Gold is a tool to help track and manage the output of visual tests over a range of commits.
A test is a bit of code that produces a PNG image which is uploaded to Gold for cataloging. The images uploaded to Gold are uniquely identified by a hash digest of their pixel content (and potentially other metadata like colorspace); thus the term digest is frequently used as a synonym of image. Digests are "triaged" to be correct (positive) or incorrect (negative); this is generally done by a human inspecting the digests. A digest that has not been marked positive or negative is said to be "untriaged" and generally means that a test has started producing different output (and a human should check if it's correct).
When a digest is uploaded to Gold, it is tagged with a number of key/value pairs. These keys (frequently called params) are generally used to label how a digest was produced, for example OS=Android or GPU=Nvidia770GTX. Params can be arbitrary and Gold will automatically identify and process any new params produced by a test - no server-side pre-configuration is necessary. There are two special keys, one for the "name" of the test and the other is "source_type" (aka corpus). Data in Gold tends to be grouped first by corpus, and then by test.
A Gold trace refers to all digests seen belonging to a unique set of params. These traces are visualized throughout Gold as lines of colored dots where a color refers to a specific digest. A trace belongs to exactly one test in exactly one corpus. A trace (or test) is allowed to have multiple positive digests; in practice this happens due to things like nondeterminism in anti-aliasing algorithms for certain GPUs.
To keep processing tractable, Gold only focuses on the last N commits for which it has seen data. N is generally between 200 and 500 depending on settings for an individual instance.
On the By Blame page, Gold shows a set of recent commits that Gold believes have first introduced on or more untriaged digests. There are links on this page that will lead to the Search Page showing all untriaged digests and the Details page showing details about individual digests.
This is one of Gold's most powerful pages, as it allows one to search and filter over all the input Gold has processed. By default, it will show traces that have untriaged digests "at head", that is to say, traces which have most recently produced an unseen image. These traces are grouped together by digest, so if multiple traces output the same digest (for example, a Release and a Debug build), they will be closer together. Digests will automatically be compared to another digest from the same test; in fact, the most similar digest.
It is common to triage digests on the search page. Additionally, there is a zoom button that can allow for a closer side-by-side view of the difference between a digest and another digest. When zoomed in, a common keyboard shortcut "u" can be used to jump to the largest difference.
The details page shows information about one digest and any traces the produce it.