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214 lines
8.2 KiB
214 lines
8.2 KiB
Xlib Software Driver |
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==================== |
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Mesa's Xlib driver provides an emulation of the GLX interface so that |
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OpenGL programs which use the GLX API can render to any X display, even |
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those that don't support the GLX extension. Effectively, the Xlib driver |
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converts all OpenGL rendering into Xlib calls. |
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The Xlib driver is the oldest Mesa driver and the most mature of Mesa's |
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software-only drivers. |
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Since the Xlib driver *emulates* the GLX extension, it's not totally |
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conformant with a true GLX implementation. The differences are fairly |
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obscure, however. |
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The unique features of the Xlib driver follows. |
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X Visual Selection |
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------------------ |
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Mesa supports RGB(A) rendering into almost any X visual type and depth. |
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The glXChooseVisual function tries to choose the best X visual for the |
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given attribute list. However, if this doesn't suit your needs you can |
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force Mesa to use any X visual you want (any supported by your X server |
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that is) by setting the **MESA_RGB_VISUAL** environment variable. When |
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a visual is requested, glXChooseVisual will first look if the |
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MESA_RGB_VISUAL variable is defined. If so, it will try to use the |
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specified visual. |
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The format of accepted values is: ``visual-class depth`` |
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Here are some examples: |
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:: |
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using csh: |
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% setenv MESA_RGB_VISUAL "TrueColor 8" // 8-bit TrueColor |
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% setenv MESA_RGB_VISUAL "PseudoColor 8" // 8-bit PseudoColor |
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using bash: |
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$ export MESA_RGB_VISUAL="TrueColor 8" |
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$ export MESA_RGB_VISUAL="PseudoColor 8" |
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Double Buffering |
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---------------- |
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Mesa can use either an X Pixmap or XImage as the back color buffer when |
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in double-buffer mode. The default is to use an XImage. The |
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**MESA_BACK_BUFFER** environment variable can override this. The valid |
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values for **MESA_BACK_BUFFER** are: **Pixmap** and **XImage** (only the |
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first letter is checked, case doesn't matter). |
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Using XImage is almost always faster than a Pixmap since it resides in |
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the application's address space. When glXSwapBuffers() is called, |
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XPutImage() or XShmPutImage() is used to transfer the XImage to the |
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on-screen window. |
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A Pixmap may be faster when doing remote rendering of a simple scene. |
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Some OpenGL features will be very slow with a Pixmap (for example, |
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blending will require a round-trip message for pixel readback.) |
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Experiment with the MESA_BACK_BUFFER variable to see which is faster for |
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your application. |
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Colormaps |
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--------- |
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When using Mesa directly or with GLX, it's up to the application writer |
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to create a window with an appropriate colormap. The GLUT toolkit tries |
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to minimize colormap *flashing* by sharing colormaps when possible. |
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Specifically, if the visual and depth of the window matches that of the |
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root window, the root window's colormap will be shared by the Mesa |
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window. Otherwise, a new, private colormap will be allocated. |
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When sharing the root colormap, Mesa may be unable to allocate the |
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colors it needs, resulting in poor color quality. This can happen when a |
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large number of colorcells in the root colormap are already allocated. |
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Gamma Correction |
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---------------- |
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To compensate for the nonlinear relationship between pixel values and |
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displayed intensities, there is a gamma correction feature in Mesa. Some |
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systems, such as Silicon Graphics, support gamma correction in hardware |
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(man gamma) so you won't need to use Mesa's gamma facility. Other |
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systems, however, may need gamma adjustment to produce images which look |
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correct. If you believe that Mesa's images are too dim, read on. |
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Gamma correction is controlled with the **MESA_GAMMA** environment |
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variable. Its value is of the form **Gr Gg Gb** or just **G** where Gr |
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is the red gamma value, Gg is the green gamma value, Gb is the blue |
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gamma value and G is one gamma value to use for all three channels. Each |
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value is a positive real number typically in the range 1.0 to 2.5. The |
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defaults are all 1.0, effectively disabling gamma correction. Examples: |
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.. code-block:: console |
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% export MESA_GAMMA="2.3 2.2 2.4" # separate R,G,B values |
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% export MESA_GAMMA="2.0" # same gamma for R,G,B |
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The ``demos/gamma.c`` program in mesa/demos repository may help you to |
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determine reasonable gamma value for your display. With correct gamma |
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values, the color intensities displayed in the top row (drawn by |
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dithering) should nearly match those in the bottom row (drawn as grays). |
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Alex De Bruyn reports that gamma values of 1.6, 1.6 and 1.9 work well on |
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HP displays using the HP-ColorRecovery technology. |
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Mesa implements gamma correction with a lookup table which translates a |
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"linear" pixel value to a gamma-corrected pixel value. There is a small |
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performance penalty. Gamma correction only works in RGB mode. Also be |
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aware that pixel values read back from the frame buffer will not be |
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"un-corrected" so glReadPixels may not return the same data drawn with |
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glDrawPixels. |
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For more information about gamma correction, see the `Wikipedia |
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article <https://en.wikipedia.org/wiki/Gamma_correction>`__ |
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Overlay Planes |
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-------------- |
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Hardware overlay planes are supported by the Xlib driver. To determine |
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if your X server has overlay support you can test for the |
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SERVER_OVERLAY_VISUALS property: |
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.. code-block:: console |
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xprop -root | grep SERVER_OVERLAY_VISUALS |
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Extensions |
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---------- |
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The following Mesa-specific extensions are implemented in the Xlib |
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driver. |
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GLX_MESA_pixmap_colormap |
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~~~~~~~~~~~~~~~~~~~~~~~~ |
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This extension adds the GLX function: |
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.. code-block:: c |
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GLXPixmap glXCreateGLXPixmapMESA( Display *dpy, XVisualInfo *visual, |
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Pixmap pixmap, Colormap cmap ) |
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It is an alternative to the standard glXCreateGLXPixmap() function. |
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Since Mesa supports RGB rendering into any X visual, not just True- |
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Color or DirectColor, Mesa needs colormap information to convert RGB |
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values into pixel values. An X window carries this information but a |
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pixmap does not. This function associates a colormap to a GLX pixmap. |
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See the xdemos/glxpixmap.c file for an example of how to use this |
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extension. |
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`GLX_MESA_pixmap_colormap |
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specification <specs/MESA_pixmap_colormap.spec>`__ |
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GLX_MESA_release_buffers |
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~~~~~~~~~~~~~~~~~~~~~~~~ |
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Mesa associates a set of ancillary (depth, accumulation, stencil and |
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alpha) buffers with each X window it draws into. These ancillary buffers |
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are allocated for each X window the first time the X window is passed to |
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glXMakeCurrent(). Mesa, however, can't detect when an X window has been |
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destroyed in order to free the ancillary buffers. |
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The best it can do is to check for recently destroyed windows whenever |
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the client calls the glXCreateContext() or glXDestroyContext() |
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functions. This may not be sufficient in all situations though. |
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The GLX_MESA_release_buffers extension allows a client to explicitly |
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deallocate the ancillary buffers by calling glxReleaseBuffersMESA() just |
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before an X window is destroyed. For example: |
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.. code-block:: c |
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#ifdef GLX_MESA_release_buffers |
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glXReleaseBuffersMESA( dpy, window ); |
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#endif |
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XDestroyWindow( dpy, window ); |
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`GLX_MESA_release_buffers |
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specification <specs/MESA_release_buffers.spec>`__ |
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This extension was added in Mesa 2.0. |
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GLX_MESA_copy_sub_buffer |
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~~~~~~~~~~~~~~~~~~~~~~~~ |
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This extension adds the glXCopySubBufferMESA() function. It works like |
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glXSwapBuffers() but only copies a sub-region of the window instead of |
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the whole window. |
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`GLX_MESA_copy_sub_buffer |
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specification <specs/MESA_copy_sub_buffer.spec>`__ |
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This extension was added in Mesa 2.6 |
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Summary of X-related environment variables |
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------------------------------------------ |
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+-----------------------------+--------------------------------------+ |
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| Environment variable | Description | |
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+=============================+======================================+ |
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| :envvar:`MESA_RGB_VISUAL` | specifies the X visual and depth for | |
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| | RGB mode (X only) | |
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+-----------------------------+--------------------------------------+ |
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| :envvar:`MESA_BACK_BUFFER` | specifies how to implement the back | |
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| | color buffer (X only) | |
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+-----------------------------+--------------------------------------+ |
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| :envvar:`MESA_GAMMA` | gamma correction coefficients | |
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| | (X only) | |
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+-----------------------------+--------------------------------------+
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