Verify the QP ceiling constrains real HEVC output under bitrate pressure

tools/chatgui/tests/test_stream_publish.cpp

@@ -309,6 +309,41 @@ private slots:
         qp.reset(24, 4000000, true, 10000000);
         QCOMPARE(qp.update(1000, 11000000), 24); QCOMPARE(qp.bitrate(), uint64_t(0));
     }
+    void maximumQpConstrainsEncodedHevc() {
+#ifdef HAVE_FFMPEG
+        auto previousDebug = g_debug_config.category_levels[DEBUG_CATEGORY_DEBUG];
+        auto cleanup = qScopeGuard([&] {
+            debug_set_log_hook(nullptr); debug_set_category_level(DEBUG_CATEGORY_DEBUG, previousDebug);
+            ffmpegDiagnosticMessages.clear();
+        });
+        debug_set_log_hook(captureFfmpegDiagnostic); debug_set_category_level(DEBUG_CATEGORY_DEBUG, DEBUG_LEVEL_TRACE);
+        QImage image(320, 192, QImage::Format_RGB32);
+        uint32_t random = 1;
+        for (int y = 0; y < image.height(); ++y) {
+            auto* row = reinterpret_cast<uint32_t*>(image.scanLine(y));
+            for (int x = 0; x < image.width(); ++x) { random = random * 1664525 + 1013904223; row[x] = 0xff000000 | (random & 0xffffff); }
+        }
+        int previousQp = -1, previousBytes = 0;
+        for (int ceiling : {18, 35}) {
+            LiveVideoConfig config; config.codec = STREAM_MEDIA_H265; config.width = 320; config.height = 192;
+            config.bitrate = 64000; config.maxQp = ceiling;
+            LiveVideoEncoder encoder;
+            QVERIFY2(encoder.open(config, LiveVideoEncoder::Backend::SoftwareOnly), qPrintable(encoder.error()));
+            ffmpegDiagnosticMessages.clear(); QList<LiveEncodedFrame> output;
+            QVERIFY(encoder.encode(image, 100000, true, output)); QCOMPARE(output.size(), 1); QVERIFY(output.front().key);
+            int initial = 0, actual = -1;
+            for (const auto& line : ffmpegDiagnosticMessages) {
+                if (line.contains("init_qp_minus26")) initial = line.section('=', -1).trimmed().toInt();
+                if (line.contains("slice_qp_delta")) actual = 26 + initial + line.section('=', -1).trimmed().toInt();
+            }
+            QVERIFY(actual >= 0); QVERIFY2(actual <= ceiling, qPrintable(QStringLiteral("QP %1 > %2").arg(actual).arg(ceiling)));
+            if (previousQp >= 0) { QVERIFY(actual > previousQp); QVERIFY(output.front().data.size() < previousBytes); }
+            previousQp = actual; previousBytes = output.front().data.size();
+        }
+#else
+        QSKIP("FFmpeg unavailable");
+#endif
+    }
     void proportionalScreenModes_data() {
         QTest::addColumn<QSize>("source"); QTest::addColumn<QSize>("maximum"); QTest::addColumn<QSize>("reduced");
         QTest::newRow("full-hd") << QSize(1920, 1080) << QSize(1920, 1080) << QSize(1280, 720);