feature: added support for Q < 128 to RTP-JPEG

For Q values less than 128, the luma and chroma
quantization tables are computed from the Q value.

lib/components/jpegdepay/make-qtable.ts

@@ -0,0 +1,50 @@
+import { clamp } from '../../utils/clamp'
+/**
+ * @function makeQtable
+ * Creating a quantization table from a Q factor
+ * Example Code from RFC 2435 Appendix A ported to TypeScript
+ *
+ * Default luminance/chrominance quantization tables in RFC example are not in zig-zag order.
+ * The RFC does mention tables should be in zig-zag ordering, but they say that about the included tables.
+ * RFC sample code appears to have a mistake.
+ * All the tested cameras and LGPL projects use zig-zag default tables.
+ * So we use zig-zaged tables from ISO/IEC 10918-1 Annex K Section K.1
+ * @see https://tools.ietf.org/html/rfc2435
+ * @see https://www.iso.org/standard/18902.html
+ */
+// prettier-ignore
+const jpegLumaQuantizer = [
+  16,  11,  12,  14,  12,  10,  16,  14,
+  13,  14,  18,  17,  16,  19,  24,  40,
+  26,  24,  22,  22,  24,  49,  35,  37,
+  29,  40,  58,  51,  61,  60,  57,  51,
+  56,  55,  64,  72,  92,  78,  64,  68,
+  87,  69,  55,  56,  80,  109, 81,  87,
+  95,  98,  103, 104, 103, 62,  77,  113,
+  121, 112, 100, 120, 92,  101, 103, 99,
+]
+// prettier-ignore
+const jpeChromaQuantizer = [
+  17,  18,  18,  24,  21,  24,  47,  26,
+  26,  47,  99,  66,  56,  66,  99,  99,
+  99,  99,  99,  99,  99,  99,  99,  99,
+  99,  99,  99,  99,  99,  99,  99,  99,
+  99,  99,  99,  99,  99,  99,  99,  99,
+  99,  99,  99,  99,  99,  99,  99,  99,
+  99,  99,  99,  99,  99,  99,  99,  99,
+  99,  99,  99,  99,  99,  99,  99,  99
+]
+
+export function makeQtable(Q: number): Buffer {
+  const factor = clamp(Q, 1, 99)
+  const buffer = Buffer.alloc(128)
+  const S = Q < 50 ? Math.floor(5000 / factor) : 200 - factor * 2
+
+  for (let i = 0; i < 64; i++) {
+    const lq = Math.floor((jpegLumaQuantizer[i] * S + 50) / 100)
+    const cq = Math.floor((jpeChromaQuantizer[i] * S + 50) / 100)
+    buffer.writeUInt8(clamp(lq, 1, 255), i)
+    buffer.writeUInt8(clamp(cq, 1, 255), i + 64)
+  }
+  return buffer
+}

lib/components/jpegdepay/parser.ts

@@ -6,6 +6,7 @@ import {
   makeFrameHeader,
 } from './headers'
 import { payload } from '../../utils/protocols/rtp'
+import { makeQtable } from './make-qtable'
 
 /**
  * Each packet contains a special JPEG header which immediately follows
@@ -88,6 +89,20 @@ export function jpegDepayFactory(defaultWidth = 0, defaultHeight = 0) {
         }
         fragment = fragment.slice(4 + length)
       }
+      // Compute Quantization Table
+      else if (Q < 128 && fragmentOffset === 0) {
+        const precision = 0
+        const qTable = makeQtable(Q)
+        metadata = {
+          typeSpecific,
+          type,
+          width,
+          height,
+          DRI,
+          precision,
+          qTable,
+        }
+      }
 
       fragments.push(fragment)
     }

lib/utils/clamp.ts

@@ -0,0 +1,3 @@
+export function clamp(val: number, min: number, max: number): number {
+  return val > max ? max : val < min ? min : val
+}