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POC# - reverse image bytes quickly?
 

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  1. POC# - reverse image bytes quickly?
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    <p>I'm trying to determine the optimal way to flip an image across the Y axis. For every pixel, there are 4 bytes, and each set of 4 bytes needs to remain together in order but get shifted. Here's the best I've come up with so far.</p> <p>This only takes .1-.2s for a 1280x960 image, but with video such performance is crippling. Any suggestions?</p> <p><strong>Initial implementation</strong></p> <pre><code> private void ReverseFrameInPlace(int width, int height, int bytesPerPixel, ref byte[] framePixels) { System.Diagnostics.Stopwatch s = System.Diagnostics.Stopwatch.StartNew(); int stride = width * bytesPerPixel; int halfStride = stride / 2; int byteJump = bytesPerPixel * 2; int length = stride * height; byte pix; for (int i = 0, a = stride, b = stride - bytesPerPixel; i &lt; length; i++) { if (b % bytesPerPixel == 0) { b -= byteJump; } if (i &gt; 0 &amp;&amp; i % halfStride == 0) { i = a; a += stride; b = a - bytesPerPixel; if (i &gt;= length) { break; } } pix = framePixels[i]; framePixels[i] = framePixels[b]; framePixels[b++] = pix; } s.Stop(); System.Console.WriteLine("ReverseFrameInPlace: {0}", s.Elapsed); } </code></pre> <p><strong>Revision #1</strong></p> <p>Revised with indexes and Buffer.BlockCopy per SLaks and Alexei. Also added a Parallel.For since the indexes allow for it.</p> <pre><code> int[] pixelIndexF = null; int[] pixelIndexB = null; private void ReverseFrameInPlace(int width, int height, int bytesPerPixel, byte[] framePixels) { System.Diagnostics.Stopwatch s = System.Diagnostics.Stopwatch.StartNew(); if (pixelIndexF == null)// || pixelIndex.Length != (width * height)) { int stride = width * bytesPerPixel; int length = stride * height; pixelIndexF = new int[width * height / 2]; pixelIndexB = new int[width * height / 2]; for (int i = 0, a = stride, b = stride, index = 0; i &lt; length; i++) { b -= bytesPerPixel; if (i &gt; 0 &amp;&amp; i % (width / 2 )== 0) { //i = a; i += width / 2; a += stride; b = a - bytesPerPixel; if (index &gt;= pixelIndexF.Length) { break; } } pixelIndexF[index] = i * bytesPerPixel; pixelIndexB[index++] = b; } } Parallel.For(0, pixelIndexF.Length, new Action&lt;int&gt;(delegate(int i) { byte[] buffer = new byte[bytesPerPixel]; Buffer.BlockCopy(framePixels, pixelIndexF[i], buffer, 0, bytesPerPixel); Buffer.BlockCopy(framePixels, pixelIndexB[i], framePixels, pixelIndexF[i], bytesPerPixel); Buffer.BlockCopy(buffer, 0, framePixels, pixelIndexB[i], bytesPerPixel); })); s.Stop(); System.Console.WriteLine("ReverseFrameInPlace: {0}", s.Elapsed); } </code></pre> <p><strong>Revision #2</strong></p> <pre><code> private void ReverseFrameInPlace(int width, int height, System.Drawing.Imaging.PixelFormat pixelFormat, byte[] framePixels) { System.Diagnostics.Stopwatch s = System.Diagnostics.Stopwatch.StartNew(); System.Drawing.Rectangle imageBounds = new System.Drawing.Rectangle(0,0,width, height); //create destination bitmap, get handle System.Drawing.Bitmap bitmap = new System.Drawing.Bitmap(width, height, pixelFormat); System.Drawing.Imaging.BitmapData bitmapData = bitmap.LockBits(imageBounds, System.Drawing.Imaging.ImageLockMode.ReadWrite, bitmap.PixelFormat); IntPtr ptr = bitmapData.Scan0; //byte[] to bmap System.Runtime.InteropServices.Marshal.Copy(framePixels, 0, ptr, framePixels.Length); bitmap.UnlockBits(bitmapData); //flip bitmap.RotateFlip(System.Drawing.RotateFlipType.RotateNoneFlipX); //get handle for bitmap to byte[] bitmapData = bitmap.LockBits(imageBounds, System.Drawing.Imaging.ImageLockMode.ReadWrite, bitmap.PixelFormat); ptr = bitmapData.Scan0; System.Runtime.InteropServices.Marshal.Copy(ptr, framePixels, 0, framePixels.Length); bitmap.UnlockBits(bitmapData); s.Stop(); System.Console.WriteLine("ReverseFrameInPlace: {0}", s.Elapsed); } </code></pre>
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    1. COThere is no point in using `ref`.
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    2. COIs `unsafe` code an option for you?
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    3. COunsafe is no problem
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