mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-11 19:06:33 +00:00
241 lines
10 KiB
Markdown
241 lines
10 KiB
Markdown
# OpenGL
|
|
|
|
OpenGL is a venerable, cross-platform 3D graphics API available for use
|
|
on Linux, Windows, MacOS, iOS and Android and is usually backed by
|
|
specialized hardware (a GPU) to accelarate rendering. There are however,
|
|
also CPU-based software implementations available for ensuring correctness and
|
|
OpenGL use without a GPU.
|
|
|
|
## Limits imposed by OpenGL
|
|
|
|
### OpenGL and Threads
|
|
|
|
A major design decision of OpenGL has an OpenGL context only being available
|
|
(or current) for use in a single thread. This is directly at odds with
|
|
GStreamer's multi-threaded design and requires guidance and/or restrictions on
|
|
how GStreamer and the application will interact with OpenGL. There are two
|
|
main models for using OpenGL from multiple threads.
|
|
|
|
The first involves creating an OpenGL context for each thread that will use
|
|
OpenGL in such a way that all the OpenGL contexts can share OpenGL resources.
|
|
While this does ensure that each thread can execute and use OpenGL resources
|
|
concurrently, it has a high resources cost for two main reasons.
|
|
|
|
1. OpenGL contexts are expensive to create and maintain. e.g. a single OpenGL
|
|
context can use multiple MB of memory for storing all the required state.
|
|
2. Transfering between different OpenGL contexts requires synchronisation.
|
|
This incurs a penalty at runtime performing the required synchronisation
|
|
as well as an increase in code complexity.
|
|
|
|
The second involves making the OpenGL context current whenever it is
|
|
needed. On some platforms this is possible to perform lazily as the context
|
|
will be uncurrented in the previous thread automatically. Regrettably this
|
|
behaviour is not guaranteed for all platforms and means that transfering an
|
|
OpenGL context between threads requires explicitly uncurrenting from the
|
|
previous thread. The other issue with this is performance related where on
|
|
some platforms, making an OpenGL current could could consume a large amount of
|
|
CPU time.
|
|
|
|
GStreamer takes a different approach to this by marshalling all the OpenGL
|
|
functionality to a dedicated OpenGL thread that is created and destroyed with
|
|
the `GstGLContext`. This removes the synchronisation requirements between
|
|
multiple GStreamer OpenGL contexts, removes the need for multiple OpenGL
|
|
contexts inside GStreamer reducing the memory requirements, and avoids the
|
|
possibly expensive currenting and uncurrenting of the OpenGL context multiple
|
|
times per frame. The marshalling is performed in the `GstGLWindow`
|
|
implementation by `gst_gl_window_send_message()` /
|
|
`gst_gl_window_send_message_async()` and is required to be re-entrant by
|
|
ensuring that marshalling from the OpenGL thread must execute the marshalled
|
|
operation immediately. The only downside of marshalling everything to a
|
|
dedicated thread has is that OpenGL in GStreamer requires two
|
|
thread switches for calling a block of OpenGL commands from outside the
|
|
OpenGL thread. Given the possibly high cost of the other approaches
|
|
this is an acceptable trade-off.
|
|
|
|
### OpenGL Function Pointers
|
|
|
|
Another design choice of OpenGL is that (at least on the windows platform)
|
|
OpenGL functions can be OpenGL context specific. This means that retrieving a
|
|
function pointer from one OpenGL context has no guarentee of doing the correct
|
|
thing with any other OpenGL context. Each `GstGLContext` has a `GstGLFuncs`
|
|
structure that contains a list of OpenGL functions known to libgstgl that is
|
|
populated by a call to `gst_gl_context_fill_info()`.
|
|
|
|
## libgstgl Library
|
|
|
|
This library provides the necessary infrastructure for integrating OpenGL usage
|
|
in GStreamer between elements in the pipeline and an application that may use
|
|
OpenGL. The library can be split into 3 main parts:
|
|
|
|
- Platform-specific wrappers (`GstGLDisplay`, `GstGLWindow`, `GstGLContext`)
|
|
- GStreamer subclasses
|
|
- Element base clases (`GstGLBaseFilter`, `GstGLFilter`, `GstGLBaseMixer`, etc)
|
|
- Data/Memory (`GstGLBuffer`, `GstGLMemory`, `GstGLBufferPool`, etc)
|
|
- Synchronisation (`GstGLSyncMeta`)
|
|
- OpenGL helpers (`GstGLShader`, `GstGLFramebuffer`, etc)
|
|
|
|
### Platform Specifics
|
|
|
|
Due to the nature of OpenGL, the use of OpenGL requires a connection to a
|
|
windowing system (X11, Wayland, Cocoa, GDI+, etc). These windowing systems
|
|
are entirely platform specific and are wrapped in the `GstGLDisplay` generic
|
|
interface. Specific functionality exposed by a windowing system may be exposed
|
|
by platform-specific `GstGLDisplay` objects. e.g. `GstGLDisplayWayland` or
|
|
`GstGLDisplayX11`.
|
|
|
|
Another platform specific part of the puzzle is the `GstGLWindow` object which
|
|
encapsulates the surface that is being rendered into by OpenGL. There are
|
|
specific windowing system implementations for each platform that are all exposed
|
|
through the generic `GstGLWindow` interface. An application generally does not
|
|
need to interact with the window used by an OpenGL context with GStreamer.
|
|
|
|
The last platform specific piece is the `GstGLContext` which represents a
|
|
OpenGL context. A `GstGLContext` has two main forms, wrapped (created with
|
|
`gst_gl_context_new_wrapped()`) vs not. Some `GstGLContext` API behaves
|
|
differently with a wrapped `GstGLContext`.
|
|
|
|
#### Wrapped `GstGLContext`s
|
|
|
|
A wrapped `GstGLContext` is simply a container to an OpenGL context handle and
|
|
does not contain any separate thread to execute the OpenGL operations on.
|
|
This means that `gst_gl_context_thread_add()` with a wrapped `GstGLContext`
|
|
will attempt to execute the function immediately and may fail in some way
|
|
(crashes, ignored) if the actual OpenGL context is not active in the current
|
|
thread. Even if the OpenGL context is active in the current thread, GStreamer
|
|
will not know that and needs to be told explicitly through the necessary calls
|
|
to `gst_gl_context_set_active()`.
|
|
|
|
### Application Integration
|
|
|
|
There are two types of resources required for integration between GStreamer
|
|
elements and the application, platform specific and generic OpenGL resources.
|
|
|
|
First, the platform specific resources are the `GstGLDisplay` for the
|
|
required connection to the windowing system and the `GstGLContext` for the
|
|
OpenGL context that the application is using. With these two pieces of
|
|
information, it is possible for certain OpenGL resources to be 'shared'
|
|
between GStreamer and the application subject to lifetime and synchronisation
|
|
requirements.
|
|
|
|
#### Sharing X11 Display With GStreamer
|
|
|
|
```c
|
|
static gboolean
|
|
sync_bus_call (GstBus *bus, GstMessage *msg, gpointer data)
|
|
{
|
|
switch (GST_MESSAGE_TYPE (msg)) {
|
|
case GST_MESSAGE_NEED_CONTEXT:
|
|
{
|
|
const gchar *context_type;
|
|
GstContext *context = NULL;
|
|
|
|
gst_message_parse_context_type (msg, &context_type);
|
|
g_print("got need context %s\n", context_type);
|
|
|
|
if (g_strcmp0 (context_type, GST_GL_DISPLAY_CONTEXT_TYPE) == 0) {
|
|
Display *x11_display; /* get this from the application somehow */
|
|
GstGLDisplay *gl_display = GST_GL_DISPLAY (gst_gl_display_x11_new_with_display (x11_display));
|
|
|
|
context = gst_context_new (GST_GL_DISPLAY_CONTEXT_TYPE, TRUE);
|
|
gst_context_set_gl_display (context, gl_display);
|
|
|
|
gst_element_set_context (GST_ELEMENT (msg->src), context);
|
|
}
|
|
if (context)
|
|
gst_context_unref (context);
|
|
break;
|
|
}
|
|
default:
|
|
break;
|
|
}
|
|
|
|
return FALSE;
|
|
}
|
|
```
|
|
|
|
#### Sharing OpenGL Context With GStreamer
|
|
|
|
```c
|
|
static gboolean
|
|
sync_bus_call (GstBus *bus, GstMessage *msg, gpointer data)
|
|
{
|
|
switch (GST_MESSAGE_TYPE (msg)) {
|
|
case GST_MESSAGE_NEED_CONTEXT:
|
|
{
|
|
const gchar *context_type;
|
|
GstContext *context = NULL;
|
|
|
|
gst_message_parse_context_type (msg, &context_type);
|
|
g_print("got need context %s\n", context_type);
|
|
|
|
if (g_strcmp0 (context_type, "gst.gl.app_context") == 0) {
|
|
GstGLContext *gl_context; /* get this from the application somehow */
|
|
GstStructure *s;
|
|
|
|
context = gst_context_new ("gst.gl.app_context", TRUE);
|
|
s = gst_context_writable_structure (context);
|
|
gst_structure_set (s, "context", GST_GL_TYPE_CONTEXT, gl_context, NULL);
|
|
|
|
gst_element_set_context (GST_ELEMENT (msg->src), context);
|
|
}
|
|
if (context)
|
|
gst_context_unref (context);
|
|
break;
|
|
}
|
|
default:
|
|
break;
|
|
}
|
|
|
|
return FALSE;
|
|
}
|
|
```
|
|
|
|
Second, other OpenGL resources such as buffers or textures require wrapping
|
|
into a GStreamer compatible format. For data-specific OpenGL resources, they
|
|
are wrapped up in some form of `GstMemory`. The application can create these
|
|
resources for pushing into GStreamer or consume these resources from GStreamer.
|
|
See the resource specific sections for more details.
|
|
|
|
### `GstGLMemory`
|
|
|
|
`GstGLMemory` is a `GstMemory` subclass that holds a single plane of a video
|
|
frame in an OpenGL texture.
|
|
|
|
### `GstGLBuffer`
|
|
|
|
`GstGLBuffer` is a `GstMemory` subclass that holds data stored in an OpenGL
|
|
buffer object.
|
|
|
|
### Automatic Transfers To/From The GPU
|
|
|
|
Both `GstGLMemory` and `GstGLBuffer` implement automatic transfers to and from
|
|
OpenGL based entirely on the sequence of calls to `gst_memory_map()` /
|
|
`gst_memory_unmap()`. Combined with a special `GstMapFlags`, `GST_MAP_GL`, the
|
|
OpenGL based `GstMemory` implementations can discern where the most recent
|
|
data was written and copy the data on-demand to where it is needed on read.
|
|
|
|
Internally this is implemented using two extra `GstMemoryFlags`,
|
|
`GST_GL_BASE_MEMORY_TRANSFER_NEED_DOWNLOAD` and
|
|
`GST_GL_BASE_MEMORY_TRANSFER_NEED_UPLOAD` that indicate that a transfer is
|
|
needed if that specific domain (system memory or OpenGL) is mapped.
|
|
The transfer flags are automatically updated and should not be modified by
|
|
elements or the application.
|
|
|
|
## Elements
|
|
|
|
There are a number of elements that make use of the OpenGL API for their
|
|
functionality. A non-comprehensive list is provided below.
|
|
|
|
- `glimagesinkelement` - Display a video using OpenGL
|
|
- `glcolorconvert` - Convert between diferent color spaces
|
|
- `glviewconvert` - Convert between different stereo view formats
|
|
- `gltransformation` - Perfom transformations in 3D space of the 2D video plane
|
|
- `gleffects` - Various OpenGL effects
|
|
- `glvideomixerelement` - Mix video using OpenGL (roughly equivalent to compositor)
|
|
- `glcolorbalance` - Color balance filtering
|
|
- `gltestsrc` - OpenGL equivalent to videotestsrc
|
|
- `glshader` - Execute an arbitrary OpenGL shader
|
|
- `gloverlay` - Overlay an image onto a video stream
|
|
- `glupload` - Upload data into OpenGL
|
|
- `gldownload` - Download data from OpenGL
|
|
|