[Vb6] AlphaBlend API function(strange result)

[joaquim]

Don't think so, you may have to manually process the pixels and then use AlphaBlend:

Option 1: test it, no guarantee it will work. I don't know if TransparentBlt modifies the alpha channel on a destination 32bpp DIB/bitmap.
1. Create offscreen 32bpp DIB section & DC
2. Fill the DIB with all zeros
3. Use TransparentBLT to draw your image to that DC
4. Now use AlphaBlend to draw that DC to your control/object
5. Destroy DC & DIB at some point

Option 2
1. Must use 32bpp DIB
2. For each pixel where the RGB value should be made transparent, set its RGBA value to zero (all 4 bytes of that pixel).
3. Now use AlphaBlend API
4. Destroy DIB at some point

Option 3
Using GDI+ this can be done. Though that does mean adding GDI+ APIs to your project and learning how to load an image into GDI+, along with using GdipSetImageAttributesColorKeys, GdipSetImageAttributesColorMatrix, and several more of the GDI+ APIs.

like you see, i have 1 sub:

'how calculate the alpha values: a*rgb1 + (1-a)rgb2
Public Sub TransparentAlphaBlend(ByRef SrcPictureBox As Control, ByRef DstPictureBox As Control, ByRef Opacy As Integer, ByRef TransparentColor As Long)
    Dim x As Long, y As Long
    Dim lngNewColor As Long
    For x = 0 To DstPictureBox.ScaleWidth - 1
        For y = 0 To DstPictureBox.ScaleHeight - 1
            If GetPixel(DstPictureBox.hdc, x, y) <> TransparentColor Then
                lngNewColor = Opacy * GetPixel(DstPictureBox.hdc, x, y) + (1 - Opacy) * GetPixel(SrcPictureBox.hdc, x, y)
                SetPixel DstPictureBox, x, y, lngNewColor
            End If
        Next y
    Next x
End Sub

(these sub is only for test, then i will convert it to DIB's)
the problem is that i recive negative values. can you advice me?
for resolve that problem i use:

Public Sub TransparentAlphaBlend(ByRef SrcPictureBox As Control, ByRef DstPictureBox As Control, ByRef Opacy As Integer, ByRef TransparentColor As Long)
    Dim X As Long, Y As Long
    Dim lngNewColor As Long
    Dim BF As BLENDFUNCTION, lBF As Long
    
    With BF
        .BlendOp = AC_SRC_OVER
        .BlendFlags = 0
        .SourceConstantAlpha = Opacy
        .AlphaFormat = 0
    End With
    RtlMoveMemory lBF, BF, 4
    For X = 0 To DstPictureBox.ScaleWidth - 1
        For Y = 0 To SrcPictureBox.ScaleHeight - 1
            If GetPixel(DstPictureBox.hdc, X, Y) <> TransparentColor Then
                AlphaBlend SrcPictureBox.hdc, X, Y, 1, 1, DstPictureBox.hdc, X, Y, 1, 1, lBF
                'lngNewColor = Opacy * GetPixel(DstPictureBox.hdc, X, Y) + (1 - Opacy) * GetPixel(SrcPictureBox.hdc, X, Y)
                ''SetPixel DstPictureBox, X, Y, lngNewColor
            End If
        Next Y
    Next X
    DstPictureBox.Picture = DstPictureBox.Image
End Sub