Android: Add support for cropping textures

webrtc/api/java/jni/native_handle_impl.cc

Index: webrtc/api/java/jni/native_handle_impl.cc
diff --git a/webrtc/api/java/jni/native_handle_impl.cc b/webrtc/api/java/jni/native_handle_impl.cc
index 1990828fa03a9b8c00f731a0c59fc64bfb920f5b..6f4e30768d60140aecf1d15af79902eee253c3d3 100644
--- a/webrtc/api/java/jni/native_handle_impl.cc
+++ b/webrtc/api/java/jni/native_handle_impl.cc
@@ -59,6 +59,37 @@ void RotateMatrix(float a[16], webrtc::VideoRotation rotation) {
   }
 }
 
+// Calculates result = a * b, in column-major order.
+void MultiplyMatrix(const float a[16], const float b[16], float result[16]) {
+  for (int i = 0; i < 4; ++i) {
+    for (int j = 0; j < 4; ++j) {
+      float sum = 0;
+      for (int k = 0; k < 4; ++k) {
+        sum += a[k * 4 + j] * b[i * 4 + k];
+      }
+      result[i * 4 + j] = sum;
+    }
+  }
+}
+
+// Center crop by keeping xFraction of the width and yFraction of the height,
+// so e.g. cropping from 640x480 to 640x360 would use
+// xFraction=1, yFraction=360/480.
+void CropMatrix(float a[16], float xFraction, float yFraction) {
+  // Move cropped area to the center of the frame by offsetting half the
+  // removed area.
+  const float xOffset = (1 - xFraction) / 2;
+  const float yOffset = (1 - yFraction) / 2;
+  const float crop_matrix[16] = {
+    xFraction, 0, 0, 0,
+    0, yFraction, 0, 0,
+    0, 0, 1, 0,
+    xOffset, yOffset, 0, 1};
+  float mul_result[16];
+  MultiplyMatrix(crop_matrix, a, mul_result);
+  memcpy(a, mul_result, sizeof(mul_result));

[nisse-webrtc, 2016/05/12 10:32:51]

It would be prettier if MultiplyMatrix allowed in-place operation. But overkill
to design great matrix handling code just for this.

Is there no useful matrix things in the c++ std library? Something like
template<type element_t, size_t columns, size_t rows, ptrdiff_t row_stride,
ptrdiff_t column_stride>?

[magjed_webrtc, 2016/05/12 10:49:51]

There is no matrix support in std, but there exists a lot of external libraries
like Eigen. We could reduce the 4x4 matrices to 2x3 instead if we are concerned
about overhead.

+}
+
 }  // anonymouse namespace
 
 namespace webrtc_jni {
@@ -146,15 +177,18 @@ AndroidTextureBuffer::NativeToI420Buffer() {
 }
 
 rtc::scoped_refptr<AndroidTextureBuffer>
-AndroidTextureBuffer::ScaleAndRotate(int dst_widht,
-                                     int dst_height,
-                                     webrtc::VideoRotation rotation) {
-  if (width() == dst_widht && height() == dst_height &&
+AndroidTextureBuffer::CropScaleAndRotate(int cropped_width,
+                                         int cropped_height,
+                                         int dst_width,
+                                         int dst_height,
+                                         webrtc::VideoRotation rotation) {
+  if (cropped_width == dst_width && cropped_height == dst_height &&
+      width() == dst_width && height() == dst_height &&
       rotation == webrtc::kVideoRotation_0) {
     return this;
   }
-  int rotated_width = (rotation % 180 == 0) ? dst_widht : dst_height;
-  int rotated_height = (rotation % 180 == 0) ? dst_height : dst_widht;
+  int rotated_width = (rotation % 180 == 0) ? dst_width : dst_height;
+  int rotated_height = (rotation % 180 == 0) ? dst_height : dst_width;
 
   // Here we use Bind magic to add a reference count to |this| until the newly
   // created AndroidTextureBuffer is destructed
@@ -163,6 +197,9 @@ AndroidTextureBuffer::ScaleAndRotate(int dst_widht,
           rotated_width, rotated_height, native_handle_,
           surface_texture_helper_, rtc::KeepRefUntilDone(this)));
 
+  CropMatrix(buffer->native_handle_.sampling_matrix,

[AlexG, 2016/05/12 23:12:44]

Call this only if (cropped_width != dst_width || cropped_height != dst_height) ?

[magjed_webrtc, 2016/05/13 07:59:58]

Done.

+             cropped_width / static_cast<float>(width()),
+             cropped_height / static_cast<float>(height()));
   RotateMatrix(buffer->native_handle_.sampling_matrix, rotation);
   return buffer;
 }