Parent and Child Posts
Parent and child posts, also known as Post Relationships, are a means of linking together groups of related posts. One post (normally the "best" version) is chosen to be the parent, while the other posts are made its children.
Parent/child relationships are typically used to group minor variants of the same image. Pools are another way of grouping posts; see the guidelines below for when to use a pool instead.
When viewing a post in a parent/child relationship, the other posts in the relationship are displayed as thumbnails at the top of the page. In search listings, parent posts are displayed with a green border, while child posts have a yellow border.
Adding a parent
First, note the parent post's ID number. This is located under the Information sidebar when viewing a post, or as part of the URL (e.g. /posts/1234).
Next, open the child post and click the Edit link just below the image. Enter the post ID of the parent post in the Parent field, then click Submit to save your changes. Repeat this process for all child posts in the relationship.
Removing a parent
Open the child post and click the Edit link just below the image. Clear the Parent field, then click Submit to save your changes. Repeat for all other child posts in the relationship as necessary.
Generally, parent/child relationships should be used for minor variations of the same image. For comics or other series of posts meant to be viewed in order, use a Pool instead.
When TO use Post relationships
- When the same image is available in multiple resolutions or pixel sizes (see post #716608).
- When cropped and whole versions of the same image has been uploaded (see post #1540326)
- Derivatives of a Post, including setting variations (post #1544682) and costume variations (post #154471). See also post #625848 (hard translation and derivative character art) and post #55525 (smaller post and vector trace).
- When identical images from different sources have been uploaded. For which posts to set parent and child, see below.
When NOT to use Post Relationships
For most other cases, it would be better to organize these images into a (Pool). Examples include:
- Pages from a manga or doujin.
- Episodes of a comic.
- Any other series of posts intended to be viewed in order.
- Large collections of posts that are too different from each other.
In such cases, Pools provide additional navigation features not available with Post Relationships.
Choosing the Best Parent
The Parent Post should be considered in this order:
For identical images
- Unsampled. Image samples should always be parented to their original image (and subsequently flagged and/or deleted).
- Most recent active revision. There are exceptional cases where the latest revision is worse than the previous image (post #2645830). See MD5 mismatch, revision, and bad revision.
- Least image artifacts (see jpeg_artifacts, scan_artifacts).
- Most lossless file format (png > jpg). See also lossy-lossless, in the case a png is more lossy than its most original jpeg counterpart.
- Highest resolution. If a picture has been upscaled, the upscaled version should be the child.
- Largest colorspace. Ex. RGBA32 encodes more data per pixel than RGB24.
- Most recent active post (post #2613788, post #2227073).
- Most unaltered metadata for exactly identical images.
- Especially interesting since Pixiv used to run uploaded JPEGs through the jpegtran optimizer (since August 2013) that reduced filesize by stripping some metadata. In those cases, the Pixiv image should be the child (see post #1502515). Pixiv no longer uses the jpegtran optimizer, so JPEG metadata should be present on most images past 2014-2015 or so.
- In fairly unique cases, the artist will have metadata present in an image from another site that needs to be manually checked. See here.
- Best compression. Whichever post has a smaller filesize while retaining the exact same image data is the parent.
For derivative posts
- Most original, as opposed to confirmed derivatives.
- Most exploitable variation of the post for creating derivatives.
- Most complete, as opposed to cropped posts.
- Cleanest, containing the least watermarking. In the case of third party detexted versions, the detexted one should be the child.
There might be exceptions with the above, for example if a picture is much higher resolution than its counterpart. If you are unsure which version should be the Parent, leave a comment on that Post or ask in the Forum.