ParaView is often used to render visualization results. While in most cases, the results are presented to the users on their screens or monitors, there are cases when this is not required. For example, if running a batch script to generate several images, one may not care to see windows flashing in front of them while the results are being generated. Another use-case is when using a pvserver or pvrenderserver to render images remotely and having them sent back to the client for viewing. The server-side windows are do not serve any useful purpose in that mode. In other cases, showing a window may not be possible at all, for example on certain Linux clusters, one may not have access to an X server at all, in which case on-screen rendering is just not possible (this is also referred to as headless operating mode).

This page documents the onscreen, offscreen, and headless rendering support in ParaView.

Caveats

In this page, we limit ourselves to a Linux-based system. On Windows and macOS, offscreen support is available, but true headless operation is currently not widely used and hence left out of this discussion.

Terminology

A brief explanation of the terms:

• Desktop Qt client: This refers to the Qt-based GUI. This is launched using the paraview executable.
• Command line executables: This refers to all other ParaView executables, e.g. pvserver, pvdataserver, pvrenderserver, pvpython, and pvbatch.
• Onscreen: Onscreen refers to the case where the rendering results are presented to the users in a viewable window. On Linux, this invariably requires a running and accessible X server. The desktop Qt client can only operate in on-screen mode and hence needs an accessible X server.
• Offscreen: Offscreen simply means that the rendering results are not presented to the user in a window. This mode is not applicable to the desktop Qt client and is only supported by the command line executables.
• Headless: Headless rendering automatically implies offscreen rendering. In addition, on Linux, it implies that the rendering does not require an accessible X server nor will it make any X calls.

Onscreen, offscreen and headless support is built by default. Thus ParaView binaries available from paraview.org support these modes.

OpenGL Implementations

ParaView uses OpenGL for rendering. OpenGL is an API specification for 2D/3D rendering. Many vendors provide implementations of this API, including those that build GPUs.

For sake of simplicity, let's classify OpenGL implementations as hardware (H/W) and software (S/W). H/W includes OpenGL implementations provided by NVIDIA, ATI, Intel, Apple and others which typically use the system hardware infrastructure for rendering. The runtime libraries needed for these are available on the system. S/W currently includes Mesa3D – a software implementation of the OpenGL standard. Despite the names, H/W doesn't necessarily imply use of GPUs, nor does S/W imply exclusion of GPUs. Nonetheless we use this naming scheme as it has been prevalent.

APIs for Headless Support

Traditionally, OpenGL implementations are coupled with the window system to provide an OpenGL context. Thus, they are designed for non-headless operation. When it comes to headless operation, there are alternative APIs that an application can use to create the OpenGL context that avoid this dependency on the window system (or X server for the sake of this discussion).

Currently, ParaView supports two distinct APIs that are available for headless operation: EGL and OSMesa (also called Offscreen Mesa). It must be noted that headless support is a rapidly evolving area and changes are expected in coming months. Modern H/W OpenGL implementations support EGL while S/W (or Mesa) supports OSMesa.

ParaView Builds

Before we look at the various ways you can build and use ParaView, let's summarize relevant CMake options available:

• VTK_USE_X: When ON, implies that ParaView can link against X libraries. This allows ParaView executables to create on-screen windows, if needed.
• VTK_OPENGL_HAS_EGL: When ON, implies that ParaView can use EGL to support headless modes of operation.
• PARAVIEW_USE_QT indicates if the desktop Qt client should be built.

Note that for binaries distributed on paraview.org the OpenGL selection is not frozen at build time. VTK is capable of detecting a suitable OpenGL context backend at runtime and fallback to pure software based rendering through OSMesa when none of the H/W implementations work. The S/W rendering path requires the presence of a libOSMesa.so or osmesa.dll in the system library directories because VTK dynamically loads OSMesa.

A few things to note:

• At least one of VTK_OPENGL_HAS_EGL, and VTK_USE_X must be ON. By default, both options are ON.
• If VTK_OPENGL_HAS_EGL is ON, the build supports H/W headless rendering, otherwise VTK_USE_X must be ON and the build does not support headless rendering with GPU, but can still support offscreen rendering or software based headless rendering. Since EGL is provided by driver packages, ParaView turns on EGL by default for convenience.
• For testing purpose, you can set the environment variable VTK_DEFAULT_OPENGL_WINDOW to create a vtkOSOpenGLRenderWindow with OSMesa or vtkEGLRenderWindow or vtkXOpenGLRenderWindow, and in windows, vtkWin32OpenGLRenderWindow.
• PARAVIEW_USE_QT=ON and VTK_USE_X=OFF has limited support. In this configuration, the command-line tools like pvserver, pvrenderserver will not depend on X. The paraview Qt client, however, is not built.

Default Rendering Modes

Depending of the enabled build options, which type of render window the ParaView executable creates by default also needs some explanation.

• The ParaView desktop Qt client always creates an onscreen window using GLX calls via Qt.
• pvserver, pvrenderserver and pvbatch always create an offscreen render window. If built with headless support, it will be an offscreen-headless window. There are a few exceptions where it will create an onscreen window instead:
  • if running in tile-display mode, i.e. -tdx or -tdy command line options are passed
  • if running in immersive mode e.g. CAVE
  • if PV_DEBUG_REMOTE_RENDERING environment is set
  • if --force-onscreen-rendering command line option is passed.
• pvpython always creates an onscreen render window if built with onscreen support. The following are the exceptions when it creates offscreen (or headless if supported) render windows.
  • if --force-offscreen-rendering command line option is passed.

--use-offscreen-rendering command line option supported by ParaView 5.4 and earlier has now been deprecated and is interpreted as --force-offscreen-rendering.