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Image scaling

Project description


This package contains image scaling logic for use in Zope environments. It supports Zope 2, grok and other systems build on using the Zope ToolKit (ZTK).

Several design goals were used when writing this package:

  • image scaling to any width, height, width&height should be supported using both up-scaling and down-scaling. Scaling parameters should never be fixed in code. This allows designers to use any image scale they want without having to modify python code.

  • the result of scaling will be an image along with its new size, not a HTML or XHTML tag. We already have excellent tools to generate tags in the form of Zope Pagetemplates, Genshi and other template languages that are much better suited for this purpose.

In addition several implementation goals were defined:

  • image scaling must happen on demand instead of up-front. This reduces initial save time and prevents unnecessary scalings from being generated.

  • image scaling parameters should not be part of the generated URL. Since the number of parameters can change and new parameters may be added in the future this would create overly complex URLs and URL parsing.

  • no HTML rewriting (such as done by repoze.bitblt) should be required.

  • it should be possibly to develop an external storage system which stores scaled images externally and returns a URL which bypasses the application server. This should be configurable via just a filesystem path & base URL.

  • minimum number of external dependencies, allowing this package to be used in many environments.

  • testable without requiring zope.testing. Running test should be sufficient.

  • URLs for scaled images should have an extension which reflects their MIME type. This is facilitates cache (and other front-end services) configuration.


The most common way to use plone.scale is from a HTML template. In TAL syntax a typical usage looks like this, assuming you have hooked up an image-scaling view:

<img tal:define="scales context/@@image-scaling;
                 thumbnail python:scales.pre_scale('logo', width=64, height=64)"
     tal:attributes="src thumbnail/url;
                     width thumbnail/width;
                     height thumbnail/height" />

With the pre_scale method, no scaling with Pillow actually happens yet. If you call the scale method instead, the scaling does happen. This generates a thumbnail of an image field called logo with a maximum size of 64x64 pixels. The dimensions of the resulting image (which might not be exactly 64x64) are set as attributes on the img tag to speed up browser rendering. Visiting the url will generate the scale if this has not happened yet.

If you prefer Genshi syntax and have the IImageScaleStorage interface in scope the syntax looks like this:

<img py:with="thumbnail=IImageScaleStorage(context).pre_scale('logo', width=64, height=64)"
     py:attributes="dict(src=thumbnail.url, width=thumbnail.width, height=thumbnail.height" />


4.1.2 (2024-03-22)

Bug fixes:

  • If width and height are given, ignore the scale parameter when determining the unique id of a scale. @wesleybl (#92)

4.1.1 (2024-01-31)

Bug fixes:

  • Fix incompatibility with Pillow 10.1+. Instead of setting image.mode, we call image.convert. This works in Pillow 9 and 10. [maurits] (#89)

4.1.0 (2023-10-25)

New features:

  • Keep scaled WEBP images in WEBP format instead of converting to JPEG. @mamico (#85)

4.0.2 (2023-10-07)

Bug fixes:

  • Fix KeyError in ScalesDict conflict resolution. @davisagli (#84)


  • Update configuration files. [plone devs] (cfffba8c)

4.0.1 (2023-03-14)


  • Update configuration files. [plone devs] (a533099d)


  • Tox: explicitly test only the plone.scale package. [maurits] (#50)

4.0.0 (2022-11-26)

Bug fixes:

  • Test with Python 3.11. [maurits] (#311)

  • Require Python 3.7 or higher. [maurits] (#600)

4.0.0b5 (2022-10-20)

New features:

  • Add support for animated GIFs @reebalazs (#69)

4.0.0b4 (2022-10-03)

Breaking changes:

  • No longer test Plone 5.2 on 3.6 and Plone 6.0 on 3.7. [maurits] (#3637)

Bug fixes:

  • Use “scale” mode as default. This cleans up more confusion between mode and direction. See also plone.namedfile issue 102. Previously our definition of the IImageScaleFactory interface had the deprecated direction="thumbnail". Other parts used mode="contain" by default, which does cropping, where in Plone we are used to simple scaling almost everywhere. [maurits] (#102)

4.0.0b3 (2022-07-19)

Bug fixes:

  • Fixed getting original data for a tile. [maurits] (#64)

4.0.0b2 (2022-07-14)

Bug fixes:

  • Fix to ensure that when a scale that was registered using pre_scale is later actually generated by get_or_generate, it is stored with the same uid as the placeholder info that was stored by pre_scale. This avoids an issue where the same scale was generated repeatedly. [davisagli] (#60)

4.0.0b1 (2022-06-23)

New features:

  • Pre scale: store non-random uid to prepare space for a scale. You call pre_scale to pre-register the scale with a unique id without actually doing any scaling with Pillow. When you later call the scale method, the scale is generated. You can still call scale directly without first calling pre_scale. [maurits] (#57)

  • Mark as plone.protect’s safeWrite AnnotationStorage. Precondition for [mamico] (#58)

Bug fixes:

  • Minor cleanup: isort, black. [maurits] (#59)

4.0.0a4 (2022-05-26)

Bug fixes:

  • Fix cropping when the height is not limited. Create a square then. Not limited means: 65000 or larger, or zero or lower. [maurits] (#53)

  • Fix cleanup of scales: only throw away outdated scales of the same field. [maurits] (#55)

4.0.0a3 (2022-05-09)

Bug fixes:

  • Fixed DeprecationWarning with Pillow 9.1.0 for ANTIALIAS. Use Resampling.LANCZOS, falling back to ANTIALIAS on older Pillows. [maurits] (#49)

4.0.0a2 (2022-03-09)

Breaking changes:

  • Removed deprecated factory argument from scale method. This is in the AnnotationStorage class and the IImageScaleStorage interface. This was already scheduled for removal in plone.scale 3.0, but was kept longer. Fixes issue 47. [maurits] (#47)

4.0.0a1 (2022-02-23)

Breaking changes:

  • Removed docs directory and sphinx extra. The docs were last updated in 2010, and the maybe still relevant parts already copied to the readme. [maurits] (#44)

  • Removed tests extra, kept only test extra and storage. Swapped packages slightly between those two extras. For storage we depend on persistent and ZODB. [maurits] (#44)

  • Depend on Pillow. Originally we did not officially depend on it (or PIL) “because not everyone can install it as an egg”. It seems time to grow up here. [maurits] (#44)

  • Removed Python 2 support. Only Python 3.6+ supported now. Still works on Plone 5.2. [maurits] (#44)

New features:

  • Add tox.ini with mxdev. Test with GitHub Actions on Plone 5.2 Py + 3.6-3.8 and Plone 6.0 + Py 3.7-3.10. [maurits] (#44)

3.1.2 (2020-09-07)

Bug fixes:

  • Resolve deprecation warning [gforcada] (#42)

3.1.1 (2020-04-22)

Bug fixes:

  • Minor packaging updates. (#1)

3.1.0 (2020-03-08)

New features:

  • The mode argument replaces the old, now deprecated, direction argument. The new names are contain or scale-crop-to-fit instead of down, cover or scale-crop-to-fill instead of up and scale instead of thumbnail. [fschulze] (#29)

  • Added calculate_scaled_dimensions function to calculate sizes from bare values without actually scaling an image. [fschulze]

    Added MAX_PIXELS constant set to 8192*8192 to prevent memory overflow while scaling. [fschulze] (#37)

Bug fixes:

  • Fix documentation of scaling modes to match it’s behavior. [thet] (#39)

3.0.3 (2018-11-04)

Bug fixes:

  • reduce warnings in tests [jensens]

3.0.2 (2018-09-28)

Bug fixes:

  • Fix cleanup of image scales in py3 [pbauer]

3.0.1 (2018-04-03)

Bug fixes:

  • Fix conflict resolution code corner case. [gforcada]

3.0 (2017-10-02)

Breaking changes:

  • Restore scale down behaviour from 1.x series without the huge memory usage. [fschulze]

New features:

  • Handle TIFF images with alpha channels. [fschulze]

2.2 (2017-08-27)

New features:

  • Python 3 compatibility. [dhavlik]

2.1.2 (2017-05-31)

Bug fixes:

  • Remove unused dependency. [gforcada]

2.1.1 (2017-03-29)

Bug fixes:

  • Only convert JPEG to greyscale if they actually are and not when the image has less than 256 colors. This bug was introduced in 2.1 with PR #13. [fschulze]

  • Preserve color profile in scaled images. [fschulze]

2.1 (2016-11-01)

New features:

  • Choose an appropriate image mode in order to reduce file size. [didrix]

Bug fixes:

  • Require the six package so we can more easily check number types. On Python 3 long has been merged into int. [maurits]

  • When getting an outdated scale, don’t throw it away when there is no factory. [maurits]

  • Avoid TypeErrors when looking for outdated scales. Fixes issue 12. [maurits]

  • Catch KeyError when deleting non existing scale. This can happen in corner cases. Fixes issue 15. [maurits]

  • Set zip_safe=False in Otherwise you cannot run the tests of the released package because the test runner does not find any tests in the egg file. Note that this is only a problem in zc.buildout 1.x: it uses unzip=False by default. zc.buildout 2.x no longer has this option and always unzips eggs. [maurits]

2.0 (2016-08-12)


  • Assume a width or height of zero is semantically the same as None already was: Use the other dimension to scale, calculate the missing one. [jensens, thet]

  • Scaled GIFs are converted to RGBA PNG images instead of converting them to JPEG. [thet, jensens]


  • Don’t scale images up for direction “down”. [thet]

  • Major housekeeping, code refactored in order to reduce complexicty. [jensens]

1.5.0 (2016-05-18)


  • Use an adapter to lookup the actual factory for scaling. Deprecated passing the factory as named parameter along, because this had not enough flexibility: If addons want to provide alternative methods to scale (i.e. cropping), now a specific adapter can perform the work. [jensens]


  • Minor housekeeping. [jensens]

1.4.1 (2016-02-12)


  • Fix KeyError in storage.AnnotationStorage._cleanup when attempting to delete the storage for the same key twice. [fulv]

1.4 (2015-12-07)


  • Resolve conflicts raised when accessing multiple scales concurrently. [gotcha]

  • Refactored scale storage. [gotcha]

1.3.5 (2015-03-10)

  • PIL thumbnail does not work for magnifying images (when scaling up). Use resize instead. [sureshvv]

1.3.4 (2014-09-07)

  • When a scale is outdated, discard all image scales that are more than a day older than the context. Refs [maurits]

  • Make sure deleting items or clearing a complete storage works. Deleting one item would often delete a linked second item, which made it hard to remove several items at once. [maurits]

1.3.3 (2014-01-27)

1.3.2 (2013-05-23)

1.3.1 (2013-04-06)

  • Cropped images are now centralised vertically as well as horizontally [mattss]

1.3 (2013-01-17)

  • Add [WouterVH]

  • Break up scaleImage, so that its scaling-related parts can be applied to instances of PIL.Image for further processing. [witsch]

1.2.2 - 2010-09-28

1.2.1 - 2010-08-18

  • Convert CMYK to RGB, allowing for web previews of print images. [tomster]

1.2 - 2010-07-18

  • Update package metadata. [hannosch]

1.1 - 2010-04-20

  • Abort if thumbnail behaviour is requested but either width or height is missing. This is nicer than confronting the caller with a PIL exception. [wichert]

  • Rename the keep direction to thumbnail to make its behaviour more intuitive, but accept keep for now. [wichert]

1.0 - 2010-04-12

  • Only pull in the uuid distribution in Python versions before 2.5. [hannosch]

  • Don’t declare dependency on PIL. [davisagli]

1.0a2 - 2010-04-10

  • Add BSD license text following board decision: [elro]

  • Allow to use PIL’s thumbnail algorithm to keep the present aspect ratio. [spamsch, witsch]

  • Allow to set the quality of the resulting image scales. [witsch]

  • Refactor storage adapter for image scales to be less dependent on the underlying content type. [witsch]

1.0a1 - 2009-11-10

  • Initial release [wichert]

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