Reading/preserving a PixelFormat.Format48bppRgb PNG Bitmap in .NET?

Question

I've been able to create a Format48bppRgb .PNG file (from some internal HDR data) using the the following C# code:

Bitmap bmp16 = new Bitmap(_viewer.Width, _viewer.Height, System.Drawing.Imaging.PixelFormat.Format48bppRgb);
System.Drawing.Imaging.BitmapData data16 = bmp16.LockBits(_viewer.ClientRectangle, System.Drawing.Imaging.ImageLockMode.WriteOnly, bmp16.PixelFormat);
unsafe {  (populates bmp16) }
bmp16.Save( "C:/temp/48bpp.png", System.Drawing.Imaging.ImageFormat.Png );

ImageMagik (and other apps) verify that this is indeed a 16bpp image:

C:\temp>identify 48bpp.png
48bpp.png PNG 1022x1125 1022x1125+0+0 DirectClass 16-bit 900.963kb

I was disappointed, however, to find that on reading the PNG back in, it had been converted to Format32bppRgb, when using:

Bitmap bmp = new Bitmap( "c:/temp/48bpp.png", false );
String info = String.Format("PixelFormat: {0}", bmp.PixelFormat );
...

Given that the PNG codec can write a Format48bppRgb, is there any way I can use .NET to read it in without the conversion? I don't mind if it does this for a DrawImage call, but I would like access to the decompressed, original data for some histogram/image processing work.

Answer 1

FYI - I did find a .NET solution to this using System.Windows.Media.Imaging (I had been using strictly WinForms/GDI+ - this requires adding WPF assemblies, but works.) With this, I get a Format64bppArgb PixelFormat, so no lost information:

using System.Windows.Media.Imaging; // Add PresentationCore, WindowsBase, System.Xaml
...

    // Open a Stream and decode a PNG image
Stream imageStreamSource = new FileStream(fd.FileName, FileMode.Open, FileAccess.Read, FileShare.Read);
PngBitmapDecoder decoder = new PngBitmapDecoder(imageStreamSource, BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.Default);
BitmapSource bitmapSource = decoder.Frames[0];

    // Convert WPF BitmapSource to GDI+ Bitmap
Bitmap bmp = _bitmapFromSource(bitmapSource);
String info = String.Format("PixelFormat: {0}", bmp.PixelFormat );
MessageBox.Show(info);

...

And this code snippet from: http://www.generoso.info/blog/wpf-system.drawing.bitmap-to-bitmapsource-and-viceversa.html

private System.Drawing.Bitmap _bitmapFromSource(BitmapSource bitmapsource) 
{ 
    System.Drawing.Bitmap bitmap; 
    using (MemoryStream outStream = new MemoryStream()) 
    { 
        // from System.Media.BitmapImage to System.Drawing.Bitmap 
        BitmapEncoder enc = new BmpBitmapEncoder(); 
        enc.Frames.Add(BitmapFrame.Create(bitmapsource)); 
        enc.Save(outStream); 
        bitmap = new System.Drawing.Bitmap(outStream); 
    } 
    return bitmap; 
} 

If anyone has knows of a way to do this that doesn't require WPF, please share!

Answer 2

Use Image.FromFile(String, Boolean) or Bitmap.FromFile(String, Boolean)

And set the Boolean value to true. All image properties will be saved in the new image.

Here String is the filename with full path...

If the image is already loaded in a program and you want to create a new Bitmap with it, you can also use

MemoryStream ms = new MemoryStream();
img.Save(ms, ImageFormat.Bmp); // img is any Image, previously opened or came as a parameter
Bitmap bmp = (Bitmap)Bitmap.FromStream(ms,true);

Common alternative is

Bitmap bmp = new Bitmap(img); // this won't preserve img.PixelFormat