# image processing

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hi i have no idea where this thread should go..but.. pls let me know how to calculate the mean and variance of an image? thanks

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Are you trying this on a bitmap, or on OpenGL/DirectX textures?

Most image processing filters are working by looping through all pixels. For each pixel (from the original image), you look at the neighbour pixels. For example, check the 8 surrounding pixels. Ussually when you adjust the "strength" or "size" in a paint program, you adjust the box size around the pixel.

With neighbour pixels and the center pixel(the one you are currently on), you can do all kind of tricks. Alot of filters are working with a "kernel" This kernel is a 2D matrix with values
You multiply the kernel values with the neighbour and central pixels, sum all results, and divide with the count of pixels you have been using (9 pixels, in case you use a 3x3 kernel) to get the average. This result is stored in a new texture.
for x:=1 to originalImage.width do   for y:=1 to originalImage.height do       newPixel.rgb = filterX( x,y, originalImage );       newImage.pixels[x,y] = newPixel.rgb;

I don't know the variance effect, but as for the mean, you pick the center pixel + neighbours. Let's say you use a small 3x3 area = 9 pixels. You'll have to order these values in an array. I'm not sure what's the best way to order though. You could calculate the luminance (1 single value) per pixel, and order on luminance -> array[0] = darkest pixel, array[MAX_SIZE] = brightes_pixel.

But you can also order per channel. Make 3 array's, 1 for red, 1 green, and 1 for blue. For each pixel, you extract the red,green and blue color. Place the red value in the red array, green in green array, etc.

In the end, you take the mid values from the array(s)
// In case you order the 3 color channels appart:result.r = red_Array[  ARRAY_SIZE / 2 ]; result.g = green_Array[ARRAY_SIZE / 2 ];result.b = blue_Array[ ARRAY_SIZE / 2 ];

Greetings,
Rick

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i am trying to calculate the mean and variance of a gray scale image..

u mean to say the mean of an image is calculated on each pixel basis??

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Depends. If you want to apply a mean filter (mainly used to remove noise from an image), you need to do it per pixel indeed. If you are using gray-scale images, the routine stays the same, although you don't have to use 3 color channels. Just order the pixel/neighbours on luminance.

However, if you want 1 single mean value over the entire image, you can forget about what I said with kernels. The only thing you'll have to do in that case, is to order ALL the pixels in 1 big array. Loop through all pixels, and place them on the right position inside that big array. The overall mean value is the value at array[ ARRAY_SIZE / 2 ], where ARRAY_SIZE = image.width * image.height.

Greetings,
Rick

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Hi,

what spek means is the median not the mean. The mean is calculated as in any other domain by summing over all pixels and dividing by the number of pixels.
For variance do the same except that you sum over (image_value - mean value)^2.

Hope this helps.

PS.: you can to the variance and the mean calculation in one go if you look up the formulas.

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Shoot. My technical english is crap. That reminds me I'll have to replace "mean" with "median" in some other applications I recently made as well. Sorry for confusing you guys :)

Rick

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