document where to find examples and guard go-gl builds

Author: soypat

README.md

@@ -21,6 +21,14 @@ Offshoot from [this project](https://github.com/soypat/sdf/pull/13). Is WIP.
 ## Part design - NPT Flange example.
 This was converted from the [original example](https://github.com/soypat/sdf/blob/main/examples/npt-flange/flange.go). See [README](https://github.com/soypat/sdf/tree/main/examples) for images.
 
+
+See working example under [examples](./examples/) directory. Run on GPU with `-gpu` flag: 
+```sh
+go run ./examples/npt-flange -gpu
+```
+
+
+
 ```go
 	const (
 		tlen             = 18. / 25.4

gleval/gpu.go

@@ -1,4 +1,4 @@
-//go:build !tinygo
+//go:build !tinygo && cgo
 
 package gleval
 

gleval/gpu_nocgo.go

@@ -0,0 +1,47 @@
+//go:build tinygo || !cgo
+
+package gleval
+
+import (
+	"errors"
+	"io"
+
+	"github.com/soypat/glgl/math/ms2"
+	"github.com/soypat/glgl/math/ms3"
+)
+
+var errNoCGO = errors.New("GPU evaluation requires CGo and is not supported on TinyGo")
+
+// NewComputeGPUSDF3 instantiates a [SDF3] that runs on the GPU.
+func NewComputeGPUSDF3(glglSourceCode io.Reader, bb ms3.Box) (*SDF3Compute, error) {
+	return nil, errNoCGO
+}
+
+type SDF3Compute struct {
+	bb ms3.Box
+}
+
+func (sdf *SDF3Compute) Bounds() ms3.Box {
+	return sdf.bb
+}
+
+func (sdf *SDF3Compute) Evaluate(pos []ms3.Vec, dist []float32, userData any) error {
+	return nil, errNoCGO
+}
+
+// NewComputeGPUSDF2 instantiates a [SDF2] that runs on the GPU.
+func NewComputeGPUSDF2(glglSourceCode io.Reader, bb ms2.Box) (*SDF2Compute, error) {
+	return nil, errNoCGO
+}
+
+type SDF2Compute struct {
+	bb ms2.Box
+}
+
+func (sdf *SDF2Compute) Bounds() ms2.Box {
+	return sdf.bb
+}
+
+func (sdf *SDF2Compute) Evaluate(pos []ms2.Vec, dist []float32, userData any) error {
+	return errNoCGO
+}