WxWidgets draw a bitmap with opacity

gdi+wxwidgets

How can I draw a wxImage, or wxBitmap to a DC with opacity? It looks like its not possible with the standard DC or with wxGraphicsContext.

BeginLayer/EndLayer is not yet implemented in wxGraphicsContext which would have been one solution. GDI+ doesn't seem to support layers, so it would be non-trivial to add this.

The only solution seems to be to programmatically alter the alpha channel pixel-by-pixel?

Best Answer

void YourCustomWindow::OnPaint( wxPaintEvent& event ) {
  wxPaintDC dc(this);
  wxImage image( width, height ); 
  image.InitAlpha();
  unsigned char* imgdata = img.GetData();
  unsigned char* imgalpha = img.GetAlpha();


  // manipulate raw memory buffers to populate
  // them with whatever image you like.
  // Putting zeros everywhere will create a fully
  // transparent image.


     // almost done, but not quite... 
     // wxDC can only draw wxBitmaps, not wxImage,
     // so one last step is required
  //
  wxBitmap bmp( image );
  dc.DrawBitmap( bmp, 0, 0 );
 }

Related Solutions

Gdiplus mask image from another image

I think you're correct in that you have to do this pixel-by-pixel. I've also searched for a more "pure" way to do it, but this is what I ended up with:

    public enum ChannelARGB
    {
        Blue = 0,
        Green = 1,
        Red = 2,
        Alpha = 3
    }

    public static void transferOneARGBChannelFromOneBitmapToAnother(
        Bitmap source,
        Bitmap dest,
        ChannelARGB sourceChannel,
        ChannelARGB destChannel )
    {
        if ( source.Size!=dest.Size )
            throw new ArgumentException();
        Rectangle r = new Rectangle( Point.Empty, source.Size );
        BitmapData bdSrc = source.LockBits( r, ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb );
        BitmapData bdDst = dest.LockBits( r, ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb );
        unsafe
        {
            byte* bpSrc = (byte*)bdSrc.Scan0.ToPointer();
            byte* bpDst = (byte*)bdDst.Scan0.ToPointer();
            bpSrc += (int)sourceChannel;
            bpDst += (int)destChannel;
            for ( int i = r.Height * r.Width; i > 0; i-- )
            {
                *bpDst = *bpSrc;
                bpSrc += 4;
                bpDst += 4;
            }
        }
        source.UnlockBits( bdSrc );
        dest.UnlockBits( bdDst );
    }

.net – How to Draw a Bitmap with 50% Opacity

Here is some code that adds an alpha channel to an image. If you want 50% alpha, you would set 128 as the alpha argument. Note this creates a copy of the bitmap...

    public static Bitmap AddAlpha(Bitmap currentImage, byte alpha)
    {
        Bitmap alphaImage;
        if (currentImage.PixelFormat != PixelFormat.Format32bppArgb)
        {
            alphaImage = new Bitmap(currentImage.Width, currentImage.Height, PixelFormat.Format32bppArgb);
            using (Graphics gr = Graphics.FromImage(tmpImage))
            {
                gr.DrawImage(currentImage, 0, 0, currentImage.Width, currentImage.Height);
            }
        }
        else
        {
            alphaImage = new Bitmap(currentImage);
        }

        BitmapData bmData = alphaImage.LockBits(new Rectangle(0, 0, alphaImage.Width, alphaImage.Height),
            ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);

        const int bytesPerPixel = 4;
        const int alphaPixel = 3;
        int stride = bmData.Stride;

        unsafe
        {
            byte* pixel = (byte*)(void*)bmData.Scan0;


            for (int y = 0; y < currentImage.Height; y++)
            {
                int yPos = y * stride;
                for (int x = 0; x < currentImage.Width; x++)
                {
                    int pos = yPos + (x * bytesPerPixel); 
                    pixel[pos + alphaPixel] = alphaByte;
                }
            }
        }

        alphaImage.UnlockBits(bmData);

        return alphaImage;
    }

Best Answer

Related Solutions

Gdiplus mask image from another image

.net – How to Draw a Bitmap with 50% Opacity

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