6.2. Built-in module imageop

The module defines the following variables and functions:

error -- exception of module imageop

This exception is raised on all errors, such as unknown number of bits per pixel, etc.

crop (image, psize, width, height, x0, y0, x1, y1) -- function of module imageop

This function takes the image in image, which should by width by height in size and consist of pixels of psize bytes, and returns the selected part of that image. X0, y0, x1 and y1 are like the lrectread parameters, i.e. the boundary is included in the new image. The new boundaries need not be inside the picture. Pixels that fall outside the old image will have their value set to zero. If x0 is bigger than x1 the new image is mirrored. The same holds for the y coordinates.

scale (image, psize, width, height, newwidth, newheight) -- function of module imageop

This function returns a image scaled to size newwidth by newheight. No interpolation is done, scaling is done by simple-minded pixel duplication or removal. Therefore, computer-generated images or dithered images will not look nice after scaling.

tovideo (image, psize, width, height) -- function of module imageop

This function runs a vertical low-pass filter over an image. It does so by computing each destination pixel as the average of two vertically-aligned source pixels. The main use of this routine is to forestall excessive flicker if the image is displayed on a video device that uses interlacing, hence the name.

grey2mono (image, width, height, threshold) -- function of module imageop

This function converts a 8-bit deep greyscale image to a 1-bit deep image by tresholding all the pixels. The resulting image is tightly packed and is probably only useful as an argument to mono2grey.

dither2mono (image, width, height) -- function of module imageop

This function also converts an 8-bit greyscale image to a 1-bit monochrome image but it uses a (simple-minded) dithering algorithm.

mono2grey (image, width, height, p0, p1) -- function of module imageop

This function converts a 1-bit monochrome image to an 8 bit greyscale or color image. All pixels that are zero-valued on input get value p0 on output and all one-value input pixels get value p1 on output. To convert a monochrome black-and-white image to greyscale pass the values 0 and 255 respectively.

grey2grey4 (image, width, height) -- function of module imageop

Convert an 8-bit greyscale image to a 4-bit greyscale image without dithering.

grey2grey2 (image, width, height) -- function of module imageop

Convert an 8-bit greyscale image to a 2-bit greyscale image without dithering.

dither2grey2 (image, width, height) -- function of module imageop

Convert an 8-bit greyscale image to a 2-bit greyscale image with dithering. As for dither2mono, the dithering algorithm is currently very simple.

grey42grey (image, width, height) -- function of module imageop

Convert a 4-bit greyscale image to an 8-bit greyscale image.

grey22grey (image, width, height) -- function of module imageop

Convert a 2-bit greyscale image to an 8-bit greyscale image.