fix: Fix cast support for HLG HDR (#7632)

Also document what is recognized, how, and why those strings are used.

.eslintrc.js

@@ -84,6 +84,7 @@ module.exports = {
     'no-empty': ['error', {'allowEmptyCatch': true}],
     'no-misleading-character-class': 'error',
     'no-template-curly-in-string': 'error',
+    'no-fallthrough': ['error', {'allowEmptyCase': true}],
     // TODO: Try to re-enable this if possible.  Right now, it produces way too
     // many false-positives with eslint 7.  It worked well enough in eslint 5.
     // 'require-atomic-updates': 'error',

lib/polyfill/media_capabilities.js

@@ -333,12 +333,43 @@ shaka.polyfill.MediaCapabilities = class {
     if (videoConfig.framerate) {
       displayType += `; framerate=${videoConfig.framerate}`;
     }
-    if (videoConfig.transferFunction === 'pq') {
-      // A "PQ" transfer function indicates this is an HDR-capable stream;
-      // "smpte2084" is the published standard. We need to inform the platform
-      // this query is specifically for HDR.
-      displayType += '; eotf=smpte2084';
+
+    // Don't trust Closure types here.  Although transferFunction is string or
+    // undefined, we don't want to count on the input type.  A switch statement
+    // will, however, differentiate between null and undefined.  So we default
+    // to a blank string.
+    const transferFunction = videoConfig.transferFunction || '';
+
+    // Based on internal sources.  Googlers, see go/cast-hdr-queries for source.
+    switch (transferFunction) {
+      // The empty case falls through to SDR.
+      case '':
+      // These are the only 3 values defined by MCap as of November 2024.
+      case 'srgb':
+        // https://en.wikipedia.org/wiki/Standard-dynamic-range_video
+        // https://en.wikipedia.org/wiki/SRGB
+        // https://en.wikipedia.org/wiki/Rec._709
+        // This is SDR, standardized in BT 709.
+        // The platform recognizes "eotf=bt709", but we can also omit it.
+        break;
+
+      case 'pq':
+        // https://en.wikipedia.org/wiki/Perceptual_quantizer
+        // This HDR transfer function is standardized as SMPTE ST 2084.
+        displayType += '; eotf=smpte2084';
+        break;
+
+      case 'hlg':
+        // https://en.wikipedia.org/wiki/Hybrid_log%E2%80%93gamma
+        // This HDR transfer function is standardized as ARIB STD-B67.
+        displayType += '; eotf=arib-std-b67';
+        break;
+
+      default:
+        // An unrecognized transfer function.  Reject this query.
+        return false;
     }
+
     let result = false;
     if (displayType in shaka.polyfill.MediaCapabilities
         .memoizedCanDisplayTypeRequests_) {