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!