDlg, %Blue, 255, %LWA_ALPHA Or %LWA_ColorKey)
Case %WM_MouseWheel
Select Case Hi(Integer,Cb.WParam) 'note the use of Integer
Case > 0 : theta = theta + 0.05 : RotateRuler
Case < 0 : theta = theta - 0.05 : RotateRuler
End Select
Case %WM_ContextMenu
Dialog End hDlg 'right mouse click to end app
Case %WM_LButtonDown
If Cb.WParam = %MK_LBUTTON Then SendMessage hDlg, %WM_NCLButtonDown, %HTCaption, ByVal %Null ' force drag
Case %WM_Paint
hDC = BeginPaint(hDlg, PS)
RotateRuler
EndPaint hDlg, PS
End Select
End Function

Sub RotateRuler
Local iMax,tempx,tempy,w,h,XCenter,YCenter,dx,dy As Long, hTemp, hTempDC, pGraphics, pImage, hBitmap As Dword

'resize dialog to enclose ruler after rotation
rotateW = RulerW*Abs(Cos(theta)) + RulerH*Abs(Sin(theta)) 'bounding W once ruler is rotated
rotateH = RulerH*Abs(Cos(theta)) + RulerW*Abs(Sin(theta)) 'bounding H once ruler is rotated
Dialog Get Loc hDlg To tempx, tempy : Dialog Get Size hDlg To w,h 'get current dialog size
Dialog Set Size hDlg, RotateW, RotateH 'set dialog size to bounding dimensions
Dialog Set Loc hDlg, tempx - (RotateW-w)/2, tempy -(RotateH-h)/2 'relocate so rotation is about center of dialog

If 1 Then 'GDI+ version of rotation
'load ruler image, which is in larger-than-needed image, surrounded by transparency color)
iMax = 300 'image containg ruler just happens to be 300x300
Graphic Bitmap Load "ruler1.bmp", iMax, iMax To hTemp 'centered ruler + blue surrounding
Graphic Attach hTemp, 0
Graphic Get DC To hTempDC

'create GDI+ object
GdipCreateFromHDC(hTempDC, pGraphics) 'get pGraphics
hBitmap = GetCurrentObject(hTempDC, %OBJ_Bitmap) 'get hBitmap
GDIpCreateBitmapFromHBITMAP(hBitmap, ByVal %Null, pImage) 'get pImage

'rotate the image
GdipTranslateWorldTransform(pGraphics, iMax/2, iMax/2, %MatrixOrderPrepend) ' Move to center
GdipRotateWorldTransform(pGraphics, theta * 57.3, %MatrixOrderPrepend) ' Rotate
GdipTranslateWorldTransform(pGraphics, -iMax/2, -iMax/2, %MatrixOrderPrepend) ' Return
GdipDrawImage(pGraphics, pImage, 0, 0)

Dialog ReDraw hdlg 'clear previous image
BitBlt hDC, 0,0,rotateW,rotateH, hTempDC, iMax/2-rotateW/2,iMax/2-rotateH/2 , %SRCCopy 'copy ruler image to dialog surface

If pImage Then GdipDisposeImage(pImage) 'cleanup
If pGraphics Then GdipDeleteGraphics(pGraphics)'cleanup

Else 'PlgBlt version of rotation
'create memory BMP containing just the ruler (222x48)
Graphic Bitmap Load "ruler2.bmp", rulerW, rulerH To hTemp 'just the ruler, no surrounding blue color
Graphic Attach hTemp, 0
Graphic Get DC To hTempDC

'get the 3 corners of the rotated image (point are located on hDlg)
dx = (rotateW - rulerW)/2 : dy = (rotateH - rulerH)/2
XCenter = rulerW/2 : YCenter = rulerH/2
Dim PlgPts(2) As PointAPI
PlgPts(0).X = XCenter + (0 - XCenter) * Cos(theta) - (0 - YCenter) * Sin(theta) + dx 'upper-left in target
PlgPts(0).Y = YCenter + (0 - XCenter) * Sin(theta) + (0 - YCenter) * Cos(theta) + dy
PlgPts(1).X = XCenter + (RulerW - XCenter) * Cos(theta) - (0 - YCenter) * Sin(theta) + dx 'upper-right in target
PlgPts(1).Y = YCenter + (RulerW - XCenter) * Sin(theta) + (0 - YCenter) * Cos(theta) + dy
PlgPts(2).X = XCenter + (0 - XCenter) * Cos(theta) - (RulerH - YCenter) * Sin(theta) + dx 'lower left in target
PlgPts(2).Y = YCenter + (0 - XCenter) * Sin(theta) + (RulerH - YCenter) * Cos(theta) + dy
Dialog ReDraw hdlg 'clear image, display blue background color
PlgBlt hDC, PlgPts(0),hTempDC, 0,0,RulerW,RulerH,0&,0,0 'copy ruler image to dialog surface
End If
End Sub
--- End code ---

Gary Beene:
Sorry, but left off a link to the two images:
http://www.garybeene.com/images/ruler1.bmp
http://www.garybeene.com/images/ruler2.bmp

Gary Beene:
More information ... conflicting results.

I'm running on Win7.

When I run under an Aero theme, the transparency works.

But when I run under a basic theme (Windows Classic), the transparency does not work.

Gary Beene:
More data ... transparency doesn't work on my wife's XP machine either.

In looking around the web, I'm seeing a number of posts who's title suggests a similar problem. But I haven't yet found a clear explanation of why, of how to resolve the problem.

Gary Beene:
Over in the PowerBASIC forums, Donald gave me some code that will work. But he took a slightly different route so I still don't know why the code I posted fails.

http://www.powerbasic.com/support/pbforums/showthread.php?p=371051#post371051

Different Question:
In these forums, how do I hide the URL above, and use it to make a highlighted link?

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