How do I colour key in gdi32?

[CodeMonkey2000]

I don't know what this is called in VB, what I want is to set transparent colours, so when I blit the image, the white pixels get ignored.

[Nick]

u mean similair's to turing's picMerge?

[Euphoracle]

No, he means set a transparency key so any pixel of that color is not drawn. PicMerge is just a method of drawing it, whereas it simply leaves the empty pixels as they were before drawing the image, thus giving you the "transparent" effect.

[CodeMonkey2000]

Well I found something on Microsoft's site that might help, you have to use vbMergeCopy. But the problem is I don't know how to use it.

EDIT: I found another function, it's called transparentblt. It's a wee bit slow though. Is there an alternative?

[Brightguy]

Yeah, there's TransparentBlt (which apparently has a memory leak on Win98) or you can do it with two calls to BitBlt. First, prepare a copy of your image with black pixels everywhere you want transparency. Then, prepare the "mask": your image with white pixels everywhere you want transparency and black pixels where you don't. You can use these images to create transparency as follows:

First, blit the mask onto the background using the raster operation AND (a bitwise AND on the pixel values). The white areas are 1 in binary so they don't change (x AND 1 = x), while the black areas are 0s in binary so are set to 0 (x AND 0 = 0). Thus, you end up with a black "hole" in the background in the shape of your image.
Then, blit the first image onto the background using the raster operation OR (a bitwise OR on the pixel values). The black areas are 0 in binary so they don't change (x OR 0 = x), so you can blit your image onto the black hole without changing anything, and without changing any pixels outside of the hole.

If want to be really 1337, you can automate the process of creating the mask, given an image and a background colour. I've updated my "double buffering" example program to do this. To try it, just change the filename and picBackground constants as appropriate (I'll attach an example image with this post). I've also made it so you can change the displayed background colour.

VisualBASIC:

Option Explicit Private 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 Private Declare Function CreateBitmap Lib "gdi32" (ByVal nWidth As Long, ByVal nHeight As Long, ByVal nPlanes As Long, ByVal nBitCount As Long, lpBits As Any) As Long Private Declare Function CreateCompatibleBitmap Lib "gdi32" (ByVal hdc As Long, ByVal nWidth As Long, ByVal nHeight As Long) As Long Private Declare Function CreateCompatibleDC Lib "gdi32" (ByVal hdc As Long) As Long Private Declare Function CreateSolidBrush Lib "gdi32" (ByVal crColor As Long) As Long Private Declare Function DeleteDC Lib "gdi32" (ByVal hdc As Long) As Long Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject As Long) As Long Private Declare Function ExtFloodFill Lib "gdi32" (ByVal hdc As Long, ByVal x As Long, ByVal y As Long, ByVal crColor As Long, ByVal wFillType As Long) As Long Private Declare Function GetDC Lib "user32" (ByVal hwnd As Long) As Long Private Declare Function GetObject Lib "gdi32.dll" Alias "GetObjectA" (ByVal hObject As Long, ByVal nCount As Long, ByRef lpObject As Any) As Long Private Declare Function SelectObject Lib "gdi32" (ByVal hdc As Long, ByVal hObject As Long) As Long Private Declare Function SetBkColor Lib "gdi32" (ByVal hdc As Long, ByVal crColor As Long) As Long Private Type BITMAP     bmType As Long     bmWidth As Long     bmHeight As Long     bmWidthBytes As Long     bmPlanes As Integer     bmBitsPixel As Integer     bmBits As Long End Type Private Const filename As String = "C:\doggie2.gif" Private Const picBackground As Long = &H000100 Private Const backColour As Long = vbBlue Private backDC As Long, picDC As Long, maskDC As Long, picInfo As BITMAP Private Sub Form_Load()     'Damn twips     Me.ScaleMode = vbPixels     'Load the picture into the device context and retrieve info     picDC = CreateCompatibleDC(GetDC(0))     SelectObject picDC, LoadPicture(filename)     GetObject LoadPicture(filename), Len(picInfo), picInfo         'Picture background colour     SetBkColor picDC, picBackground     'Create mask     Dim maskBitmap As Long     maskDC = CreateCompatibleDC(GetDC(0))     maskBitmap = CreateBitmap(picInfo.bmWidth, picInfo.bmHeight, 1, 1, ByVal 0)     SelectObject maskDC, maskBitmap     BitBlt maskDC, 0, 0, picInfo.bmWidth, picInfo.bmHeight, picDC, 0, 0, vbSrcCopy     BitBlt picDC, 0, 0, picInfo.bmWidth, picInfo.bmHeight, maskDC, 0, 0, vbSrcInvert     DeleteObject maskBitmap         'Create device context for the background     backDC = CreateCompatibleDC(GetDC(0)) End Sub Private Sub Form_MouseDown(Button As Integer, Shift As Integer, x As Single, y As Single)     'Draw picture on background     Form_MouseMove Button, Shift, x, y End Sub Private Sub Form_MouseMove(Button As Integer, Shift As Integer, x As Single, y As Single)     If Button > 0 Then         'Draw picture on background         BitBlt backDC, x - Int(picInfo.bmWidth / 2), y - Int(picInfo.bmHeight / 2), picInfo.bmWidth, picInfo.bmHeight, maskDC, 0, 0, vbSrcAnd         BitBlt backDC, x - Int(picInfo.bmWidth / 2), y - Int(picInfo.bmHeight / 2), picInfo.bmWidth, picInfo.bmHeight, picDC, 0, 0, vbSrcPaint         Form_Paint     End If End Sub Private Sub Form_Paint()     'Draw background on form     BitBlt Me.hdc, 0, 0, Me.ScaleWidth, Me.ScaleHeight, backDC, 0, 0, vbSrcCopy End Sub Private Sub Form_Resize()     'Create bitmap of proper size for the background     Dim backBitmap As Long     backBitmap = CreateCompatibleBitmap(GetDC(0), Me.ScaleWidth, Me.ScaleHeight)     SelectObject backDC, backBitmap     'Create background colour brush     Dim backBrush As Long     backBrush = CreateSolidBrush(backColour)     SelectObject backDC, backBrush     'Fill black area with brush colour     ExtFloodFill backDC, 0, 0, vbBlack, 1     'Draw picture in the middle of background     BitBlt backDC, Int((Me.ScaleWidth - picInfo.bmWidth) / 2), Int((Me.ScaleHeight - picInfo.bmHeight) / 2), picInfo.bmWidth, picInfo.bmHeight, maskDC, 0, 0, vbSrcAnd     BitBlt backDC, Int((Me.ScaleWidth - picInfo.bmWidth) / 2), Int((Me.ScaleHeight - picInfo.bmHeight) / 2), picInfo.bmWidth, picInfo.bmHeight, picDC, 0, 0, vbSrcPaint     Form_Paint     'Cleanup memory     DeleteObject backBitmap     DeleteObject backBrush End Sub Private Sub Form_Unload(Cancel As Integer)     'Cleanup memory     DeleteDC picDC     DeleteDC maskDC     DeleteDC backDC End Sub

[bellopapo]

hi
i tried the example ...
it's ok but image has a lot of "gosts", that are the old images when doggie2.gif go in another location.
so .. is it possible to display on screen ONLY ONE doggie2.gif after it is moved ?
many thanks

peppe

[Brightguy]

Sure, you just want to clear the background before each new doggie is drawn. Add something like...

VisualBASIC:

BitBlt backDC, 0, 0, Me.ScaleWidth, Me.ScaleHeight, 0, 0, 0, &H42         ExtFloodFill backDC, 0, 0, vbBlack, 1

...right before "Draw picture on background" in Form_MouseMove.

And I just realized ExtFloodFill is really slow (that's "onboard graphics accelerator" for you). If you don't care about the background colour you can remove that.