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307 lines
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307 lines
12 KiB
3 years ago
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Frequently Asked Questions
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==========================
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Last updated: 19 September 2018
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1. High-level Questions and Answers
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-----------------------------------
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1.1 What is Mesa?
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~~~~~~~~~~~~~~~~~
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Mesa is an open-source implementation of the OpenGL specification.
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OpenGL is a programming library for writing interactive 3D applications.
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See the `OpenGL website <https://www.opengl.org/>`__ for more
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information.
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Mesa 9.x supports the OpenGL 3.1 specification.
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1.2 Does Mesa support/use graphics hardware?
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--------------------------------------------
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Yes. Specifically, Mesa serves as the OpenGL core for the open-source
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DRI drivers for X.org.
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- See the `DRI website <https://dri.freedesktop.org/>`__ for more
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information.
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- See `01.org <https://01.org/linuxgraphics>`__ for more information
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about Intel drivers.
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- See `nouveau.freedesktop.org <https://nouveau.freedesktop.org>`__ for
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more information about Nouveau drivers.
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- See
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`www.x.org/wiki/RadeonFeature <https://www.x.org/wiki/RadeonFeature>`__
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for more information about Radeon drivers.
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1.3 What purpose does Mesa serve today?
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---------------------------------------
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Hardware-accelerated OpenGL implementations are available for most
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popular operating systems today. Still, Mesa serves at least these
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purposes:
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- Mesa is used as the core of the open-source X.org DRI hardware
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drivers.
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- Mesa is quite portable and allows OpenGL to be used on systems that
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have no other OpenGL solution.
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- Software rendering with Mesa serves as a reference for validating the
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hardware drivers.
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- A software implementation of OpenGL is useful for experimentation,
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such as testing new rendering techniques.
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- Mesa can render images with deep color channels: 16-bit integer and
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32-bit floating point color channels are supported. This capability
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is only now appearing in hardware.
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- Mesa's internal limits (max lights, clip planes, texture size, etc)
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can be changed for special needs (hardware limits are hard to
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overcome).
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1.4 What's the difference between "Stand-Alone" Mesa and the DRI drivers?
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-------------------------------------------------------------------------
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*Stand-alone Mesa* is the original incarnation of Mesa. On systems
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running the X Window System it does all its rendering through the Xlib
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API:
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- The GLX API is supported, but it's really just an emulation of the
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real thing.
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- The GLX wire protocol is not supported and there's no OpenGL
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extension loaded by the X server.
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- There is no hardware acceleration.
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- The OpenGL library, ``libGL.so``, contains everything (the
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programming API, the GLX functions and all the rendering code).
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Alternately, Mesa acts as the core for a number of OpenGL hardware
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drivers within the DRI (Direct Rendering Infrastructure):
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- The ``libGL.so`` library provides the GL and GLX API functions, a GLX
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protocol encoder, and a device driver loader.
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- The device driver modules (such as ``r200_dri.so``) contain a
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built-in copy of the core Mesa code.
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- The X server loads the GLX module. The GLX module decodes incoming
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GLX protocol and dispatches the commands to a rendering module. For
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the DRI, this module is basically a software Mesa renderer.
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1.5 How do I upgrade my DRI installation to use a new Mesa release?
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-------------------------------------------------------------------
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This wasn't easy in the past. Now, the DRI drivers are included in the
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Mesa tree and can be compiled separately from the X server. Just follow
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the Mesa :doc:`compilation instructions <install>`.
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1.6 Are there other open-source implementations of OpenGL?
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----------------------------------------------------------
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Yes, SGI's `OpenGL Sample Implementation
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(SI) <http://web.archive.org/web/20171010115110_/http://oss.sgi.com/projects/ogl-sample/index.html>`__
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is available. The SI was written during the time that OpenGL was
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originally designed. Unfortunately, development of the SI has stagnated.
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Mesa is much more up to date with modern features and extensions.
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`Vincent <https://sourceforge.net/projects/ogl-es/>`__ is an open-source
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implementation of OpenGL ES for mobile devices.
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`miniGL <http://web.archive.org/web/20130830162848/http://www.dsbox.com/minigl.html>`__
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is a subset of OpenGL for PalmOS devices. The website is gone, but the
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source code can still be found on
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`sourceforge.net <https://sourceforge.net/projects/minigl/>`__.
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`TinyGL <http://bellard.org/TinyGL/>`__ is a subset of OpenGL.
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`SoftGL <https://sourceforge.net/projects/softgl/>`__ is an OpenGL
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subset for mobile devices.
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`Chromium <http://chromium.sourceforge.net/>`__ isn't a conventional
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OpenGL implementation (it's layered upon OpenGL), but it does export the
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OpenGL API. It allows tiled rendering, sort-last rendering, etc.
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`ClosedGL <http://www.ticalc.org/archives/files/fileinfo/361/36173.html>`__
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is an OpenGL subset library for TI graphing calculators.
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There may be other open OpenGL implementations, but Mesa is the most
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popular and feature-complete.
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2. Compilation and Installation Problems
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----------------------------------------
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2.1 What's the easiest way to install Mesa?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If you're using a Linux-based system, your distro CD most likely already
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has Mesa packages (like RPM or DEB) which you can easily install.
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2.2 I get undefined symbols such as bgnpolygon, v3f, etc...
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You're application is written in IRIS GL, not OpenGL. IRIS GL was the
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predecessor to OpenGL and is a different thing (almost) entirely. Mesa's
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not the solution.
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2.3 Where is the GLUT library?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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GLUT (OpenGL Utility Toolkit) is no longer in the separate
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``MesaGLUT-x.y.z.tar.gz`` file. If you don't already have GLUT
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installed, you should grab
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`freeglut <http://freeglut.sourceforge.net/>`__.
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2.4 Where is the GLw library?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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GLw (OpenGL widget library) is now available from a separate `git
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repository <https://gitlab.freedesktop.org/mesa/glw>`__. Unless you're
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using very old Xt/Motif applications with OpenGL, you shouldn't need it.
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2.5 What's the proper place for the libraries and headers?
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----------------------------------------------------------
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On Linux-based systems you'll want to follow the `Linux
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ABI <https://www.khronos.org/registry/OpenGL/ABI/>`__ standard.
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Basically you'll want the following:
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``/usr/include/GL/gl.h``
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the main OpenGL header
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``/usr/include/GL/glu.h``
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the OpenGL GLU (utility) header
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``/usr/include/GL/glx.h``
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the OpenGL GLX header
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``/usr/include/GL/glext.h``
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the OpenGL extensions header
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``/usr/include/GL/glxext.h``
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the OpenGL GLX extensions header
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``/usr/include/GL/osmesa.h``
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the Mesa off-screen rendering header
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``/usr/lib/libGL.so``
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a symlink to ``libGL.so.1``
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``/usr/lib/libGL.so.1``
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a symlink to ``libGL.so.1.xyz``
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``/usr/lib/libGL.so.xyz``
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the actual OpenGL/Mesa library. xyz denotes the Mesa version number.
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When configuring Mesa, there are three meson options that affect the
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install location that you should take care with: ``--prefix``,
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``--libdir``, and ``-D dri-drivers-path``. To install Mesa into the
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system location where it will be available for all programs to use, set
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``--prefix=/usr``. Set ``--libdir`` to where your Linux distribution
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installs system libraries, usually either ``/usr/lib`` or
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``/usr/lib64``. Set ``-D dri-drivers-path`` to the directory where your
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Linux distribution installs DRI drivers. To find your system's DRI
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driver directory, try executing ``find /usr -type d -name dri``. For
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example, if the ``find`` command listed ``/usr/lib64/dri``, then set
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``-D dri-drivers-path=/usr/lib64/dri``.
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After determining the correct values for the install location, configure
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Mesa with
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``meson configure --prefix=/usr --libdir=xxx -D dri-drivers-path=xxx``
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and then install with ``sudo ninja install``.
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3. Runtime / Rendering Problems
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-------------------------------
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3.1 Rendering is slow / why isn't my graphics hardware being used?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If Mesa can't use its hardware accelerated drivers it falls back on one
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of its software renderers. (e.g. classic swrast, softpipe or llvmpipe)
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You can run the ``glxinfo`` program to learn about your OpenGL library.
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Look for the ``OpenGL vendor`` and ``OpenGL renderer`` values. That will
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identify who's OpenGL library with which driver you're using and what
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sort of hardware it has detected.
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If you're using a hardware accelerated driver you want
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``direct rendering: Yes``.
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If your DRI-based driver isn't working, go to the `DRI
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website <https://dri.freedesktop.org/>`__ for trouble-shooting
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information.
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3.2 I'm seeing errors in depth (Z) buffering. Why?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Make sure the ratio of the far to near clipping planes isn't too great.
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Look
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`here <https://www.opengl.org/archives/resources/faq/technical/depthbuffer.htm#0040>`__
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for details.
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Mesa uses a 16-bit depth buffer by default which is smaller and faster
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to clear than a 32-bit buffer but not as accurate. If you need a deeper
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you can modify the parameters to ``glXChooseVisual`` in your code.
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3.3 Why Isn't depth buffering working at all?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Be sure you're requesting a depth buffered-visual. If you set the
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``MESA_DEBUG`` environment variable it will warn you about trying to
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enable depth testing when you don't have a depth buffer.
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Specifically, make sure ``glutInitDisplayMode`` is being called with
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``GLUT_DEPTH`` or ``glXChooseVisual`` is being called with a non-zero
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value for ``GLX_DEPTH_SIZE``.
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This discussion applies to stencil buffers, accumulation buffers and
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alpha channels too.
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3.4 Why does ``glGetString()`` always return ``NULL``?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Be sure you have an active/current OpenGL rendering context before
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calling ``glGetString``.
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3.5 ``GL_POINTS`` and ``GL_LINES`` don't touch the right pixels
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If you're trying to draw a filled region by using ``GL_POINTS`` or
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``GL_LINES`` and seeing holes or gaps it's because of a float-to-int
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rounding problem. But this is not a bug. See Appendix H of the OpenGL
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Programming Guide - "OpenGL Correctness Tips". Basically, applying a
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translation of (0.375, 0.375, 0.0) to your coordinates will fix the
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problem.
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4. Developer Questions
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----------------------
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4.1 How can I contribute?
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~~~~~~~~~~~~~~~~~~~~~~~~~
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First, join the :doc:`mesa-dev mailing list <lists>`. That's where
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Mesa development is discussed.
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The `OpenGL Specification <https://www.opengl.org/documentation>`__ is
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the bible for OpenGL implementation work. You should read it.
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Most of the Mesa development work involves implementing new OpenGL
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extensions, writing hardware drivers (for the DRI), and code
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optimization.
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4.2 How do I write a new device driver?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Unfortunately, writing a device driver isn't easy. It requires detailed
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understanding of OpenGL, the Mesa code, and your target
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hardware/operating system. 3D graphics are not simple.
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The best way to get started is to use an existing driver as your
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starting point. For a classic hardware driver, the i965 driver is a good
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example. For a Gallium3D hardware driver, the r300g, r600g and the i915g
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are good examples.
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The DRI website has more information about writing hardware drivers. The
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process isn't well document because the Mesa driver interface changes
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over time, and we seldom have spare time for writing documentation. That
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being said, many people have managed to figure out the process.
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Joining the appropriate mailing lists and asking questions (and
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searching the archives) is a good way to get information.
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4.3 Why isn't ``GL_EXT_texture_compression_s3tc`` implemented in Mesa?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Oh but it is! Prior to 2nd October 2017, the Mesa project did not
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include S3TC support due to intellectual property (IP) and/or patent
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issues around the S3TC algorithm.
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As of Mesa 17.3.0, Mesa now officially supports S3TC, as the patent has
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expired.
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In versions prior to this, a 3rd party `plug-in
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library <https://dri.freedesktop.org/wiki/S3TC>`__ was required.
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