While porting GTK to GskRenderer we noticed that the current fallback
code for widgets using Cairo to draw is not enough to cover all the
possible cases.
For instance, if a container widget still uses GtkWidget::draw to render
its children, and at least one of them has been ported to using render
nodes instead, the container won't know how to draw it.
For this reason we want to provide to layers above GSK the ability to
create a "fallback" renderer instance, created using a "parent"
GskRenderer instance, but using a Cairo context as the rendering target
instead of a GdkDrawingContext.
GTK will use this inside the gtk_widget_draw() implementation, if a
widget implements GtkWidgetClass.get_render_node().
We're going to need to allow rendering on a specific cairo_t in order to
implement fallback code paths inside GTK; this means that there will be
times when we have a transient GskRenderer instance that does not have a
GdkDrawingContext to draw on.
Instead of adding a new render() implementation for those cases and then
decide which one to use, we can remove the drawing context argument from
the virtual function itself, and allow using a NULL GdkDrawingContext
when calling gsk_renderer_render(). A later commit will add a generic
function to create a transient GskRenderer with a cairo_t attached to
it.
Renderers inside GSK will have to check whether we have access to a
GdkDrawingContext, in which case we're going to use it; or if we have
access to a cairo_t and a window.
GskRenderNode is, at its core, a write-only API; you're supposed to set
up the render nodes instead of querying them for state.
Querying render nodes is left to the GskRenderer implementation.
We store the vertices in (unscaled) window coords (but the item size
is still scaled to match the texture size). Also, the
projection/model-view multiplication order is switched so that the scale
is applied at the right place.
The renderer will always use nearest-neighbor filters because it renders
at 1:1 pixel to texel ratio.
On the other hand, render nodes may be scaled, so we need to offer a way
to control the minification and magnification filters.
If we already have a GL texture we definitely don't want to use
gdk_cairo_draw_from_gl() to draw on a Cairo context if we're going
to take the Cairo surface to which we draw and put it into an OpenGL
texture.
The details of the modelview and projection matrices are only useful for
the GL renderer; there's really no point in having those details
available in the generic API — especially as the Cairo fallback renderer
cannot really set up a complex modelview or a projection matrix.
Just like we reuse texture ids with the same size we can, at the expense
of a little memory, reuse vertex buffers if they reference the same
attributes and contain the same data.
Each VAO is marked as free at the end of the frame, and if it's not
reused in the following frame, it gets dropped.
The child-transform is useful only if we also provide clipping to the
parent nodes, otherwise children will just be drawn outside of the
parent's bounds.
We'll introduce child transforms either at a higher layer, or once we
add clipping support to GskRenderNode.
I don't think this should stay in the code long-term, but it
is useful for debugging. It helped me track down some suspicious
placements of render nodes.
Instead of passing the size of the buffer, we should pass the number of
quads; we know what the size of a single quad structure is, so we can do
the multiplication internally when creating the VAO.
This allows us to print the quads for debugging purposes.
The naming is consistent with other scene graph libraries, as it
represents an additional translation transformation applied on top of
the provided transformation matrices.
We can also simplify the implementation by applying the translation when
we compute the world matrix.
We keep the textures used inside a frame around until the end of the
following frame; whenever we need a texture with the same size, and
it's not marked in use, then we just reuse the existing texture.
This was overwhelming other useful debug output, so make it
opt-in. We print the render items for both opengl and transforms,
since the matrices bleed into each other, otherwise.
Since we use an FBO to render the contents of the render node tree, the
coordinate space is going to be flipped in GL. We can undo the flip by
using an appropriate projection matrix, instead of changing the sampling
coordinates in the shaders and updating all our coordinates at render
time.
We need to apply a scaling factor whenever we deal with user-supplied
coordinates, like:
- when creating textures
- when setting up the viewport
- when submitting the scene
Render nodes need access to rendering information like scaling factors.
If we keep render nodes separate from renderers until we submit a nodes
tree for rendering we're going to have to duplicate all that information
in a way that makes the API more complicated and fuzzier on its
semantics.
By having GskRenderer create GskRenderNode instances we can tie nodes
and renderers together; since higher layers will also have access to
the renderer instance, this does not add any burden to callers.
Additionally, if memory measurements indicate that we are spending too
much time in the allocation of new render nodes, we can now easily
implement a free-list or a renderer-specific allocator without breaking
the API.
If a node is non-opaque and has a non-zero opacity we need to paint its
contents and children first to an off screen buffer, and then render the
resulting texture at the desired opacity — otherwise the opacities will
combine and result in the wrong rendering.
We're not going to use separate rendering lists soon, and the way we
render items is less similar to a gaming engine and more similar to a
simpler compositor. This means we don't need to perform a two pass
rendering — opaque items first, transparent items later.
Use appropriate names, and annotate the names with the types — 'u' for
uniforms, 'a' for attributes. The common preambles for shaders are split
from the bodies, so we need some way to distinguish the uniforms and the
attributes just from their name.
We want the GL driver to cache as many resources as possible, so we can
always ensure that we're in a consistent state, and we can handle state
transitions more appropriately.
Drop the texture parameters handling from the texture creation, and
associate them with the contents upload. Also, rename the function to
something more in line with what it does.
We want to add the list of FBOs tied to a texture; this means we cannot
trivally copy the Texture structure when adding it to a GArray. We're
also going to have more textures than VAOs, so it makes more sense to
use a O(1) access data structure for them.
We can use the GL_ARB_timer_query extension (available since OpenGL
3.2, and part of the OpenGL specification since version 3.3) to query
the time elapsed when drawing each frame. This allows us to gather
timing information on our use of the GPU.
