This extension exposes the ability to subscribe to a set of uniform targets which can be used to populate uniforms within shader programs. This extension is generic, but currently only supports mouse position as a subscription target.
Background:
The depth of the web pipeline makes it difficult to support low latency interaction as event information retrieved via javascript is outdated by the time it's displayed to clients. By populating event information later in the pipeline one can reduce perceived input latency.
This extension creates a new buffer type 'Valuebuffer' to
maintain the active state for predefined subscription targets. Since a
mechanism for buffering uniform information isn't available pre 2.0 (UBOs)
an additional data type was needed. See 'New Types' for additional
information.
When this extension is enabled:
Valuebuffer object.Valuebuffer
object.Valuebuffer object.Valuebuffer object.Valuebuffer object to a subscription target.Valuebuffer object with the state of the subscriptions to
which it is subscribed.Valuebuffer object.This interface is used to maintain a reference to internal
Valuebuffer subscription states.
A Valuebuffer abstracts the mapping of subscription targets to internal
state and acts as a single storage object for subscription information (e.g. current
mouse position). Clients can then use the objects data to populate uniform variables.
Post WebGL API 2.0, this abstraction could exist as a layer ontop of UBOs
which managers the mapping of subscription targets to internal state and the mapping
of subscription targets to offsets within the buffer. The UBO would be used to store the
active buffer state as well as the uniform location mapping. Clients would be required to
state all their subscription targets at once to allocate the appropriate amount of memory.
Aside from this small change the implementation is essentially the same, with UBOs replacing
Valuebuffers and relevant create, delete, bind methods being replaced.
Additionally, the inclusion of UBOs would replace the need for
uniformValueBuffer(...).
SUBSCRIBED_VALUES_BUFFER
is accepted as the target parameter to bindValuebuffer
SUBSCRIBED_VALUES_BUFFER
is accepted as the target parameter to subscribeValuebuffer
MOUSE_POSITION
is accepted as the subscription parameter to subscribeValuebuffer
SUBSCRIBED_VALUES_BUFFER
is accepted as the target parameter to populateSubscribedValues
SUBSCRIBED_VALUES_BUFFER
is accepted as the target parameter to uniformValuebuffer
MOUSE_POSITION
is accepted as the subscription parameter to uniformValuebuffer
<script id="vshader" type="x-shader/x-vertex">
uniform ivec2 uMousePosition;
void main()
{
gl_Position = vec4(uMousePosition, 0, 1);
}
</script>
function init(gl) {
shader.uMousePosition = gl.getUniformLocation(shader, "uMousePosition");
...
var ext = gl.getExtension('WEBGL_subscribe_uniform');
// Create the value buffer and subscribe.
var valuebuffer = ext.createValuebuffer();
ext.bindValuebuffer(SUBSCRIBED_VALUES_BUFFER, valuebuffer);
ext.subscribeValue(MOUSE_POSITION);
...
}
function draw(gl) {
// Populate buffer and populate uniform
ext.bindValuebuffer(SUBSCRIBED_VALUES_BUFFER, valuebuffer);
ext.populateSubscribedValues(SUBSCRIBED_VALUES_BUFFER);
ext.uniformValuebuffer(shader.uMousePosition,
SUBSCRIBED_VALUES_BUFFER,
MOUSE_POSITION);
gl.drawElements(...);
}