Resuming a render that has crashed or aborted is an advanced feature which requires setting some options to tell Arnold to pick up where it left off.  This is also known as checkpoint renders, or append mode renders.  This feature only applies to renders writing to the file system, which means it will only work for disk/batch renders and not preview or live renders.

When append mode is active, Arnold will skip the rendering of any compatible tiles in the image file that are complete, instead of starting over.  It will only skip finished tiles; partial tiles will be re-rendered.

Requirements

In order to resume a render, it must be previously set up properly so that Arnold can pick up where it left off in the even a render is aborted for any reason in the middle.

  • Each output channel must use either the EXR or TIFF driver (via the ArnoldOutputChannelDefine node)
  • Each driver must be set to tiled mode
  • Each driver must have append enabled

We will examine these in detail below.

Output Channel Setup

For each AOV you intend to include in your render, you will need an output channel set up via the ArnoldOutputChannelDefine node. Among other settings, in that node you can adjust the driver used to write the image.  For renders to resume they must be written using a tiling image format, requiring the use of either the EXR or TIFF driver.  The tiled parameter must be enabled for resuming a render (as scanline renders wait to write the image until entire scanlines are done, and often this means the image won't be written at all until it is nearly finished anyway). And finally append must be enabled or else Arnold will always restart the render regardless of what tiles were already in the rendered image on disk.

If the image file was not on disk yet, or was otherwise empty (no tiles or only partial tiles rendered), Arnold will just start from the beginning of the render, and as such it doesn't harm anything to have it on for a new image.  Turning off append mode makes sense if you intend to overwrite a previous render that changed – you don't want to keep any tiles already written to disk. Append mode generally makes sense when a render crashed or was aborted by the user or farm management software for some reason and you want to avoid losing as much render progress as possible.

Render Output Setup

Each render output (which has one output channel it pulls from) will need an explicit RenderOutputDefine node.  In it, you will need to select the channel defined in the corresponding ArnoldOutputChannelDefine. You can set the tempRenderLocation parameter to anything you like or even leave it empty.  When that temporary location is left empty, Katana auto-generates a location within its temp session directory (for example, located in /tmp/katana_tmpdir_6142), but when the render exits in any way it will delete the temporary render file. KtoA attempts to ensure any already-rendered tiles in the temporary file are copied to the final location so any partial progress will be kept.

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