As New Bitmap Alters Image

Question

4.00/5 (1 vote)

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I need to know why the following code alters the image.

Dim a = New Bitmap( _
My.Application.Info.DirectoryPath & "\a.png")
Dim b As New Bitmap(a)
b.Save("b.png", System.Drawing.Imaging.ImageFormat.Png)

And please don't reply

Dim b = a

I need to know why the colors in the resulting image are altered (i.e. white becomes 254, and other colors and alpha values change). I need to write this so that the resulting picture's colors match those of the original.

Posted 16-Jan-13 7:35am

kamikazehighland

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Thomas Daniels · Accepted Answer · 2013-01-16T07:40:00

Solution 1

Hi,

Have a look here:
http://en.wikipedia.org/wiki/Lossy_compression[^]

Quote from Wikipedia page:
In information technology, "lossy" compression is a data encoding method that compresses data by discarding (losing) some of it.

Posted 16-Jan-13 7:40am

Thomas Daniels

Comments

kamikazehighland 16-Jan-13 13:46pm

Hmm. Thanks. I was hoping to avoid that by using PNG. I'll have to look into it further.

Thomas Daniels 16-Jan-13 13:48pm

Thanks.
You're welcome!

Sergey Alexandrovich Kryukov 16-Jan-13 19:03pm

And now, I suggest how to resolve this problem by using lossless compression. Please see my solution.
—SA

Sergey Alexandrovich Kryukov 16-Jan-13 18:51pm

Very good, a 5.
—SA

Thomas Daniels 17-Jan-13 11:40am

Thank you!

Sergey Alexandrovich Kryukov · Accepted Answer · 2013-01-16T13:02:00

You can use lossless compression, for example, in the TIFF container, which can support a number of different compression algorithms, most of them are lossless. PNG supports lossless compression as well. Please see:
http://en.wikipedia.org/wiki/Portable_Network_Graphics[^],
http://en.wikipedia.org/wiki/Tagged_Image_File_Format[^],
http://en.wikipedia.org/wiki/Lossless_data_compression[^].

Now, .NET classes do not support PNG compression parameters. Instead, you can consider using this library: http://optipng.sourceforge.net/[^].
Unfortunately, this is a C library. You can use it in your .NET projects via P/Invoke or C++/CLI (mixed mode).
You might want to search more in hope to find something better.

So, you might be better off with TIFF, as .NET FCL offers a class for TIFF support:
http://msdn.microsoft.com/en-us/library/system.windows.media.imaging.tiffbitmapencoder%28v=vs.90%29.aspx[^] (and later versions of .NET).

Good luck,

—SA