Capture full YUV planes and add YUV420->ARGB converter

- VideoDecoder.Frame and PlaybackEngine.VideoFrame now carry U and V
  chroma planes plus their strides, in addition to luma.
- YuvConverter converts YUVJ420P frames to packed 0xAARRGGBB pixels for
  the renderer, with unit tests.

core/src/main/java/dev/quoije/umd/media/VideoDecoder.java

@@ -95,16 +95,33 @@ public final class VideoDecoder implements AutoCloseable {
 
         int width = frame.width();
         int height = frame.height();
-        BytePointer y = frame.data(0);
+        int chromaWidth = (width + 1) / 2;
-        int stride = frame.linesize(0);
+        int chromaHeight = (height + 1) / 2;
-        byte[] yPlane = new byte[height * stride];
-        y.position(0).get(yPlane);
 
-        return new Frame(width, height, frame.format(), yPlane, stride);
+        BytePointer y = frame.data(0);
+        BytePointer u = frame.data(1);
+        BytePointer v = frame.data(2);
+
+        int yStride = frame.linesize(0);
+        int uStride = frame.linesize(1);
+        int vStride = frame.linesize(2);
+
+        byte[] yPlane = new byte[height * yStride];
+        byte[] uPlane = new byte[chromaHeight * uStride];
+        byte[] vPlane = new byte[chromaHeight * vStride];
+
+        y.position(0).get(yPlane);
+        u.position(0).get(uPlane);
+        v.position(0).get(vPlane);
+
+        return new Frame(width, height, frame.format(), yPlane, yStride, uPlane, uStride, vPlane, vStride);
     }
 
-    /** A decoded video frame. */
+    /** A decoded video frame (planar YUV 4:2:0). */
-    public record Frame(int width, int height, int pixelFormat, byte[] yPlane, int yStride) {
+    public record Frame(int width, int height, int pixelFormat,
+                        byte[] yPlane, int yStride,
+                        byte[] uPlane, int uStride,
+                        byte[] vPlane, int vStride) {
     }
 
     /** Returns the next decoded frame, or {@code null} if none is buffered. */

core/src/main/java/dev/quoije/umd/media/YuvConverter.java

@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+package dev.quoije.umd.media;
+
+/**
+ * Converts decoded video frames (planar YUV 4:2:0, full range as used by
+ * JPEG/YUVJ420P) into packed 0xAARRGGBB pixels.
+ */
+public final class YuvConverter {
+
+    private YuvConverter() {
+    }
+
+    /**
+     * Converts a YUVJ420P frame to ARGB. The returned array has
+     * {@code width * height} packed pixels.
+     */
+    public static int[] yuvj420ToArgb(int width, int height,
+                                      byte[] yPlane, int yStride,
+                                      byte[] uPlane, int uStride,
+                                      byte[] vPlane, int vStride) {
+        int[] argb = new int[width * height];
+
+        for (int row = 0; row < height; row++) {
+            int yRow = row * yStride;
+            int uvRow = (row >> 1) * uStride;
+            int outRow = row * width;
+
+            for (int col = 0; col < width; col++) {
+                int y = yPlane[yRow + col] & 0xFF;
+                int u = uPlane[uvRow + (col >> 1)] & 0xFF;
+                int v = vPlane[uvRow + (col >> 1)] & 0xFF;
+
+                int du = u - 128;
+                int dv = v - 128;
+
+                int r = y + ((359 * dv) >> 8);
+                int g = y - ((88 * du) >> 8) - ((183 * dv) >> 8);
+                int b = y + ((454 * du) >> 8);
+
+                argb[outRow + col] = 0xFF000000
+                        | (clamp(r) << 16)
+                        | (clamp(g) << 8)
+                        | clamp(b);
+            }
+        }
+
+        return argb;
+    }
+
+    private static int clamp(int value) {
+        if (value < 0) {
+            return 0;
+        }
+        return Math.min(value, 255);
+    }
+}

core/src/main/java/dev/quoije/umd/player/PlaybackEngine.java

@@ -31,7 +31,10 @@ public final class PlaybackEngine implements AutoCloseable {
     public static final long AUDIO_FRAME_TICKS = (long) AUDIO_FRAME_SAMPLES * MPEG_TIMESTAMP / AUDIO_SAMPLE_RATE;
 
     /** A decoded video frame tagged with its presentation timestamp. */
-    public record VideoFrame(long pts, int width, int height, int pixelFormat, byte[] yPlane, int yStride) {
+    public record VideoFrame(long pts, int width, int height, int pixelFormat,
+                             byte[] yPlane, int yStride,
+                             byte[] uPlane, int uStride,
+                             byte[] vPlane, int vStride) {
     }
 
     /** A chunk of interleaved S16 PCM tagged with its presentation timestamp. */
@@ -136,7 +139,8 @@ public final class PlaybackEngine implements AutoCloseable {
             while ((frame = videoDecoder.receive()) != null) {
                 videoQueue.add(new VideoFrame(
                         assignVideoPts(), frame.width(), frame.height(),
-                        frame.pixelFormat(), frame.yPlane(), frame.yStride()));
+                        frame.pixelFormat(), frame.yPlane(), frame.yStride(),
+                        frame.uPlane(), frame.uStride(), frame.vPlane(), frame.vStride()));
             }
         }
     }
@@ -202,7 +206,8 @@ public final class PlaybackEngine implements AutoCloseable {
         while ((frame = videoDecoder.receive()) != null) {
             videoQueue.add(new VideoFrame(
                     assignVideoPts(), frame.width(), frame.height(),
-                    frame.pixelFormat(), frame.yPlane(), frame.yStride()));
+                    frame.pixelFormat(), frame.yPlane(), frame.yStride(),
+                    frame.uPlane(), frame.uStride(), frame.vPlane(), frame.vStride()));
         }
 
         if (audioDecoder != null) {

core/src/test/java/dev/quoije/umd/media/YuvConverterTest.java

@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+package dev.quoije.umd.media;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class YuvConverterTest {
+
+    @Test
+    void convertsKnownColours() {
+        // 2x2 frame; Y=255 everywhere (white), U=V=128 (no chroma) -> white.
+        byte[] y = new byte[4];
+        byte[] u = new byte[]{(byte) 128};
+        byte[] v = new byte[]{(byte) 128};
+        java.util.Arrays.fill(y, (byte) 255);
+
+        int[] argb = YuvConverter.yuvj420ToArgb(2, 2, y, 2, u, 1, v, 1);
+        assertThat(argb).containsExactly(0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF);
+
+        // Y=0, U=V=128 -> black.
+        java.util.Arrays.fill(y, (byte) 0);
+        argb = YuvConverter.yuvj420ToArgb(2, 2, y, 2, u, 1, v, 1);
+        assertThat(argb).containsExactly(0xFF000000, 0xFF000000, 0xFF000000, 0xFF000000);
+    }
+
+    @Test
+    void respectsChroma() {
+        // Y=0, U=128, V=255 -> strong red (V - 128 = 127 => r = 0 + 1.402*127 ~ 178).
+        byte[] y = new byte[]{0, 0, 0, 0};
+        byte[] u = new byte[]{-128}; // 128
+        byte[] v = new byte[]{-1};   // 255
+
+        int[] argb = YuvConverter.yuvj420ToArgb(2, 2, y, 2, u, 1, v, 1);
+        int r = (argb[0] >> 16) & 0xFF;
+        int g = (argb[0] >> 8) & 0xFF;
+        int b = argb[0] & 0xFF;
+        assertThat(r).isGreaterThan(150);
+        assertThat(b).isEqualTo(0);
+        assertThat(g).isLessThanOrEqualTo(0);
+    }
+}