fix difference op

README.md

@@ -10,35 +10,43 @@ Offshoot from [this project](https://github.com/soypat/sdf/pull/13). Is WIP.
     - Design your part using one API, switch between CPU and GPU after design.
 - Extremely coherent API design.
 
-## NPT Flange example.
+## 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.
-```go
-var (
-    npt    threads.NPT
-    flange glbuild.Shader3D
-)
-npt.SetFromNominal(1.0 / 2.0)
-pipe, err := threads.Nut(threads.NutParms{
-    Thread: npt,
-    Style:  threads.NutCircular,
-})
-if err != nil {
-    panic(err)
-}
-
-flange, err = gsdf.NewCylinder(flangeD/2, flangeH, flangeH/8)
-return makeSDF(flange)
-if err != nil {
-    return nil, err
-}
-flange = gsdf.Translate(flange, 0, 0, -tlen/2)
-flange = gsdf.SmoothUnion(pipe, flange, 0.2)
-hole, err := gsdf.NewCylinder(internalDiameter/2, 4*flangeH, 0)
-if err != nil {
-    return nil, err
-}
-flange = gsdf.Difference(flange, hole) // Make through-hole in flange bottom
-flange = gsdf.Scale(flange, 25.4)      // convert to millimeters
 
-render(flange) // Do something with it.
+```go
+    const (
+		tlen             = 18. / 25.4 // thread length
+		internalDiameter = 1.5 / 2.
+		flangeH          = 7. / 25.4
+		flangeD          = 60. / 25.4
+	)
+	var (
+		npt    threads.NPT
+		flange glbuild.Shader3D
+		err    error
+	)
+	err = npt.SetFromNominal(1.0 / 2.0)
+	if err != nil {
+		return nil, err
+	}
+	pipe, err := threads.Nut(threads.NutParms{
+		Thread: npt,
+		Style:  threads.NutCircular,
+	})
+	if err != nil {
+		return nil, err
+	}
+	flange, err = gsdf.NewCylinder(flangeD/2, flangeH, flangeH/8)
+	if err != nil {
+		return nil, err
+	}
+	flange = gsdf.Translate(flange, 0, 0, -tlen/2)
+	flange = gsdf.SmoothUnion(pipe, flange, 0.2)
+	hole, err := gsdf.NewCylinder(internalDiameter/2, 4*flangeH, 0)
+	if err != nil {
+		return nil, err
+	}
+	flange = gsdf.Difference(flange, hole) // Make through-hole in flange bottom
+	flange = gsdf.Scale(flange, 25.4)      // convert to millimeters
+	renderSDF(flange)
 ```

cpu_evaluators.go

@@ -181,7 +181,7 @@ func (u *diff) Evaluate(pos []ms3.Vec, dist []float32, userData any) error {
 		return err
 	}
 	for i := range dist {
-		dist[i] = maxf(-d1[i], d2[i])
+		dist[i] = maxf(d1[i], -d2[i])
 	}
 	return nil
 }

examples/npt-flange/flange.go

@@ -17,30 +17,20 @@ import (
 )
 
 func init() {
+	flag.BoolVar(&useGPU, "gpu", useGPU, "Enable GPU usage")
+	flag.Parse()
 	if useGPU {
+		fmt.Println("enable GPU usage")
 		runtime.LockOSThread() // For when using GPU this is required.
 	}
 }
 
 var useGPU = false
 
-const (
-	// visualization is the name of the file with a GLSL
-	// generated visualization of the SDF which can be visualized in https://www.shadertoy.com/
-	// or using VSCode's ShaderToy extension. If visualization=="" then no file is generated.
-	visualization = "nptflange.glsl"
-	// thread length
-	tlen             = 18. / 25.4
-	internalDiameter = 1.5 / 2.
-	flangeH          = 7. / 25.4
-	flangeD          = 60. / 25.4
-)
+const visualization = "nptflange.glsl"
 
 func main() {
-	flag.BoolVar(&useGPU, "gpu", useGPU, "Enable GPU usage")
-	flag.Parse()
 	if useGPU {
-		fmt.Println("enable GPU usage")
 		_, terminate, err := glgl.InitWithCurrentWindow33(glgl.WindowConfig{
 			Title:   "compute",
 			Version: [2]int{4, 6},
@@ -84,21 +74,33 @@ func main() {
 }
 
 func scene() (gleval.SDF3, error) {
+	const (
+		// visualization is the name of the file with a GLSL
+		// generated visualization of the SDF which can be visualized in https://www.shadertoy.com/
+		// or using VSCode's ShaderToy extension. If visualization=="" then no file is generated.
+		// thread length
+		tlen             = 18. / 25.4
+		internalDiameter = 1.5 / 2.
+		flangeH          = 7. / 25.4
+		flangeD          = 60. / 25.4
+	)
 	var (
 		npt    threads.NPT
 		flange glbuild.Shader3D
+		err    error
 	)
-	npt.SetFromNominal(1.0 / 2.0)
+	err = npt.SetFromNominal(1.0 / 2.0)
+	if err != nil {
+		return nil, err
+	}
 	pipe, err := threads.Nut(threads.NutParms{
 		Thread: npt,
 		Style:  threads.NutCircular,
 	})
 	if err != nil {
-		panic(err)
+		return nil, err
 	}
-
 	flange, err = gsdf.NewCylinder(flangeD/2, flangeH, flangeH/8)
-	return makeSDF(flange)
 	if err != nil {
 		return nil, err
 	}

operations.go

@@ -84,11 +84,9 @@ func (s *diff) AppendShaderName(b []byte) []byte {
 }
 
 func (s *diff) AppendShaderBody(b []byte) []byte {
-	b = append(b, "return max(-"...)
-	b = s.s1.AppendShaderName(b)
-	b = append(b, "(p),"...)
-	b = s.s2.AppendShaderName(b)
-	b = append(b, "(p));"...)
+	b = appendDistanceDecl(b, s.s1, "a", "p")
+	b = appendDistanceDecl(b, s.s2, "b", "p")
+	b = append(b, "return max(a,-b);"...)
 	return b
 }