Older GPUs without usable DirectX 12 Feature Level 12_0 or working
Vulkan support (e.g. legacy Intel iGPUs and other old parts), and old
GLES drivers whose shader compiler rejects iced's own shaders, crash
at startup or on the first drawing viewport: StagingBelt aborts in
Buffer::get_mapped_range (panic=abort, silent exit) on the DX12 path,
and shader compilation fails inside iced's gradient pipeline on the
GLES path. Selecting a working backend via WGPU_BACKEND before iced
boots avoids both classes.
- Probe each candidate backend (Windows: dx12,vulkan,gl; Linux:
vulkan,gl) in an isolated --gpu-probe child: device request,
staging-style mapped upload, render pipeline creation (its fragment
shader reads a private vec4 array, catching old-Mesa compile
failures), offscreen render and readback. First fully-passing
backend wins; explicit --backend / WGPU_BACKEND / --safe-mode
always win, and Windows keeps preferring DX12/Vulkan (#55).
- Software adapters (WARP/llvmpipe/SwiftShader) are the last resort
before a best-effort GL attempt: slow but alive.
- The crash sentinel names the attempted backend so only implicated
backends are skipped next launch.
- Winning probes report adapter limits; GPUs without shader storage
buffers get the packed compatibility renderer automatically, with
a startup command-line notice so the switch is never silent.
