Converting Bitmap PixelFormats in C#

Question

I need to convert a Bitmap from PixelFormat.Format32bppRgb to PixelFormat.Format32bppArgb.

I was hoping to use Bitmap.Clone, but it does not seem to be working.

Bitmap orig = new Bitmap("orig.bmp");
Bitmap clone = orig.Clone(new Rectangle(0,0,orig.Width,orig.Height), PixelFormat.Format24bppArgb);

If I run the above code and then check clone.PixelFormat it is set to PixelFormat.Format32bppRgb. What is going on/how do I convert the format?

Victorie

Does this only happen on XP? I found the problem on XP but it seems to work fine on Windows 7. Hans' solution fixed it for me. – Hammerhead

That's kind of funny. I have a solution that works fine on XP but breaks, giving this error, on Windows 7... – Ary

I can confirm that only XP is affected. – Wittol

You should watch out on systems that aren't XP too. I haven't tested this spesific code, but working on the pixel level when comparing two images on Windows 7 I ran into a similar problem that was caused from this bug/feature. Dan7 explained exactly what is happening and a more general approach to fixing it. – Refrigerator

This works on all platforms (disclaimer: written by me). Can be downloaded as a NuGet. – Rabbitfish

Answer 1

Sloppy, not uncommon for GDI+. This fixes it:

Bitmap orig = new Bitmap(@"c:\temp\24bpp.bmp");
Bitmap clone = new Bitmap(orig.Width, orig.Height,
    System.Drawing.Imaging.PixelFormat.Format32bppPArgb);

using (Graphics gr = Graphics.FromImage(clone)) {
    gr.DrawImage(orig, new Rectangle(0, 0, clone.Width, clone.Height));
}

// Dispose orig as necessary...

Lunula

I had a similar issue to the OP, except I had problems with a 48BppRgb TIFF that was saved as 32BppArgb PNG. GDI+ couldn't convert properly and the resulting image was very dark. This solution allowed me to solve that problem by manually redrawing the image as 32BppRgb, so +1. – Infracostal

Answer 2

For some reason if you create a Bitmap from a file path, i.e. Bitmap bmp = new Bitmap("myimage.jpg");, and call Clone() on it, the returned Bitmap will not be converted.

However if you create another Bitmap from your old Bitmap, Clone() will work as intended.

Try something like this:

using (Bitmap oldBmp = new Bitmap("myimage.jpg"))
using (Bitmap newBmp = new Bitmap(oldBmp))
using (Bitmap targetBmp = newBmp.Clone(new Rectangle(0, 0, newBmp.Width, newBmp.Height), PixelFormat.Format32bppArgb))
{
    // targetBmp is now in the desired format.
}

Know

+1 Thanks Dan7 for explaining what the trigger was. I've seen new Bitmap(new Bitmap("image.jpg")) before but didn't know why it worked. – Ary

is there an easy way to Clone and convert all the pages of the bitmap ? I've noticed that targtBmp only has the active frame in oldBmp. – Caddish

This better than marked "Solution" - it's worked with Indexed PixelFormat 4bpp, 8bpp – Doorstep

Answer 3

using (var bmp = new Bitmap(width, height, PixelFormat.Format24bppArgb))
using (var g = Graphics.FromImage(bmp)) {
  g.DrawImage(..);
}

Should work like that. Maybe you want to set some parameters on g to define the interpolation mode for quality etc.

Omnivorous

InterpolationMode is for scaling, i don't really think that he wants to scale it. Maybe do something like bmp.SetResolution(old.HorizontalResolution, old.VerticalResolution) and call g.DrawImageUnscaled(old, 0,0). But its just my opinion :) – Ribbing

@Elmue - do you have any reference to your statement?MSDN states other.. – Leg