Transformer GL: Clarify variables and comments.

Simplifying and clarifying variables, and adding comments.

Tested by confirming demo-gl and demo-transformer both
correctly display videos

PiperOrigin-RevId: 421792079

demos/gl/src/main/java/com/google/android/exoplayer2/gldemo/BitmapOverlayVideoProcessor.java

@@ -86,23 +86,9 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
       throw new IllegalStateException(e);
     }
     program.setBufferAttribute(
-        "a_position",
-        new float[] {
-          -1, -1, 0, 1,
-          1, -1, 0, 1,
-          -1, 1, 0, 1,
-          1, 1, 0, 1
-        },
-        4);
+        "a_position", GlUtil.getNormalizedCoordinateBounds(), GlUtil.RECTANGLE_VERTICES_COUNT);
     program.setBufferAttribute(
-        "a_texcoord",
-        new float[] {
-          0, 0, 0, 1,
-          1, 0, 0, 1,
-          0, 1, 0, 1,
-          1, 1, 0, 1
-        },
-        4);
+        "a_texcoord", GlUtil.getTextureCoordinateBounds(), GlUtil.RECTANGLE_VERTICES_COUNT);
     GLES20.glGenTextures(1, textures, 0);
     GLES20.glBindTexture(GL10.GL_TEXTURE_2D, textures[0]);
     GLES20.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_MIN_FILTER, GL10.GL_NEAREST);

library/common/src/main/java/com/google/android/exoplayer2/util/GlUtil.java

@@ -202,6 +202,9 @@ public final class GlUtil {
   /** Whether to throw a {@link GlException} in case of an OpenGL error. */
   public static boolean glAssertionsEnabled = false;
 
+  /** Number of vertices in a rectangle. */
+  public static final int RECTANGLE_VERTICES_COUNT = 4;
+
   private static final String TAG = "GlUtil";
 
   private static final String EXTENSION_PROTECTED_CONTENT = "EGL_EXT_protected_content";
@@ -210,6 +213,26 @@ public final class GlUtil {
   /** Class only contains static methods. */
   private GlUtil() {}
 
+  /** Bounds of normalized device coordinates, commonly used for defining viewport boundaries. */
+  public static float[] getNormalizedCoordinateBounds() {
+    return new float[] {
+      -1, -1, 0, 1,
+      1, -1, 0, 1,
+      -1, 1, 0, 1,
+      1, 1, 0, 1
+    };
+  }
+
+  /** Typical bounds used for sampling from textures. */
+  public static float[] getTextureCoordinateBounds() {
+    return new float[] {
+      0, 0, 0, 1,
+      1, 0, 0, 1,
+      0, 1, 0, 1,
+      1, 1, 0, 1
+    };
+  }
+
   /**
    * Returns whether creating a GL context with {@value #EXTENSION_PROTECTED_CONTENT} is possible.
    * If {@code true}, the device supports a protected output path for DRM content when using GL.

library/transformer/src/main/assets/shaders/vertex_shader.glsl

@@ -11,12 +11,12 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-attribute vec4 aPosition;
+attribute vec4 aFramePosition;
 attribute vec4 aTexCoords;
 uniform mat4 uTexTransform;
 uniform mat4 uTransformationMatrix;
 varying vec2 vTexCoords;
 void main() {
-  gl_Position = uTransformationMatrix * aPosition;
+  gl_Position = uTransformationMatrix * aFramePosition;
   vTexCoords = (uTexTransform * aTexCoords).xy;
 }

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/FrameEditor.java

@@ -131,24 +131,11 @@ import java.util.concurrent.atomic.AtomicInteger;
     GlUtil.Program glProgram =
         new GlUtil.Program(context, VERTEX_SHADER_FILE_PATH, FRAGMENT_SHADER_FILE_PATH);
 
+    // Draw the frame on the entire normalized device coordinate space, from -1 to 1, for x and y.
     glProgram.setBufferAttribute(
-        "aPosition",
-        new float[] {
-          -1.0f, -1.0f, 0.0f, 1.0f,
-          1.0f, -1.0f, 0.0f, 1.0f,
-          -1.0f, 1.0f, 0.0f, 1.0f,
-          1.0f, 1.0f, 0.0f, 1.0f,
-        },
-        /* size= */ 4);
+        "aFramePosition", GlUtil.getNormalizedCoordinateBounds(), GlUtil.RECTANGLE_VERTICES_COUNT);
     glProgram.setBufferAttribute(
-        "aTexCoords",
-        new float[] {
-          0.0f, 0.0f, 0.0f, 1.0f,
-          1.0f, 0.0f, 0.0f, 1.0f,
-          0.0f, 1.0f, 0.0f, 1.0f,
-          1.0f, 1.0f, 0.0f, 1.0f,
-        },
-        /* size= */ 4);
+        "aTexCoords", GlUtil.getTextureCoordinateBounds(), GlUtil.RECTANGLE_VERTICES_COUNT);
     glProgram.setSamplerTexIdUniform("uTexSampler", textureId, /* unit= */ 0);
 
     float[] transformationMatrixArray = getGlMatrixArray(transformationMatrix);

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/TransformationRequest.java

@@ -61,6 +61,9 @@ public final class TransformationRequest {
      * <p>This can be used to perform operations supported by {@link Matrix}, like scaling and
      * rotating the video.
      *
+     * <p>The video dimensions will be on the x axis, from -aspectRatio to aspectRatio, and on the y
+     * axis, from -1 to 1.
+     *
      * <p>For now, resolution will not be affected by this method.
      *
      * @param transformationMatrix The transformation to apply to video frames.
@@ -71,6 +74,10 @@ public final class TransformationRequest {
       // allow transformations to change the resolution, by scaling to the appropriate min/max
       // values. This will also be required to create the VertexTransformation class, in order to
       // have aspect ratio helper methods (which require resolution to change).
+
+      // TODO(b/213198690): Consider changing how transformationMatrix is applied, so that
+      // dimensions will be from -1 to 1 on both x and y axes, but transformations will be applied
+      // in a predictable manner.
       this.transformationMatrix = transformationMatrix;
       return this;
     }

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/VideoSamplePipeline.java

@@ -63,12 +63,12 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
     // Scale width and height to desired transformationRequest.outputHeight, preserving aspect
     // ratio.
     // TODO(internal b/209781577): Think about which edge length should be set for portrait videos.
-    float inputAspectRatio = (float) inputFormat.width / inputFormat.height;
+    float inputFormatAspectRatio = (float) inputFormat.width / inputFormat.height;
     int outputWidth = inputFormat.width;
     int outputHeight = inputFormat.height;
     if (transformationRequest.outputHeight != C.LENGTH_UNSET
         && transformationRequest.outputHeight != inputFormat.height) {
-      outputWidth = Math.round(inputAspectRatio * transformationRequest.outputHeight);
+      outputWidth = Math.round(inputFormatAspectRatio * transformationRequest.outputHeight);
       outputHeight = transformationRequest.outputHeight;
     }
 
@@ -82,14 +82,17 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
     } else {
       outputRotationDegrees = inputFormat.rotationDegrees;
     }
-    if ((inputFormat.rotationDegrees % 180) != 0) {
-      inputAspectRatio = 1.0f / inputAspectRatio;
-    }
+    float displayAspectRatio =
+        (inputFormat.rotationDegrees % 180) == 0
+            ? inputFormatAspectRatio
+            : 1.0f / inputFormatAspectRatio;
 
-    // Scale frames by input aspect ratio, to account for FrameEditor normalized device coordinates
-    // (-1 to 1) and preserve frame relative dimensions during transformations (ex. rotations).
-    transformationRequest.transformationMatrix.preScale(inputAspectRatio, 1);
-    transformationRequest.transformationMatrix.postScale(1.0f / inputAspectRatio, 1);
+    // Scale frames by input aspect ratio, to account for FrameEditor's square normalized device
+    // coordinates (-1 to 1) and preserve frame relative dimensions during transformations
+    // (ex. rotations). After this scaling, transformationMatrix operations operate on a rectangle
+    // for x from -displayAspectRatio to displayAspectRatio, and y from -1 to 1
+    transformationRequest.transformationMatrix.preScale(displayAspectRatio, 1);
+    transformationRequest.transformationMatrix.postScale(1.0f / displayAspectRatio, 1);
 
     // The decoder rotates videos to their intended display orientation. The frameEditor rotates
     // them back for improved encoder compatibility.