Fix WebXR and WebVR viewport scaling on desktop

compositor_base was confusing top/bottom of image with DX vs. GL
coordinates.

Mirroring UVs were making a similar error, measuring from top instead of
bottom.

This change makes WebVR and WebXR behave similarly on desktop and
Android.

Bug: 949594
Change-Id: Ie4ead47d6582fcd3578d037a30380ec1f2e9f44d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1566424Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#650555}

device/vr/windows/compositor_base.cc

@@ -168,6 +168,13 @@ void XRCompositorCommon::UpdateLayerBounds(int16_t frame_id,
   // merge with vr_shell_gl eventually.
   left_webxr_bounds_ = left_bounds;
   right_webxr_bounds_ = right_bounds;
+
+  // Swap top/bottom to account for differences between D3D and GL coordinates.
+  left_webxr_bounds_.set_y(
+      1 - (left_webxr_bounds_.y() + left_webxr_bounds_.height()));
+  right_webxr_bounds_.set_y(
+      1 - (right_webxr_bounds_.y() + right_webxr_bounds_.height()));
+
   source_size_ = source_size;
 
   OnLayerBoundsChanged();

third_party/blink/renderer/modules/xr/xr_webgl_layer.cc

@@ -227,12 +227,10 @@ void XRWebGLLayer::UpdateViewports() {
     // When mirroring make sure to also update the mirrored canvas UVs so it
     // only shows a single eye's data, cropped to display proportionally.
     if (session()->outputContext()) {
-      float left = 0;
-      float top = 0;
-      float right = static_cast<float>(left_viewport_->width()) /
-                    static_cast<float>(framebuffer_width);
-      float bottom = static_cast<float>(left_viewport_->height()) /
-                     static_cast<float>(framebuffer_height);
+      float source_pixels_left = left_viewport_->x();
+      float source_pixels_right = left_viewport_->x() + left_viewport_->width();
+      float source_pixels_bottom = left_viewport_->y();
+      float source_pixels_top = left_viewport_->y() + left_viewport_->height();
 
       // Adjust the UVs so that the mirrored content always fills the canvas
       // and is centered while staying proportional.
@@ -242,23 +240,32 @@ void XRWebGLLayer::UpdateViewports() {
                                static_cast<float>(left_viewport_->height());
 
       if (output_aspect > viewport_aspect) {
-        float viewport_scale = bottom;
-        output_aspect = viewport_aspect / output_aspect;
-        top = 0.5 - (output_aspect * 0.5);
-        bottom = top + output_aspect;
-        top *= viewport_scale;
-        bottom *= viewport_scale;
+        // Output is wider than rendered image, scale to height and chop off top
+        // and bottom.
+        float cropped_image_height = left_viewport_->width() / output_aspect;
+        float crop_amount = (left_viewport_->height() - cropped_image_height);
+        source_pixels_top -= crop_amount / 2;
+        source_pixels_bottom += crop_amount / 2;
+
       } else {
-        float viewport_scale = right;
-        output_aspect = output_aspect / viewport_aspect;
-        left = 0.5 - (output_aspect * 0.5);
-        right = left + output_aspect;
-        left *= viewport_scale;
-        right *= viewport_scale;
+        // Output is taller relatively than rendered image, scale to width and
+        // chop of left and right.
+        float cropped_image_width = left_viewport_->height() * output_aspect;
+        float crop_amount = (left_viewport_->width() - cropped_image_width);
+        source_pixels_left += crop_amount / 2;
+        source_pixels_right -= crop_amount / 2;
       }
 
-      session()->outputContext()->SetUV(FloatPoint(left, top),
-                                        FloatPoint(right, bottom));
+      float uv_left = source_pixels_left / framebuffer_width;
+      float uv_right = source_pixels_right / framebuffer_width;
+      float uv_top = source_pixels_top / framebuffer_height;
+      float uv_bottom = source_pixels_bottom / framebuffer_height;
+
+      // Finally, in UV space (0, 0) is top-left corner, so we need to flip.
+      uv_top = 1 - uv_top;
+      uv_bottom = 1 - uv_bottom;
+      session()->outputContext()->SetUV(FloatPoint(uv_left, uv_top),
+                                        FloatPoint(uv_right, uv_bottom));
     }
   } else {
     left_viewport_ = MakeGarbageCollected<XRViewport>(