[vlc-devel] [PATCH v2 21/22] opengl: document filters implementation

[Romain Vimont]

On Thu, Jul 02, 2020 at 05:12:03PM +0200, Alexandre Janniaux wrote:
> The documentation is not up-to-date anymore with the option.

Good catch, thanks!

> I feel that we also didn't choose about how to handle --gl-filter
> addition/removal which would change the documentation here.
> 
> What do you suggest currently?

I agree with your suggestion here:

https://mailman.videolan.org/pipermail/vlc-devel/2020-June/134926.html

> I'd suggest that we keep it until the opengl filter_t implementation
> is merged though, so as to be able to test and review the patches
> correctly.

Once we can run them in the existing filtering system (filter_t), we
could consider removing it. However, in my opinion, keeping it doesn't
hurt, and it could help to investigate filter issues (like: when the
result of a filter is black, does it work when executed in the vout?)

> 
> Regards,
> --
> Alexandre Janniaux
> Videolabs
> 
> On Wed, Jul 01, 2020 at 12:30:22PM +0200, Romain Vimont wrote:
> > Add an overview of the implementation of OpenGL filters.
> >
> > Co-authored-by: Alexandre Janniaux <ajanni at videolabs.io>
> > ---
> >  modules/video_output/opengl/filters.c | 78 +++++++++++++++++++++++++++
> >  1 file changed, 78 insertions(+)
> >
> > diff --git a/modules/video_output/opengl/filters.c b/modules/video_output/opengl/filters.c
> > index 66084b81cb..aefb8bff15 100644
> > --- a/modules/video_output/opengl/filters.c
> > +++ b/modules/video_output/opengl/filters.c
> > @@ -32,6 +32,84 @@
> >  #include "renderer.h"
> >  #include "sampler_priv.h"
> >
> > +/* The filter chain contains the sequential list of filters, typically given
> > + * via command-line arguments "--gl-filters=filter1:filter2:...:filtern".
> > + *
> > + * There are two types of filters:
> > + *  - blend filters just draw over the provided framebuffer (containing the
> > + *    result of the previous filter), without reading the input picture.
> > + *  - non-blend filters read their input picture and draw whatever they want to
> > + *    their own output framebuffer.
> > + *
> > + * For convenience, the filter chain does not store the filters as a single
> > + * sequential list, but as a list of non-blend filters, each containing the
> > + * list of their associated blend filters.
> > + *
> > + * For example, given:
> > + *
> > + *    --gl-filters=draw:triangle:triangle_mask:triangle_clock:triangle:renderer
> > + *
> > + * the filters chain stores the filters as follow:
> > + *
> > + *     +- draw               (non-blend)
> > + *     |  +- triangle        (blend)
> > + *     +- triangle_mask      (non-blend)
> > + *     |  +- triangle_clock  (blend)
> > + *     |  +- triangle        (blend)
> > + *     +- renderer           (non-blend)
> > + *
> > + * An output framebuffer is created for each non-blend filters. It is used as
> > + * draw framebuffer for that filter and all its associated blend filters.
> > + *
> > + * If the first filter is a blend filter, then a "draw" filter is automatically
> > + * inserted. If the renderer does not appear in the filter list, it is
> > + * automatically added at the end.
> > + *
> > + *
> > + * ## Multisample anti-aliasing (MSAA)
> > + *
> > + * Each filter may also request multisample anti-aliasing, by providing a MSAA
> > + * level during its Open(), for example:
> > + *
> > + *     filter->config.msaa_level = 4;
> > + *
> > + * For example:
> > + *
> > + *     +- draw               msaa_level=0
> > + *     |  +- triangle        msaa_level=4
> > + *     |  +- triangle_clock  msaa_level=2
> > + *     +- renderer           msaa_level=0
> > + *
> > + * Among a "group" of one non-blend filter and its associated blend filters,
> > + * the highest MSAA level (or 0 if multisampling is not supported) is assigned
> > + * both to the non-blend filter, to configure its MSAA framebuffer, and to the
> > + * blend filters, just for information and consistency:
> > + *
> > + *     +- draw               msaa_level=4
> > + *     |  +- triangle        msaa_level=4
> > + *     |  +- triangle_clock  msaa_level=4
> > + *     +- renderer           msaa_level=0
> > + *
> > + * Some platforms (Android) do not support resolving multisample to the default
> > + * framebuffer. Therefore, the msaa_level must always be 0 on the last filter.
> > + * If this is not the case, a "draw" filter is automatically appended.
> > + *
> > + * For example:
> > + *
> > + *     +- draw               msaa_level=0
> > + *     |  +- triangle        msaa_level=4
> > + *     +- renderer           msaa_level=0
> > + *        +- triangle_clock  msaa_level=2
> > + *
> > + * will become:
> > + *
> > + *     +- draw               msaa_level=4
> > + *     |  +- triangle        msaa_level=4
> > + *     +- renderer           msaa_level=2
> > + *     |  +- triangle_clock  msaa_level=2
> > + *     +- draw               msaa_level=0
> > + */
> > +
> >  struct vlc_gl_filters {
> >      struct vlc_gl_t *gl;
> >      const struct vlc_gl_api *api;
> > --
> > 2.27.0
> >
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