A RECTANGULAR collection of picture elements. Each pixel is a 'cell', with an (i,j) index running from (0,0) to (xres-1,yres-1)

pixel array

Point ops involve a calculation on JUST one pixel at a time, repeated for all pixels in an img.

Brightness: color <- color*brightness_value.

brighness enhancement - linear

The brightness factor can be made non-linear first, using a bias() function:

bias(b,t) = t^(ln(b)/ln(0.5))

When brightness is non-linearly adjusted this way, it's called "gamma correction".

gamma correction

Contrast: use a s-curve, eg. a gain() function:

gain(g,t)= if (t<0.5) then bias(1-g,2*t) else 1-bias(1-g,2-2*2t)/2

contrast enhancement

Notice that we're brightening the bright regions and darkening the dark..

Color correction - modify R,G,B values of pixels, often using a lookup table or a 'transfer curve'

Here is another point process:

solarization

Color correction demo

Likewise, converting RGB image to "black and white" (grayscale, really) is also a point operation.

RGB -> gray

Operations such as color cycling, posterization (quantization), histogram equalization, etc. are also point processes.

Here we compute a new value (color/grayscale) for each pixel by considering pixels in its neighborhood also, not just the pixel being processed. The rectangular grid of multiplication (scale) factors around a pixel's neighborhood (including itself) is called a 'convolution kernel' or 'mask'. You can imagine this mask being slid over every pixel to alter its value (compute a replacement).

Convolution - 1

Convolution - 2

Convolution - 3

Convolution - 4

When it comes to non-point/non-kernel processes (eg. image distortion), 'forward mapping' (calculating where source pixel ends up on a target grid) can cause overlaps/holes! So what's commonly done is 'inverse mapping', where every target pixel is synthesized from source pixels.

inverse mapping

While resizing images, we use filters such as 'box', 'triangle' and 'bicubic'.

Bicubic interpolation

Lecture from Princeton U.

Another page on warping

One more..

These are special warps meant to (de)warp imagery..

Dewarping

Warping/Dewarping

Warping/Dewarping - more

Texture mapping - 1

Texture mapping - 2

Texture mapping - 3

Texture mapping - 4

Texture mapping - 5

Texture mapping - 6

Texture mapping - 7

Displacement mapping

In normal mapping, we use an image (normal map) to look up normals for lighting purposes.. So it is really 'texture based bump mapping"..

Normal mapping

Compositing

Link: http://en.wikipedia.org/wiki/Mandelbrot_set