About PPM Parser

Python class for parsing PPM files.

Requirements:

• numpy

Usage

Get the x/y/z/nx/ny/nz for a given u/v

with PPMParser(ppm_filename).open() as ppm:
    x, y, z, nx, ny, nz = ppm.get_3d_coords(1000, 2000)
    print(x, y, z, nx, ny, nz)

Note that this operation is very fast because PPMParser uses file seek operation to read the correct coordinates.

Traverse over ppm

with PPMParser(ppm_filename).open() as ppm:
    for imx, imy, x, y, z, nx, ny, nz in ppm.read_next_coords(skip_empty=True):
        print(x, y, z)

Using step

We often want to get only every n-th point in both x and y direction. Parameter step allows us to do this:

with PPMParser(ppm_filename, step=4).open() as ppm:
    for imx, imy, x, y, z, nx, ny, nz in ppm.read_next_coords(skip_empty=True):
        print(imx, imy)  # note that you need to multiply both with 4 to get the original coordinates

To avoid possible mistakes, we can use im_shape() like this:

with PPMParser(ppm_filename).open() as ppm:
    a = np.zeros(ppm.im_shape(), dtype=...)
    for imx, imy, x, y, z, nx, ny, nz in ppm.read_next_coords(skip_empty=True):
        a[imx, imy] = ...

Or we can let PPMParser allocate numpy array for us:

with PPMParser(ppm_filename).open() as ppm:
    a = ppm.im_zeros(dtype=...)
    for imx, imy, x, y, z, nx, ny, nz in ppm.read_next_coords(skip_empty=True):
        a[imx, imy] = ...

Utility: cropppm.py

Utility cropppm crops the input PPM file and writes the result to output PPM file. In the process stride can be applied. The output filename, if not specified, is constructed automatically.

Usage:

$ pip install pipenv
$ pipenv install
$ pipenv shell
<pipenv> $ python cropppm.py -i /path/to/input.ppm -o /path/to/output.ppm --stride 2 -roi 1000-2000,4000-5000