A still car screen gets a fresh frame every second

A surface encoder only makes a frame when the screen changes, and the
repeat-last-frame setting should cover a still one. On a Pixel 10 it
didn't: a LuChi box got 11 frames, then nothing, and stayed black
(#58). When the encoder has been quiet for a second, Gearslip now
draws its screen again, which is a new frame for the car.

gearslip/src/main/java/app/seb3thehacker/gearslip/GearslipActivity.kt

@@ -526,6 +526,10 @@ class GearslipActivity : ComponentActivity(), Projection {
         }
     }
 
+    override fun onStill() {
+        screenProjector?.redraw()
+    }
+
     override fun onProjectionStopped() {
         // Before the car UI goes, while the media connection is still up.
         app.seb3thehacker.gearslip.car.CarServices.pausePhoneMedia()

gearslip/src/main/java/app/seb3thehacker/gearslip/GearslipRunner.kt

@@ -1189,6 +1189,7 @@ class GearslipRunner(
             onFrame = { data, presentationTimeUs, keyFrame ->
                 sendVideoFrame(data, presentationTimeUs, keyFrame)
             },
+            onStill = { projection?.onStill() },
         ).also { source ->
             runCatching {
                 source.start()

gearslip/src/main/java/app/seb3thehacker/gearslip/Projection.kt

@@ -24,6 +24,12 @@ interface Projection {
     fun onTouch(action: Int, actionIndex: Int, points: List<TouchPoint>)
 
     fun onProjectionStopped()
+
+    /**
+     * The screen has been still for a moment and the encoder has sent nothing since. Draw it once
+     * more, so the car gets a fresh frame (see [VideoSource]'s onStill).
+     */
+    fun onStill() {}
 }
 
 /** One finger on the car's screen: a stable [id] for as long as it stays down, and where it is. */

gearslip/src/main/java/app/seb3thehacker/gearslip/ScreenProjector.kt

@@ -223,6 +223,11 @@ class ScreenProjector(private val context: Context) {
      * Points the display at another surface, or none, without restarting what it shows. The
      * preview's SurfaceView comes and goes as the phone turns; the car UI on it carries on.
      */
+    /** Draws the screen again as it is, which hands the display's consumer a new frame. */
+    fun redraw() {
+        root?.let { it.post { it.invalidate() } }
+    }
+
     fun setSurface(surface: Surface?) {
         virtualDisplay?.surface = surface
     }

gearslip/src/main/java/app/seb3thehacker/gearslip/VideoSource.kt

@@ -27,6 +27,12 @@ class VideoSource(
     private val wireless: Boolean = false,
     private val onCodecConfig: (ByteArray) -> Unit,
     private val onFrame: (data: ByteArray, presentationTimeUs: Long, keyFrame: Boolean) -> Unit,
+    /**
+     * SURFACE mode, nothing encoded for [STILL_MS]. The repeat setting below should cover a still
+     * screen, but on a Pixel 10 it didn't: a LuChi box got 11 frames, then none, and stayed black
+     * (issue #58). Whoever draws the screen can draw it again, and that is a frame.
+     */
+    private val onStill: () -> Unit = {},
 ) {
     private val log = GearslipLog.tagged("VIDEO")
 
@@ -170,6 +176,8 @@ class VideoSource(
     private fun pump(encoder: MediaCodec) {
         val info = MediaCodec.BufferInfo()
         val frameIntervalUs = 1_000_000L / frameRate
+        var lastOutputAt = 0L
+        var lastNudgeAt = 0L
 
         while (running) {
             // SURFACE mode has no input buffers to fill - frames arrive from the display.
@@ -208,8 +216,16 @@ class VideoSource(
                     }
                 }
                 encoder.releaseOutputBuffer(outIndex, false)
+                lastOutputAt = System.currentTimeMillis()
                 outIndex = encoder.dequeueOutputBuffer(info, 0)
             }
+            if (mode == Mode.SURFACE && lastOutputAt > 0) {
+                val now = System.currentTimeMillis()
+                if (now - lastOutputAt > STILL_MS && now - lastNudgeAt > STILL_MS) {
+                    lastNudgeAt = now
+                    onStill()
+                }
+            }
 
             if (mode != Mode.SURFACE) Thread.sleep(maxOf(1L, frameIntervalUs / 2000))
         }
@@ -256,6 +272,7 @@ class VideoSource(
         const val MIME = MediaFormat.MIMETYPE_VIDEO_AVC
         const val TIMEOUT_US = 10_000L
         const val SURFACE_TIMEOUT_US = 100_000L
+        private const val STILL_MS = 1_000L
 
         /**
          * 800x480 at 30fps gets 8 Mbps: about the most that size can show, enough that a moving