Sequence

Note

The image sequence-animation.gif used in this docs has been released into the public domain by its author, C6541 at Wikipedia project. This applies worldwide. (Source)

New in version 0.3.0.

Some images may actually consist of two or more images. For example, animated image/gif images consist of multiple frames. Some image/ico images have different sizes of icons.

sequence-animation.gif

For example, the above image sequence-animation.gif consists of the following frames (actually it has 60 frames, but we sample only few frames to show here):

frames of sequence-animation.gif

sequence is a Sequence

If we open this image, Image object has sequence. It’s a list-like object that maintain its all frames.

For example, len() for this returns the number of frames:

>>> from wand.image import Image
>>> with Image(filename='sequence-animation.gif') as image:
...     len(image.sequence)
...
60

You can get an item by index from sequence:

>>> with Image(filename='sequence-animation.gif') as image:
...     image.sequence[0]
...
<wand.sequence.SingleImage: ed84c1b (256x256)>

Or slice it:

>>> with Image(filename='sequence-animation.gif') as image:
...     image.sequence[5:10]
...
[<wand.sequence.SingleImage: 0f49491 (256x256)>,
 <wand.sequence.SingleImage: 8eba0a5 (256x256)>,
 <wand.sequence.SingleImage: 98c10fa (256x256)>,
 <wand.sequence.SingleImage: b893194 (256x256)>,
 <wand.sequence.SingleImage: 181ce21 (256x256)>]

Image versus SingleImage

Note that each item of sequence is a SingleImage instance, not Image.

Image is a container that directly represents image files like sequence-animation.gif, and SingleImage is a single image that represents frames in animations or sizes in image/ico files.

They both inherit BaseImage, the common abstract class. They share the most of available operations and properties like resize() and size, but some are not. For example, save() and mimetype are only provided by Image. delay and index are only available for SingleImage.

In most cases, images don’t have multiple images, so it’s okay if you think that Image and SingleImage are the same, but be careful when you deal with animated image/gif files or image/ico files that contain multiple icons.

Manipulating SingleImage

When working with sequence, it’s important to remember that each instance of SingleImage holds a copy of image data from the stack. Altering the copied data will not automatically sync back to the original image-stack.

>>> with Image(filename='animation.gif') as image:
...     # Changes on SingleImage are invisible to `image` container.
...     image.sequence[2].negate()
...     image.save(filename='output.gif')  # Changes ignored.

If you intended to alter a SingleImage, and have changes synchronized back to the parent image-stack, use an additional with-statement context manager.

>>> with Image(filename='animation.gif') as image:
...     # Changes on SingleImage are sync-ed after context manager closes.
...     with image.sequence[2] as frame:
...         frame.negate()
...     image.save(filename='output.gif')  # Changes applied.

Working directly with Image-Stack Iterators

A faster way to work with images in a sequence is to use the internal stack iterator. This does not create copies, or generate Sequence / SingleImage instances.

Warning

Users should NOT mix Image.sequence code with direct iterator methods.

When reading a image file, the internal iterator is pointing to the last frame read. To iterate over all frames, use Image.iterator_reset() and Image.iterator_next() methods.

>>> with Image(filename='link_to_the_past.gif') as img:
...     img.iterator_reset()
...     print("First frame", img.size)
...     while img.iterator_next():
...         print("Additional frame", img.size)
First frame (300, 289)
Additional frame (172, 128)
Additional frame (172, 145)
Additional frame (123, 112)
Additional frame (144, 182)
Additional frame (107, 117)
Additional frame (171, 128)
Additional frame (123, 107)

You can also iterate backwards with Image.iterator_last() and Image.iterator_previous() methods.

>>> with Image(filename='link_to_the_past.gif') as img:
...     img.iterator_last()
...     print("End frame", img.size)
...     while img.iterator_previous():
...         print("Previous frame", img.size)
End frame (123, 107)
Previous frame (171, 128)
Previous frame (107, 117)
Previous frame (144, 182)
Previous frame (123, 112)
Previous frame (172, 145)
Previous frame (172, 128)
Previous frame (300, 289)

Method Image.iterator_first() is like Image.iterator_reset(), but allows the next image read to be prepended at the start of the stack.

>>> with Image(filename='support/link_net.gif') as img:
...     img.iterator_first()
...     img.pseudo(1, 1, 'xc:gold')
...     img.iterator_set(0)
...     print(img.size)
...     img.iterator_set(1)
...     print(img.size)
(1, 1)
(300, 289)

Note

The “image-stack” is general term for a linked list of sub-images in a file. The nomenclature varies between industries & users. You may find documents referencing sub-images as:

  • Frame for animated formats (GIF)

  • Page for document based formats (PDF)

  • Layer for publishing formats (PSD, TIFF)