how to compare two bitmaps?
Jianren Lu
I have two bitmaps, rawBitmap and newBitmap, one is another's
copy. But later the newBitmap may have some change, how can I compare
them the same picture or not?
the following is my code.
rawBitmap = new Bitmap(newBitmap.Width, newBitmap.Height);
Graphics g=Graphics.FromImage(rawBitmap);
g.DrawImage(newBitmap, 0, 0, newBitmap.Width,
newBitmap.Height);
g.Dispose();
later, newBitmap may have some change, for example, change to gray
scale, something like that, how can I compare them still the same
picture?
jianren
Chris Hornberger
sampling. Say, every 10th byte and see if it matches from one to the
other. Or a random sampling of byte matches.
l...@ils.uec.ac.jp (Jianren Lu) wrote in message news:<150b8982.03062...@posting.google.com>...
Chris R. Timmons
Jianren,
I've appended a class (BitmapHasher) to the end of this message that
can be used to get a unique hash value for a bitmap. Here's how to
use it:
System.UInt32 rawHash = BitmapHasher.GetBitmapHashValue(rawBitmap);
System.UInt32 newHash = BitmapHasher.GetBitmapHashValue(newBitmap);
if (rawHash != newHash)
// The bitmaps are different.
Hope this helps.
Chris.
-------------
C.R. Timmons Consulting, Inc.
http://www.crtimmonsinc.com/
///////////////////////////////////////////
using System;
using System.Drawing;
namespace CrtInc.Utils
{
/// <summary>
/// BitmapHasher provides one static method to determine the
/// 32-bit hash (unique) value of a given bitmap.
/// <para>
/// The code for the class was ported directly from a C code
/// algorithm for the PNG graphics format at
/// <see href="http://www.libpng.org/pub/png/spec/PNG-
/// CRCAppendix.html">
/// http://www.libpng.org/pub/png/spec/PNG-CRCAppendix.html</see>.
/// A few C# specific constructs were added, and the code was
/// shortened somewhat, but otherwise remains as close as
/// possible to the original C code.
/// </para>
/// </summary>
public sealed class BitmapHasher
{
// Don't allow instances of this class to be created.
private BitmapHasher() : base() {}
// Use a lookup table for speed.
private static readonly uint[] CRCTable = new uint[256];
static BitmapHasher()
{
uint c;
// Populate the CRC table.
for (uint n = 0; n < 256; n++)
{
c = n;
for (uint k = 0; k < 8; k++)
{
if ((c & 1) == 1)
c = 0xEDB88320 ^ (c >> 1);
else
c = c >> 1;
}
BitmapHasher.CRCTable[n] = c;
}
}
/// <summary>
/// Static method which returns the unsigned 32-bit integer
/// hash value of a bitmap.
/// </summary>
/// <param name="image">A <see cref="System.Drawing.Bitmap"/>
/// object.</param>
/// <returns>A <c>uint (System.UInt32)</c> hash value for the
/// <c>image</c> bitmap.</returns>
/// <remarks>
/// I'm not going to pretend that I know exactly how this
/// code works.
/// I didn't write it (I only ported it from C), and all
/// the bit-shifting and exclusive-OR (^) operations
/// leaves me dizzy.
/// Therefore, I can safely claim ignorance :).
/// <para>
/// However, it does appear to work. My testing so far has
/// been limited to the code's functionality, and not its speed.
/// </para>
/// <para>
/// Note that this method is not Common Language
/// Specification (CLS) compliant because it returns an
/// unsigned 32-bit integer. If you want to call
/// this code from a non-C# language that does not support the
/// System.UInt32 data type, the call probably won't work.
/// </para>
/// </remarks>
/// <example>
/// <code lang="C#">
/// // Example 1:
/// System.UInt32 hash = BitmapHasher.GetBitmapHashValue(
/// myImageList.Images[1] as Bitmap);
/// <para>
/// // Example 2:
/// Icon icon = new Icon("C:\IMAGES\FLOWER.ICO");
/// System.UInt32 hash = BitmapHasher.GetBitmapHashValue(
/// icon.ToBitmap());
/// </para>
/// </code>
/// </example>
[CLSCompliant(false)]
public static uint GetBitmapHashValue(
Bitmap image)
{
uint result = 0xFFFFFFFF;
for (int x = 0; x < image.Width; x++)
for (int y = 0; y < image.Height; y++)
result =
BitmapHasher.CRCTable[
(result ^ image.GetPixel(x, y).ToArgb()) & 0xFF] ^
(result >> 8);
return result ^ 0xFFFFFFFF;
}
}
}
Bill Woodruff
Do a search on messages in this newsgroup with the word "checksum" in the title
and you will find how to use the cryptography namespace in C# and source code :
Looks like these solutions require that you get your bitmap into a byte array format.
best, Bill Woodruff
dotScience
Chaing Mai, Thailand
"Jianren Lu" <l...@ils.uec.ac.jp> wrote in message news:150b8982.03062...@posting.google.com...