Leela is a strong Go playing program combining advances in Go programming and further original research into a small, easy to use graphical interface.

It is freely available, making it an ideal introduction to the game.

Analyzing a game, showing the most probable moves.

What's this Deep Learning technology? Leela contains an AI technique modeled after the human visual system and brain, a deep stack of artificial neurons processing visual input with each layer combining the previous ones into higher level features (a so called DCNN, deep convolutional neural network). This "neural network" has been trained with more than 32 million positions from high level go games and taught to predict which moves a pro player would most likely consider. In 19x19 games the engine will query this deep "neural network" during the search to focus on the most critical variations. The result is a substantial raise in playing strength (about 6 stones), and a more human-like playstyle, while still allowing the engine to innovate of its own.

What's the benefit of GPU Acceleration? A GPU is typically much faster than a normal processor at doing the kind of calculations the Deep Learning algorithms need. For example, an AMD R9 390 (mid-range videocard) can find the worthwhile moves in a Go position about 15 times faster than an Intel Core i5-6600 (mid-range CPU). Leela makes use of this to explore variations deeper and in more detail. You can notice that in analysis mode the critical variations displayed by the engine are much longer when GPU Acceleration is used.

What do I need to run the "GPU Accelerated" version? You need a video card with OpenCL 1.1 (or later) capability. This is supported on all recent AMD and NVIDIA cards. Make sure the latest drivers for the video card are installed. The detected video card can be seen in the Help->About menu.

Can I run the "GPU Accelerated" version on the built-in graphics of my CPU? This only makes sense if the built-in GPU is faster at doing the Deep Learning calculations than the CPU itself. Intel CPUs are typically very fast, with a weak built-in GPU, so this is not recommended. AMD APU's typically have a slow CPU but a fast GPU, so it would be worthwhile on those.

My Linux distribution is not listed.

You have several options:

The "engine only" download has Linux binaries included that should run on almost all distributions, and which can be used with any Go GUI that supports the GTP protocol (almost all of them do). A good looking one is Sabaki. To use the OpenCL version you must have OpenCL drivers and an OpenCL ICD loader installed, please see the included README.TXT.

The Windows installer and graphical interface work and are fully functional under wine-1.9.23 or later, although GPU acceleration will not work.

Many Linux distributions have some way to take in Ubuntu/Debian packages and convert them to the distributions' own format. Perhaps someone has already done this for you. Look around!

Why does Leela report >100% or negative winrates?

Being sure of winning the game with 0.5 points corresponds to 100% winrate. Bigger margins of victory are represented by >100%.

How do I analyse games?

Enable the Analysis Window via Analyze→Analysis Window. Start analysis with F2 (or the toolbar) and move through the game with the arrow keys, looking for jumps in the Win% of the best move (the one on top). These represent mistakes by one of the players. Look at what moves Leela suggests instead. Show Network Probabilities (F3) and Show Best Moves (F4) can help understand direction of play mistakes and Show Territory (F5) can help understand the "size" and "temperature" of plays.

You can "save" the position by pressing the +, then try out a variation on the board. Then do Analyze→Revert to Stored (Alt-M) to go back to the position you saved. The position is also saved whenever you enter Analyze mode.

Enabling the Histogram (Ctrl-H) shows a graphical display of who was ahead at what point of the game. It plots winrate versus move number. A steep vertical bank in the line represents a likely mistake according to Leela. You can go to that move by clicking inside the Histogram window.

What do the columns in the Analysis Window mean?

Leela always orders the moves in this window from best to worst, i.e. the top move is the best move (so far). You can click in this window to show the principal variation for the selected move on the board.

Simulations: the number of Monte Carlo playouts used to investigate the move. More simulations means more confidence in the winrate as the move has been investigated deeper.

Effort%: the percentual amount of search effort Leela has spent on a move. A large search effort means that the move has consistently looked good throughout the search. Leela generally prefers such moves, unless a new very promising candidate has popped up.

Win%: this represents Leela's best guess as to how likely it is for the player to move to win the game. If there is a move that is not investigated deeply yet, but appears to provide a higher chance of winning than the current best move, it is bolded. You might want to let Leela analyze a bit longer if this happens.

MC Win%: the likelyhood that the player to move wins the game, as determined by randomized Monte Carlo playouts from the current position. This is a factor in the Win% calculation.

Net Win%: the likelyhood that the player to move wins the game, as determined by analyzing the position with a neural network. This is a factor in the Win% calculation.

Net Prob%: the probability that a pro player would play this move, as estimated by the Neural Network.

Eval: displayed in non-19x19 games instead of Net Prob%. A dimensionless number expressing the likelihood of the move being good, based on a pattern database and the moves' features.

PV: the principal variation. The sequence of suggested moves for both players that Leela believes is optimal.

My Mac is complaining that Leela can't be opened because it is from an unidentified developer.

Right click the download and select Open. You'll get the option to bypass the warning. Explanation from Apple.

Leela says that white is more likely to win when black gets handicap stones.

Leela's value network is generated from real human games, not idealized computer opponents. In handicap games, the stronger (white) player wins more often. You can look at the Monte Carlo evaluation for an assement with equally strong players.