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authorJohn Firebaugh <john.firebaugh@gmail.com>2017-03-31 15:10:20 -0700
committerJohn Firebaugh <john.firebaugh@gmail.com>2017-04-03 09:12:46 -0700
commitd092841b19e0e235681f8e9ef3cd13ad1b4df149 (patch)
tree7cc3985659041faef2e8df8cf0211fbf9af02354 /src
parentda35f64f09c8dc77b8bbc4add5189ada388aa27e (diff)
downloadqtlocation-mapboxgl-d092841b19e0e235681f8e9ef3cd13ad1b4df149.tar.gz
[core] Add another explanatory comment to PaintPropertyBinder
Diffstat (limited to 'src')
-rw-r--r--src/mbgl/style/paint_property_binder.hpp31
1 files changed, 31 insertions, 0 deletions
diff --git a/src/mbgl/style/paint_property_binder.hpp b/src/mbgl/style/paint_property_binder.hpp
index e6e957958c..7e44b509a1 100644
--- a/src/mbgl/style/paint_property_binder.hpp
+++ b/src/mbgl/style/paint_property_binder.hpp
@@ -49,6 +49,37 @@ std::array<float, N*2> zoomInterpolatedAttributeValue(const std::array<float, N>
return result;
}
+/*
+ PaintPropertyBinder is an abstract class serving as the interface definition for
+ the strategy used for constructing, uploading, and binding paint property data as
+ GLSL attributes.
+
+ It has three concrete subclasses, one for each of the three strategies we use:
+
+ * For _constant_ properties -- those whose value is a constant, or the constant
+ result of evaluating a camera function at a particular camera position -- we
+ don't need a vertex buffer, and can instead use a constant attribute binding
+ via the `glVertexAttrib*` family of functions.
+ * For source functions, we use a vertex buffer with a single attribute value,
+ the evaluated result of the source function for the given feature.
+ * For composite functions, we use a vertex buffer with two attributes: min and
+ max values covering the range of zooms at which we expect the tile to be
+ displayed. These values are calculated by evaluating the composite function for
+ the given feature at strategically chosen zoom levels. In addition to this
+ attribute data, we also use a uniform value which the shader uses to interpolate
+ between the min and max value at the final displayed zoom level. The use of a
+ uniform allows us to cheaply update the value on every frame.
+
+ Note that the shader source is the same regardless of the strategy used to bind
+ the attribute -- in all cases the attribute is declared as a vec2, in order to
+ support composite min and max values (color attributes use a vec4 with special
+ packing). When the constant or source function strategies are used, the
+ interpolation uniform value is set to zero, and the second attribute element is
+ unused. This differs from the GL JS implementation, which dynamically generates
+ shader source based on the strategy used. We found that in WebGL, using
+ `glVertexAttrib*` was unnacceptably slow. Additionally, in GL Native we have
+ implemented binary shader caching, which works better if the shaders are constant.
+*/
template <class T, class A>
class PaintPropertyBinder {
public: