A generic error occurred in GDI+ During a Bitmap Save

Question

When I run this code

Bitmap im = new Bitmap(600, 600, PixelFormat.Format16bppGrayScale);
int height = im.Height;
int width = im.Width;
Point p1 = new Point(0, 0);
Point p2 = new Point(im.Size.Width, 0);
Point p3 = new Point(im.Width / 2, im.Height);
Random r = new Random();
Point p = new Point(r.Next(0, im.Size.Width), r.Next(0, im.Size.Height));
BitmapData data = im.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.ReadWrite, PixelFormat.Format16bppGrayScale);
const int stride = 2;
int wsrtide = data.Stride;
unsafe
{
   IntPtr osc0 = data.Scan0;
   ushort* sc0 = (ushort*)data.Scan0;
   for (int y = 0; y < height; y++)
   {
      for (int x = 0; x < width; x++)
      {
         *sc0 = 0;
         sc0 += 1;
      }
   }
   for (long i = 0; i < width * height; i++)
   {
      if (i % 1_000_000 == 0)
      {
         Console.WriteLine(i);
      }
      var ran = r.Next(0, 3);
      Point tp;
      switch (ran)
      {
         case 0:
            tp = new Point((p1.X + p.X) / 2, (p1.Y + p.Y) / 2);
            sc0 = (ushort*)((int)osc0 + (wsrtide * tp.Y + (tp.X * stride)));
            *sc0 = ushort.MaxValue;
            p = tp;
            break;
         case 1:
            tp = new Point((p2.X + p.X) / 2, (p2.Y + p.Y) / 2);
            sc0 = (ushort*)((int)osc0 + (wsrtide * tp.Y + (tp.X * stride)));
            *sc0 = ushort.MaxValue;
            p = tp;
            break;
         case 2:
            tp = new Point((p3.X + p.X) / 2, (p3.Y + p.Y) / 2);
            sc0 = (ushort*)((int)osc0 + (wsrtide * tp.Y + (tp.X * stride)));
            *sc0 = ushort.MaxValue;
            p = tp;
            break;
      }
   }
   im.UnlockBits(data);
   im.Save(Environment.CurrentDirectory + "\\img.png");
   im.Dispose();

It throws where I save the bitmap

System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI+.`

I am almost certain that this isn't caused by file permissions, when I give the program admin permission and delete the image to reset permissions it still throws. I can confirm that this code swapped with BitmapSetPixel() on RGB works.

My suspicion is that I'm messing up the pointers, but I'm not really sure. Also, curiously enough, it makes empty png files even though it throws.

The purpose of this code is to generate a Sierpinski triangle using the chaos game method.

Answer 1

Your problem is the Format16bppGrayScale i don't think the GDI supports it very well.

Basically if you just create the Bitmap in Format16bppGrayScale, and save it with nothing else, it still gives the error.

I have taken the liberty to rewrite your method Format32bppPArgb

private unsafe static void Main(string[] args)
{
   var height = 600;
   var width = 600;
   var p1 = new Point(0, 0);
   var p2 = new Point(width, 0);
   var p3 = new Point(width / 2, height);
   var r = new Random();
   var p = new Point(r.Next(0, width), r.Next(0, width));

   using (var im = new Bitmap(width, height, PixelFormat.Format32bppPArgb))
   {

      var data = im.LockBits(new Rectangle(0, 0, width, height), ImageLockMode.ReadWrite, PixelFormat.Format32bppPArgb);
      var sc0 = (int*)data.Scan0;
      var pLen = sc0 + height * width;

      var black = Color.Black.ToArgb();
      var white = Color.White.ToArgb();

      for (var pI = sc0; pI < pLen; pI++)
         *pI = black;

      for (long i = 0; i < width * height; i++)
      {
         Point tp;
         switch (r.Next(0, 3))
         {
            case 0:
               tp = new Point((p1.X + p.X) / 2, (p1.Y + p.Y) / 2);
               *(sc0 + tp.Y + tp.X * width) = white;
               p = tp;
               break;
            case 1:
               tp = new Point((p2.X + p.X) / 2, (p2.Y + p.Y) / 2);
               *(sc0 + tp.Y + tp.X * width) = white;
               p = tp;
               break;
            case 2:
               tp = new Point((p3.X + p.X) / 2, (p3.Y + p.Y) / 2);
               *(sc0 + tp.Y + tp.X * width) = white;
               p = tp;
               break;
         }
      }
      im.UnlockBits(data);
      im.Save(@"D:\img.png", ImageFormat.Png);
   }
}

Result

You can convert it after the fact if you want, add pepper and salt to taste

Also if you get rid of all the points and cache the colors this will be a bit faster