Examples 3 libvips

## libvips examples

A few example Python programs using libvips

This page shows a few libvips examples using Python. They will work with small syntax changes in any language with a libvips binding.

The libvips test suite is written in Python and exercises every operation in the API. It’s also a useful source of examples.

## Average a region of interest box on an image

    #!/usr/bin/env python

    import sys
    import gi
    gi.require_version('Vips', '8.0')
    from gi.repository import Vips

    left = 10
    top = 10
    width = 64
    height = 64

    image = Vips.Image.new_from_file(sys.argv[1])
    roi = image.crop(left, top, width, height)
    print 'average:', roi.avg()

## libvips and numpy

You can use `Vips.Image.new_from_memory_copy()` to make a vips image from an area of memory. The memory array needs to be laid out band-interleaved, as a set of scanlines, with no padding between lines.

This example moves an image from numpy to vips, but it’s simple to move the other way (use `Vips.Image.write_to_memory()`) to to move images into or out of PIL.

    #!/usr/bin/python

    import numpy
    import scipy.ndimage
    import gi
    gi.require_version('Vips', '8.0')
    from gi.repository import Vips

    def np_dtype_to_vips_format(np_dtype):
        '''
        Map numpy data types to VIPS data formats.

        Parameters
        ----------
        np_dtype: numpy.dtype

        Returns
        -------
        gi.overrides.Vips.BandFormat
        '''
        lookup = {
            numpy.dtype('int8'): Vips.BandFormat.CHAR,
            numpy.dtype('uint8'): Vips.BandFormat.UCHAR,
            numpy.dtype('int16'): Vips.BandFormat.SHORT,
            numpy.dtype('uint16'): Vips.BandFormat.USHORT,
            numpy.dtype('int32'): Vips.BandFormat.INT,
            numpy.dtype('float32'): Vips.BandFormat.FLOAT,
            numpy.dtype('float64'): Vips.BandFormat.DOUBLE
        }
        return lookup[np_dtype]

    def np_array_to_vips_image(array):
        '''
        Convert a `numpy` array to a `Vips` image object.

        Parameters
        ----------
        nparray: numpy.ndarray

        Returns
        -------
        gi.overrides.Vips.image
        '''
        # Look up what VIPS format corresponds to the type of this np array
        vips_format = np_dtype_to_vips_format(array.dtype)
        dims = array.shape
        height = dims[0]
        width = 1
        bands = 1
        if len(dims) > 1:
            width = dims[1]
        if len(dims) > 2:
            bands = dims[2]
        img = Vips.Image.new_from_memory_copy(array.data,
            width, height, bands, vips_format)

        return img

    array = numpy.random.random((10,10))
    vips_image = np_array_to_vips_image(array)
    print 'avg =', vips_image.avg()

    array = scipy.ndimage.imread("test.jpg")
    vips_image = np_array_to_vips_image(array)
    print 'avg =', vips_image.avg()
    vips_image.write_to_file("test2.jpg")

## Watermarking

This example renders a simple watermark on an image. Use it like this:

    ./watermark.py somefile.png output.jpg "hello <i>world</i>"

The text is rendered in transparent red pixels all over the image. It knows about transparency, CMYK, and 16-bit images.

    #!/usr/bin/python

    import sys
    import gi
    gi.require_version('Vips', '8.0')
    from gi.repository import Vips

    im = Vips.Image.new_from_file(sys.argv[1], access = Vips.Access.SEQUENTIAL)

    text = Vips.Image.text(sys.argv[3], width = 500, dpi = 300)
    text = (text * 0.3).cast("uchar")
    text = text.embed(100, 100, text.width + 200, text.width + 200)
    text = text.replicate(1 + im.width / text.width, 1 + im.height / text.height)
    text = text.crop(0, 0, im.width, im.height)

    # we want to blend into the visible part of the image and leave any alpha
    # channels untouched ... we need to split im into two parts

    # 16-bit images have 65535 as white
    if im.format == Vips.BandFormat.USHORT:
        white = 65535
    else:
        white = 255

    # guess how many bands from the start of im contain visible colour information
    if im.bands >= 4 and im.interpretation == Vips.Interpretation.CMYK:
        # cmyk image ... put the white into the magenta channel
        n_visible_bands = 4
        text_colour = [0, white, 0, 0]
    elif im.bands >= 3:
        # colour image ... put the white into the red channel
        n_visible_bands = 3
        text_colour = [white, 0, 0]
    else:
        # mono image
        n_visible_bands = 1
        text_colour = white

    # split into image and alpha
    if im.bands - n_visible_bands > 0:
        alpha = im.extract_band(n_visible_bands, n = im.bands - n_visible_bands)
        im = im.extract_band(0, n = n_visible_bands)
    else:
        alpha = None

    # blend means do a smooth fade using the 0 - 255 values in the condition channel
    # (test in this case) ... this will render the anit-aliasing
    im = text.ifthenelse(text_colour, im, blend = True)

    # reattach alpha
    if alpha:
        im = im.bandjoin(alpha)

    im.write_to_file(sys.argv[2])

## Build huge image mosaic

This makes a 100,000 x 100,000 black image, then inserts all the images you pass on the command-line into it at random positions. libvips is able to run this program in sequential mode: it’ll open all the input images at the same time, and stream pixels from them as it needs them to generate the output.

To test it, first make a large 1-bit image. This command will take the green channel and write as a 1-bit fax image. `wtc.jpg` is a test 10,000 x 10,000 jpeg:

    $ vips extract_band wtc.jpg x.tif[squash,compression=ccittfax4,strip] 1

Now make 1,000 copies of that image in a subdirectory:

    $ mkdir test
    $ for i in {1..1000}; do cp x.tif test/$i.tif; done

And run this Python program on them:

    $ time ./try255.py x.tif[squash,compression=ccittfax4,strip,bigtif] test/*
    real    1m59.924s
    user    4m5.388s
    sys 0m8.936s

It completes in just under two minutes on this laptop, and needs about 7gb of RAM to run. It would need about the same amount of memory for a full-colour RGB image, I was just keen to keep disc usage down.

If you wanted to handle transparency, or if you wanted mixed CMYK and RGB images, you’d need to do some more work to convert them all into the same colourspace before inserting them.

    #!/usr/bin/env python

    import sys
    import random

    import gi
    gi.require_version('Vips', '8.0')
    from gi.repository import Vips

    # turn on progress reporting
    Vips.progress_set(True)

    # this makes a 8-bit, mono image of 100,000 x 100,000 pixels, each pixel zero
    im = Vips.Image.black(100000, 100000)

    for filename in sys.argv[2:]:
        tile = Vips.Image.new_from_file(filename, access = Vips.Access.SEQUENTIAL)

        im = im.insert(tile,
                       random.randint(0, im.width - tile.width),
                       random.randint(0, im.height - tile.height))

    im.write_to_file(sys.argv[1])

## Rename DICOM images using header fields

DICOM images commonly come in an awful directory hierarchy named as something like `images/a/b/e/z04`. There can be thousands of files and it can be very hard to find the one you want.

This utility copies files to a single flat directory, naming them using fields from the DICOM header. You can actually find stuff! Useful.

    #!/usr/bin/env python

    import sys
    import re
    import os
    import shutil

    import gi
    gi.require_version('Vips', '8.0')
    from gi.repository import Vips

    if len(sys.argv) != 3:
        print 'rename DICOM files using tags from the header'
        sys.exit(1)

    srcdir = sys.argv[1]
    destdir = sys.argv[2]

    if not os.access(destdir, os.F_OK | os.R_OK | os.W_OK | os.X_OK):
        os.mkdir(destdir)

    def get_field(vim, field):
        result = vim.get_value(field)

        # remove any \n etc.
        result = re.sub("\n", "", result)

        # remove any leading or trailing spaces
        result = re.sub(" $", "", result)
        result = re.sub("^ ", "", result)

        return result

    modality_name = "magick-dcm:Modality"
    series_name = "magick-dcm:SeriesNumber"
    instance_name = "magick-dcm:Instance(formerlyImage)Number"
    date_name = "magick-dcm:ImageDate"

    for(dirpath, dirnames, filenames) in os.walk(srcdir):
        for file in filenames:
            path = os.path.join(dirpath, file)

            try:
                vim = Vips.Image.new_from_file(path)
            except Vips.Error, e:
                print 'unable to open', path
                print e
                continue

            try:
                modality = get_field(vim, modality_name)
                series = get_field(vim, series_name)
                instance = get_field(vim, instance_name)
                date = get_field(vim, date_name)
            except Vips.Error, e:
                print 'unable to get fields from header', path
                print e
                continue

            match = re.match("(\d\d\d\d)(\d\d)(\d\d)", date)
            date = match.group(1) + "." + match.group(2) + "." + match.group(3)

            newname = "lan." + modality + "." + instance + "." + date + ".IMA"

            shutil.copyfile(path, os.path.join(destdir, newname))