Specifying a Primary Point of Interest (PPOI)¶
The crop Sizer is super-useful for creating images at a specific size/aspect-ratio however, sometimes you want the ‘crop centerpoint’ to be somewhere other than the center of a particular image. In fact, the initial inspiration for django-versatileimagefield came as a result of tackling this very problem.
The crop Sizer’s core functionality (located in the versatileimagefield.versatileimagefield.CroppedImage.crop_on_centerpoint method) was inspired by PIL’s ImageOps.fit function (by Kevin Cazabon) which takes an optional keyword argument, centering, that expects a 2-tuple comprised of floats which are greater than or equal to 0 and less than or equal to 1. These two values together form a cartesian coordinate system that dictates what percentage of pixels to ‘trim’ off each of the long sides (i.e. left/right or top/bottom, depending on the aspect ratio of the cropped size vs. the original size):
|Top||(0.0, 0.0)||(0.0, 0.5)||(0.0, 1.0)|
|Middle||(0.5, 0.0)||(0.5, 0.5)||(0.5, 1.0)|
|Bottom||(1.0, 0.0)||(1.0, 0.5)||(1.0, 1.0)|
The crop Sizer works in a similar way but converts the 2-tuple into an exact (x, y) pixel coordinate which is then used as the ‘centerpoint’ of the crop. This approach gives significantly more accurate results than using ImageOps.fit, especially when dealing with PPOI values located near the edges of an image or aspect ratios that differ significantly from the original image.
Even though the PPOI value is used as a crop ‘centerpoint’, the pixel it corresponds to won’t necessarily be in the center of the cropped image, especially if its near the edges of the original image.
At present, only the crop Sizer changes how it creates images based on PPOI but a VersatileImageField makes its PPOI value available to ALL its attached Filters and Sizers. Get creative!
Each image managed by a VersatileImageField can store its own, unique PPOI in the database via the easy-to-use PPOIField. Here’s how to integrate it into our example model (relevant lines highlighted in the code block below):
# models.py with `VersatileImageField` & `PPOIField` from django.db import models from versatileimagefield.fields import VersatileImageField, \ PPOIField class ImageExampleModel(models.Model): name = models.CharField( 'Name', max_length=80 ) image = VersatileImageField( 'Image', upload_to='images/testimagemodel/', width_field='width', height_field='height', ppoi_field='ppoi' ) height = models.PositiveIntegerField( 'Image Height', blank=True, null=True ) width = models.PositiveIntegerField( 'Image Width', blank=True, null=True ) ppoi = PPOIField( 'Image PPOI' ) class Meta: verbose_name = 'Image Example' verbose_name_plural = 'Image Examples'
As you can see, you’ll need to add a new PPOIField field to your model and then include the name of that field in the VersatileImageField‘s ppoi_field keyword argument. That’s it!
PPOIField is fully-compatible with south so migrate to your heart’s content!
How PPOI is Stored in the Database¶
The Primary Point of Interest is stored in the database as a string with the x and y coordinates limited to two decimal places and separated by an ‘x’ (for instance: '0.5x0.5' or '0.62x0.28').
PPOI is set via the ppoi attribute on a VersatileImageField. You should always set an image’s PPOI here (as opposed to directly on a PPOIField attribute) since a VersatileImageField will ensure updated values are passed-down to all its attached Filters & Sizers.
When you save a model instance, VersatileImageField will ensure its currently-assigned PPOI value is ‘sent’ to the PPOIField associated with it (if any) prior to writing to the database.
Via The Shell¶
# Importing our example Model >>> from someapp.models import ImageExampleModel # Retrieving a model instance >>> example = ImageExampleModel.objects.all() # Retrieving the current PPOI value associated with the image field # A `VersatileImageField`'s PPOI value is ALWAYS associated with the `ppoi` # attribute, irregardless of what you named the `PPOIField` attribute on your model >>> example.image.ppoi (0.5, 0.5) # Creating a cropped image >>> example.image.crop['400x400'].url u'/media/__sized__/images/testimagemodel/test-image-crop-c0-5__0-5-400x400.jpg' # Changing the PPOI value >>> example.image.ppoi = (1, 1) # Creating a new cropped image with the new PPOI value >>> example.image.crop['400x400'].url u'/media/__sized__/images/testimagemodel/test-image-crop-c1__1-400x400.jpg' # PPOI values can be set as either a tuple or a string >>> example.image.ppoi = '0.1x0.55' >>> example.image.ppoi (0.1, 0.55) >>> example.image.ppoi = (0.75, 0.25) >>> example.image.crop['400x400'].url u'/media/__sized__/images/testimagemodel/test-image-crop-c0-75__0-25-400x400.jpg' # u'0.75x0.25' is written to the database in the 'ppoi' column associated with # our example model >>> example.save()
As you can see, changing an image’s PPOI changes the filename of the cropped image. This ensures updates to a VersatileImageField‘s PPOI value will result in unique cache entries for each unique image it creates.
Each time a field’s PPOI is set, its attached Filters & Sizers will be immediately updated with the new value.
It’s pretty hard to accurately set a particular image’s PPOI when working in the Python shell so django-versatileimagefield ships with an admin-ready formfield. Simply add an image, click ‘Save and continue editing’, click where you’d like the PPOI to be and then save your model instance again. A helpful translucent red square will indicate where the PPOI value is currently set to on the image: