Symptom Index

Start from what you see. Each cause has its own tell, so work down the list rather than guessing.

Nothing Appears

An output that stays empty or black.

The node never ran. Execution is driven backwards from the outputs that are requested, so a node whose output nothing consumes and which publishes no RenderVar is not executed at all. Check the output is connected to a RenderVar and that the RenderVar is in orderedVars. Refer to how execution order is decided.

The RenderVar is not published. It needs a sourceName as well as the connection.

The name collides with a built-in AOV. The renderer’s own output shadows the node’s. Refer to name collisions.

A node is wired to another product’s node instead of to a published AOV. A shader-to-shader connection only works inside one product. Reaching across a product boundary needs the producing node’s output published as a RenderVar and listed in that product’s orderedVars. The tell is one warning per frame naming the upstream port, with the output AOV never written:

resolveInputShape: Upstream output GrayscaleKernel.LdrGrayscale not resolved yet

Refer to Work Across Render Products.

The shader is under a Material prim. Those are ignored. Author it under a plain scope or the RenderProduct.

The shader is under an instance prototype. A prim marked instanceable, and everything beneath it, is skipped before the kernel is ever compiled. Move the shader out, or drop the instanceable flag on the ancestor.

A Slang node had no thread group size to work from. A .spv whose launch script states neither numthreads nor grid leaves nothing to derive a grid from, so the node reports what is missing and does not run. Refer to with a group size from neither place.

A composite channel or parameter was asked for by a name it does not carry. The node fails without publishing, and the message names the port, the name used and the nearest match.

The launch script was rejected. Scripts run in a sandbox and the source is scanned before it runs. The scan is a plain substring match over the whole file, including comments, so a comment mentioning a forbidden construct is enough. A rejected script produces no GPU work.

An output was never described. Every declared output must be assigned a descriptor before the launch script returns.

The shader failed to compile. Check the log. A Slang compile failure reports that it will use the existing shader and the run continues with an empty AOV and a zero exit code, so assert on content rather than on the process exiting cleanly.

The sensor returned nothing. For a composite, Counts of zero means the sweep produced no points, not that the node is wrong. Refer to bounding by the count channel. The composite’s status does not answer this and should not be used for it.

Wrong Pixels Rather Than No Pixels

Something is produced, but it is not what you expected. Almost always a mismatch between the launch script and the GPU code, because both are matched by position.

The argument or bind order does not match. In CUDA, args is matched against the C signature by position and a mismatch is not diagnosed: the kernel reads whatever occupies that slot. In Slang, the Nth bind entry is matched against the Nth declared resource per descriptor kind.

The shape order is reversed. Shapes are height-first while the image functions take width first.

A value is in the wrong field. For a Slang parameter block, a wrong count is an error but a wrong order is only a warning, and a value carrying no name cannot be cross-checked.

A value-input was used as a number. It is a wrapper; arithmetic on it fails. Refer to value-inputs are wrappers.

A tiled product was treated as one image. A product bound to more than one camera renders one camera per tile, and pixel coordinates alone do not say which view a pixel belongs to.

Something Is Stale or Late

A cross-product read is one frame behind. Expected when the consuming product renders first. Refer to cross-product staleness.

The first frame of a stateful node reads zeros. Defined behaviour. Refer to stateful zero-initialisation.

A scene edit had no effect. The write did not land before the step.

Reading the Log

SPG reports through the renderer’s log, which the renderer writes to standard output: node loading, shader compilation and binding diagnostics, most of them naming the port concerned. When a node produces nothing and the scene looks right, the log is the fastest way to tell a rejected script from a node that was never asked to run.

Messages a launch script writes with warning appear there too, prefixed Lua:. Those written with info or print do not, at the default log level. Refer to The Lua Launch Script.