feat: support OpenGL EGL and GLX contexts

.github/workflows/ci.yml

@@ -16,6 +16,13 @@ defaults:
   run:
     shell: bash
 
+env:
+  # Headless OpenGL EGL: render without an X server / display surface, and force
+  # the llvmpipe software rasterizer (CI runners have no GPU). Harmless for the
+  # other backends.
+  EGL_PLATFORM: surfaceless
+  LIBGL_ALWAYS_SOFTWARE: "1"
+
 jobs:
   lint:
     name: Lint
@@ -94,6 +101,10 @@ jobs:
             backend: vulkan
             precompiled: 0
             amalgam: 0
+          - runs-on: ubuntu-latest
+            backend: glx
+            precompiled: 0
+            amalgam: 0
           - runs-on: ubuntu-latest
             backend: vulkan
             precompiled: 1 # also cover the prebuilt amalgam path
@@ -139,11 +150,11 @@ jobs:
           echo "VK_ICD_FILENAMES=$(brew --prefix molten-vk)/etc/vulkan/icd.d/MoltenVK_icd.json" >> $GITHUB_ENV
           echo "DYLD_LIBRARY_PATH=$(brew --prefix vulkan-loader)/lib" >> $GITHUB_ENV
       - run: rustup update stable && rustup default stable
-      - name: Run CI tests (Linux OpenGL via Xvfb)
-        if: startsWith(matrix.runs-on, 'ubuntu') && matrix.backend == 'opengl'
+      - name: Run CI tests (Linux OpenGL GLX via Xvfb)
+        if: startsWith(matrix.runs-on, 'ubuntu') && matrix.backend == 'glx'
         run: xvfb-run -a --server-args="-screen 0 1280x1024x24" just ci-test ${{ matrix.backend }}
       - name: Run CI tests
-        if: ${{ !(startsWith(matrix.runs-on, 'ubuntu') && matrix.backend == 'opengl') }}
+        if: ${{ !(startsWith(matrix.runs-on, 'ubuntu') && matrix.backend == 'glx') }}
         run: just ci-test ${{ matrix.backend }}
 
   build-msrv:

Cargo.toml

@@ -24,9 +24,15 @@ metal = []
 opengl = []
 vulkan = []
 wgpu = ["dep:webgpu-shim", "dep:wgpu"]
+
+# Use a GLX context for OpenGL (Linux)
+glx = ["opengl"]
+
 # Enables parsing style-spec layer JSON via `AnyLayer::from_json_str`/`from_json_value`.
 json = ["dep:serde_json"]
+# Interoperate with the `geojson` crate
 geojson = ["dep:geojson"]
+# Logging via the `log` crate.
 log = ["dep:log"]
 
 [dependencies]

README.md

@@ -19,14 +19,16 @@ Instructions for this are below.
 This crate supports multiple rendering backends:
 
 - `vulkan` (default on Linux/Windows): `cargo build --features vulkan`
-- `opengl` (cross-platform): `cargo build --features opengl`
 - `metal` (default on macOS/iOS): `cargo build --features metal`
+- `opengl` (Linux/Windows): `cargo build --features opengl`
 
-If no feature is specified, the crate will automatically select the platform-appropriate default backend.
+On Linux, OpenGL uses EGL by default for headless rendering. Use the `glx` feature only when you explicitly need rendering through GLX/X11.
 
-We also support the following other features:
+We also support the following other features (all enabled by default):
 
-- `log` logging via the [`log` library](https://lib.rs/log)
+- `json`: load styles and layers from JSON via [`serde_json`](https://lib.rs/serde_json)
+- `geojson`: interoperate with the [`geojson`](https://lib.rs/geojson) crate
+- `log`: logging via the [`log`](https://lib.rs/log) crate
 
 At its core, we work as follows:
 
@@ -75,7 +77,7 @@ The following platform and rendering-API combinations are supported and tested i
 ### Dependencies
 
 This command will install the required dependencies on Linux or macOS for the `vulkan` backend.
-You may also use it with `opengl` parameter on Linux.
+You may also use it with the `opengl` or `glx` parameter on Linux.
 It is OK to run this command multiple times for each backend.
 
 ```shell