I wanted to make a small image resize function in my asp.net website, and everything worked fine, but i faced a problem resizing transparent .png and .gif images.

After resizing done, i find that the output resized images transparent parts become white color and not transparent anymore.

This was my code:

Graphics objGraphics;

objGraphics = System.Drawing.Graphics.FromImage(imgFinal);
objGraphics.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic; /* new way */
objGraphics.SmoothingMode = SmoothingMode.AntiAlias;
objGraphics.DrawImage(imgOriginal, 0, 0, iNewWidth, iNewHeight);
objGraphics.Dispose();

imgFinal.Save("filename");

And searched to find a solution for this problem with no hope for a complete easy solution.

And while trying to find a way, i remembered that i did a small resize tool “windows application” 4 years ago, i decided to try it and see the output, and i found after resize the image, the output is very nice and preserve transparent :).

I traced the code, and found the only difference is that in my windows application i was using the PictureBox control not the Graphics class.

I copies the code from there and put in my asp.net “sure needed to import System.Windows.Forms library in my asp.net to use the PictureBox control”.

And the Code became:

PictureBox pic = new PictureBox();
pic.Image = imgOriginal;
imgFinal = new System.Drawing.Bitmap(pic.Image, iTargetWidth, iTargetHeight);
imgFinal.Save("filename");

I believe that using Windows form control in asp.net is odd, but it solved my current problem, it may need some more testing for performance issues.