1 files changed, 45 insertions, 0 deletions
diff --git a/gfx/wr/swgl/README.md b/gfx/wr/swgl/README.md
new file mode 100644
index 0000000000..eac1ec3798
--- /dev/null
+++ b/gfx/wr/swgl/README.md
@@ -0,0 +1,45 @@
+# swgl
+
+Software OpenGL implementation for WebRender
+
+## Overview
+This is a relatively simple single threaded software rasterizer designed
+for use by WebRender. It will shade one quad at a time using a 4xf32 vector
+with one vertex per lane. It rasterizes quads usings spans and shades that
+span 4 pixels at a time.
+
+## Building
+clang-cl is required to build on Windows. This can be done by installing
+the llvm binaries from https://releases.llvm.org/ and adding the installation
+to the path with something like `set PATH=%PATH%;C:\Program Files\LLVM\bin`.
+Then `set CC=clang-cl` and `set CXX=clang-cl`. That should be sufficient
+for `cc-rs` to use `clang-cl` instead of `cl`.
+
+## Extensions
+SWGL contains a number of OpenGL and GLSL extensions designed to both ease
+integration with WebRender and to help accelerate span rasterization.
+
+GLSL extension intrinsics are generally prefixed with `swgl_` to distinguish
+them from other items in the GLSL namespace.
+
+Inside GLSL, the `SWGL` preprocessor token is defined so that usage of SWGL
+extensions may be conditionally compiled.
+
+```
+void swgl_clipMask(sampler2D mask, vec2 offset, vec2 bb_origin, vec2 bb_size);
+```
+
+When called from the the vertex shader, this specifies a clip mask texture to
+be used to mask the currently drawn primitive while blending is enabled. This
+mask will only apply to the current primitive.
+
+The mask must be an R8 texture that will be interpreted as alpha weighting
+applied to the source pixel prior to the blend stage. It is sampled 1:1 with
+nearest filtering without any applied transform. The given offset specifies
+the positioning of the clip mask relative to the framebuffer's viewport.
+
+The supplied bounding box constrains sampling of the clip mask to only fall
+within the given rectangle, specified relative to the clip mask offset.
+Anything falling outside this rectangle will be clipped entirely. If the
+rectangle is empty, then the clip mask will be ignored.
+