How do you Draw Transparent Image using System.Drawing?

Question

I\'m trying to return a transparent GIF from an .aspx page for display within a web page. I am trying to get the image to have transparency, but I just keep getting Black be

Answer 1

here is some code to have a gif (that already have transparency in it) transformed (supposed you want to resize it) in bitmap and then can be showed properly with it's transparency.

imagePath = System.Web.HttpContext.Current.Request.MapPath(libraryPath + reqImageFile);
System.Drawing.Image image = null;
Bitmap resizedImage = null;

if (reqWidth == 0) { reqWidth = image.Width; }
if (reqHeight == 0) { reqHeight = image.Height; }
image = System.Drawing.Image.FromFile(imagePath);
reqWidth = image.Width;
reqHeight = image.Height;

//here is the transparency 'special' treatment
resizedImage = new Bitmap(reqWidth, reqHeight, PixelFormat.Format8bppIndexed);
ColorPalette pal = resizedImage.Palette;
for (int i = 0; i < pal.Entries.Length; i++)
{
       Color col = pal.Entries[i];
       pal.Entries[i] = Color.FromArgb(0, col.R, col.G, col.B);
}
resizedImage.Palette = pal;
BitmapData src = ((Bitmap)image).LockBits(new Rectangle(0, 0, reqWidth, reqHeight),  ImageLockMode.ReadOnly, image.PixelFormat);
BitmapData dst = resizedImage.LockBits(new Rectangle(0, 0, resizedImage.Width, resizedImage.Height),
ImageLockMode.WriteOnly, resizedImage.PixelFormat);
((Bitmap)image).UnlockBits(src);
resizedImage.UnlockBits(dst);

Good luck !

Grégoire Lafortune