"Anders Spliid Hansen" <no@spam.for.me> wrote in message
news:Xns9755730AA7673nospamforme@130.225.247.90...
> I've got a bunch of scanned microfiches in 1bpp format, all rather big,
> about 6000 x 4000 pixels. Some of the fiches are in handwriting, and this
> is why I want to make an algorithm that converts and smooths them into
> 8bpp
> grayscale. After reading Bob Powells excellent "Understanding LockBits" I
> was able to come up with this (initialization details omitted):
>
> Bitmap bmOrg; // Format1bppIndexed image
> Bitmap bmGray = new Bitmap(bmOrg.Width, bmOrg.Height,
> PixelFormat.Format8bppIndexed);
> // convert 1bpp to 8bpp, palette of bmGray is set to a grayscale one.
> unsafe
> {
> for (y = 0; y < bmGray.Height; y++)
> {
> byte* pixelGray = (byte*)(void*)Scan0_gray + (y * strideGray);
> byte* pixelOrg = (byte*)(void*)Scan0_org + (y * strideOrg);
>
> for (x = 0; x < bmGray.Width; x++)
> {
> //ANDs between 128, 64, 32, 16, 8, 4, 2, 1 and the original image byte,
> //to obtain the individual bits.
> if ((1 << (7 - (x & 7)) & pixelOrg[x / 8]) != 0)
> pixelGray[x] = 255;
> else
> pixelGray[x] = 0;
> }
> }
> }
> // smoothing algorithm. Idea is to calculate a mean value for a 2x2
> // window, and set the four pixels to this mean value. In this way every
> // pixel except those in the first row and the first column, will have it
> // value changed twice.
> unsafe
> {
> for (y = 0; y < bmGray.Height - 1; y++)
> {
> byte* pixel = (byte*)(void*)Scan0_gray + (y * strideGray);
> for (x = 0; x < bmGray.Width - 1; x++)
> {
> mean = (pixel[x] + pixel[x + 1] + pixel[x + skip] + pixel[x + 1 +
> skip]) / 4;
> pixel[x] = pixel[x + 1] = pixel[x + skip] = pixel[x + 1 + skip] =
> (byte)mean;
> }
> }
> }
>
> It works as intended, OK, but I'm not happy with the performance. On an
> old
> Athlon 1800XP bitbucket the algorithm's running time on a 6000x4000 image
> is about 8 seconds.
>
> Any ideas how to tweak this mother?
>
>
> Thanks in advance,
> Anders Spliid Hansen, Denmark

> oh yeah...
> I have a very quick fix.
> chache bitmap.Width and bitmap.Height and use the cached value.
> as follow
> int w = img.Width;
> int h = img.Height;
>
> from now on, never again access the Width and Height property and use
> the w & h variable.
> try it on, you'll be surprise by the results.
>
> In such cases you thanks your perfmance tool for finding this out!
>