docs(webxr): prefer VR over AR session in XR documentation

Updates the default XR session type preference in WebXRVolume and GeometryViewer examples to better
reflect our current XR supported devices list. Previously single-screen mobile devices were the only
devices supporting both VR (Cardboard) and AR sessions with AR generally favored, so AR mode was
made the default. Recently vtk.js added support for HMDs compatible with both VR and AR
(passthrough) mode, which require stereoscopic AR rendering different from the mobile AR approach.
Under the status quo the default behavior on such an HMD was to attempt to render in AR mobile as if
it were a mobile device, which would produce incorrect renderings. This change prefers rendering in
VR mode such that default HMD behavior will be rendering in full virtual reality and some mobile
devices may render in Cardboard VR mode by default. AR HMD and AR mobile experiences may still be
accessed through explicit, non-default links in the WebXR documentation landing page.

re #2862

Documentation/content/docs/develop_webxr.md

@@ -72,7 +72,7 @@ The following examples target a head-mounted virtual reality device, such as the
 [![VR Cone Example][VrCone]](../examples/VR.html)
 [![SkyboxViewer Example][SkyboxViewerVR]](../examples/SkyboxViewer.html?fileURL=https://data.kitware.com/api/v1/file/5ae8a89c8d777f0685796bae/download)
 [![GeometryViewer Brain Blood Vessels][GeometryViewerBrainBloodVessels]](../examples/GeometryViewer/GeometryViewer.html?xrSessionType=0&fileURL=[https://kitware.github.io/vtk-js/examples/GeometryViewer/GeometryViewer.html?xrSessionType=0&fileURL=%5Bhttps://data.kitware.com/api/v1/file/61f041f14acac99f42c2ff9a/download,https://data.kitware.com/api/v1/file/61f042024acac99f42c2ffa6/download,https://data.kitware.com/api/v1/file/61f042b74acac99f42c30079/download%5D])
-[![XR Volume Example][WebXRVolume]](../examples/WebXRVolume/WebXRVolume.html?xrSessionType=0&fileURL=https://data.kitware.com/api/v1/file/63fe3f237b0dfcc98f66a857/download&colorPreset=CT-AAA&rotateX=90&rotateY=180)
+[![XR Volume Example][WebXRVolumeHeadAndNeck]](../examples/WebXRVolume/WebXRVolume.html?xrSessionType=0&fileURL=https://data.kitware.com/api/v1/file/63fe3f237b0dfcc98f66a857/download&colorPreset=CT-AAA&rotateX=90&rotateY=180)
 
 </div>
 
@@ -84,7 +84,7 @@ The following examples target a handheld or head-mounted mobile augmented realit
 
 [![AR Cone Example][ArCone]](../examples/AR.html)
 [![GeometryViewer Brain Blood Vessels][GeometryViewerBrainBloodVessels]](../examples/GeometryViewer/GeometryViewer.html?xrSessionType=1&fileURL=[https://kitware.github.io/vtk-js/examples/GeometryViewer/GeometryViewer.html?xrSessionType=0&fileURL=%5Bhttps://data.kitware.com/api/v1/file/61f041f14acac99f42c2ff9a/download,https://data.kitware.com/api/v1/file/61f042024acac99f42c2ffa6/download,https://data.kitware.com/api/v1/file/61f042b74acac99f42c30079/download%5D])
-[![XR Volume Example][WebXRVolume]](../examples/WebXRVolume/WebXRVolume.html?xrSessionType=1&fileURL=https://data.kitware.com/api/v1/file/63fe3f237b0dfcc98f66a857/download&colorPreset=CT-AAA&rotateX=90&rotateY=180)
+[![XR Volume Example][WebXRVolumeHeadAndNeck]](../examples/WebXRVolume/WebXRVolume.html?xrSessionType=1&fileURL=https://data.kitware.com/api/v1/file/63fe3f237b0dfcc98f66a857/download&colorPreset=CT-AAA&rotateX=90&rotateY=180)
 
 </div>
 
@@ -98,7 +98,7 @@ Microsoft HoloLens 2 users should run the [virtual reality examples](#Virtual-Re
 
 [![AR Cone Example][ArCone]](../examples/AR/index.html?xrSessionType=3)
 [![GeometryViewer Brain Blood Vessels][GeometryViewerBrainBloodVessels]](../examples/GeometryViewer/GeometryViewer.html?xrSessionType=3&fileURL=[https://kitware.github.io/vtk-js/examples/GeometryViewer/GeometryViewer.html?xrSessionType=0&fileURL=%5Bhttps://data.kitware.com/api/v1/file/61f041f14acac99f42c2ff9a/download,https://data.kitware.com/api/v1/file/61f042024acac99f42c2ffa6/download,https://data.kitware.com/api/v1/file/61f042b74acac99f42c30079/download%5D])
-[![XR Volume Example][WebXRVolume]](../examples/WebXRVolume/WebXRVolume.html?xrSessionType=3&fileURL=https://data.kitware.com/api/v1/file/63fe3f237b0dfcc98f66a857/download&colorPreset=CT-AAA&rotateX=90&rotateY=180)
+[![XR Volume Example][WebXRVolumeHeadAndNeck]](../examples/WebXRVolume/WebXRVolume.html?xrSessionType=3&fileURL=https://data.kitware.com/api/v1/file/63fe3f237b0dfcc98f66a857/download&colorPreset=CT-AAA&rotateX=90&rotateY=180)
 
 </div>
 
@@ -110,7 +110,7 @@ The following examples target holographic displays from Looking Glass Factory. M
 
 [![Looking Glass Cone Example][LookingGlassCone]](../examples/LookingGlass.html)
 [![GeometryViewer Brain Blood Vessels][GeometryViewerBrainBloodVessels]](../examples/GeometryViewer/GeometryViewer.html?xrSessionType=2&fileURL=[https://kitware.github.io/vtk-js/examples/GeometryViewer/GeometryViewer.html?xrSessionType=0&fileURL=%5Bhttps://data.kitware.com/api/v1/file/61f041f14acac99f42c2ff9a/download,https://data.kitware.com/api/v1/file/61f042024acac99f42c2ffa6/download,https://data.kitware.com/api/v1/file/61f042b74acac99f42c30079/download%5D])
-[![XR Volume Example][WebXRVolume]](../examples/WebXRVolume/WebXRVolume.html?xrSessionType=2&fileURL=https://data.kitware.com/api/v1/file/63fe3f237b0dfcc98f66a857/download&colorPreset=CT-AAA&rotateX=90&rotateY=180)
+[![XR Volume Example][WebXRVolumeHeadAndNeck]](../examples/WebXRVolume/WebXRVolume.html?xrSessionType=2&fileURL=https://data.kitware.com/api/v1/file/63fe3f237b0dfcc98f66a857/download&colorPreset=CT-AAA&rotateX=90&rotateY=180)
 
 </div>
 

Examples/Applications/GeometryViewer/index.js

@@ -101,14 +101,14 @@ if (requestedXrSessionType === XrSessionTypes.LookingGlassVR) {
     // eslint-disable-next-line no-new
     new obj.LookingGlassWebXRPolyfill();
   });
-} else if (requestedXrSessionType === null && navigator.xr !== undefined) {
-  // Determine supported session type
-  navigator.xr.isSessionSupported('immersive-ar').then((arSupported) => {
-    if (arSupported) {
-      requestedXrSessionType = XrSessionTypes.MobileAR;
+} else if (requestedXrSessionType === null) {
+  // Guess the session type based on what XR session(s) the device supports
+  navigator.xr.isSessionSupported('immersive-vr').then((vrSupported) => {
+    if (vrSupported) {
+      requestedXrSessionType = XrSessionTypes.HmdVr;
     } else {
-      navigator.xr.isSessionSupported('immersive-vr').then((vrSupported) => {
-        requestedXrSessionType = vrSupported ? XrSessionTypes.HmdVR : null;
+      navigator.xr.isSessionSupported('immersive-ar').then((arSupported) => {
+        requestedXrSessionType = arSupported ? XrSessionTypes.MobileAR : null;
       });
     }
   });

Examples/Volume/WebXRVolume/index.js

@@ -85,13 +85,13 @@ if (requestedXrSessionType === XrSessionTypes.LookingGlassVR) {
     new obj.LookingGlassWebXRPolyfill();
   });
 } else if (requestedXrSessionType === null) {
-  // Determine supported session type
-  navigator.xr.isSessionSupported('immersive-ar').then((arSupported) => {
-    if (arSupported) {
-      requestedXrSessionType = XrSessionTypes.MobileAR;
+  // Guess the session type based on what XR session(s) the device supports.
+  navigator.xr.isSessionSupported('immersive-vr').then((vrSupported) => {
+    if (vrSupported) {
+      requestedXrSessionType = XrSessionTypes.HmdVr;
     } else {
-      navigator.xr.isSessionSupported('immersive-vr').then((vrSupported) => {
-        requestedXrSessionType = vrSupported ? XrSessionTypes.HmdVR : null;
+      navigator.xr.isSessionSupported('immersive-ar').then((arSupported) => {
+        requestedXrSessionType = arSupported ? XrSessionTypes.MobileAR : null;
       });
     }
   });