What are kudos and why do they exist?

Kudos are essentially a point tracking system implemented within the backend of Stable Horde in order to determine queue priority and prevent general abuse of the service. Those who are logged in with a Stable Horde API key will get higher priority than anonymous users. Those who are logged in and have more kudos will get even higher priority in the queue.

** They are not a currency and cannot be bought. **

They are awarded to users for contributing GPU power to the distributed cluster. Other ways to get kudos are to rate images for aesthetics training , participate in the Stable Horde Discord channel -- by being a helpful member of the community or sharing neat art that you've generated.

Some image generation parameters (e.g., those determined to be more computationaly expensive operations, such as higher resolution images or more steps) require an upfront kudos balance. However, most operations can be completed without needing the required amount of kudos upfront. (Most requests that are less than ~17 kudos can be completed without issue.)

More information about how kudos are used and the philosophy behind them can be found here.

Is there any sort of syntax (prompt weights or prompt matrix)?

Prompt weighting is available on ArtBot thanks to the Stable Horde. To utilize prompt weighting, use the following syntax: subject in parenthesis separated by a colon, followed by a number. e.g., (flower:1.2) Example:

A beautiful photograph of a (blue:1.2) and (red:0.8) flower

Any integer greater than 1.0 denotes something you want to give extra strength to (e.g., 1.2 = 120%). Anything below 1.0 is something you want to deemphasize. Other Stable Diffusion UIs, such as Automatic1111, utilize multiple parens and brackets (e.g., ((stronger)) and [[[weaker]]]). That syntax is not supported on ArtBot.

ArtBot also supports using a prompt matrix -- anything inside brackets separated by a pipe character. That looks like this:

{thing one|thing two|thing three}

Each value will be run separately (and against any other values, if provided).For example, the following query will yield 6 images:

A beautiful forest full of {bears|clowns}, painted by {Bob Ross|Thomas Kinkade|Maurice Sendak}

Why does the estimated time remaining to generate an image seem to fluctuate so much?
This generally happens if you're an anonymous user. Stable Horde uses a priority queue system. Users who provide an API key and have kudos are given priority access to the job queue ahead of anonymous users. As such, your image request might be shuffled back in the queue if a valid registered user creates an image request.
Why do my pending image requests sometimes disappear without generating an image?
In order to keep image generation requests moving along, Stable Horde drops any outstanding request after about 10 minutes. If your job is older than 20 minutes, it may disappear.
Why am I getting a black image?
You're probably running into the Stable Diffusion / Stable Horde NSFW filter. It can sometimes be a bit too aggressive. Try disabling it in the settings panel.