diff --git a/aom/aom_image.h b/aom/aom_image.h
index 03bc73e..9ec3225 100644
--- a/aom/aom_image.h
+++ b/aom/aom_image.h
@@ -300,7 +300,8 @@
 /*!\brief Set the rectangle identifying the displayed portion of the image
  *
  * Updates the displayed rectangle (aka viewport) on the image surface to
- * match the specified coordinates and size.
+ * match the specified coordinates and size. Specifically, sets img->d_w,
+ * img->d_h, and elements of the img->planes[] array.
  *
  * \param[in]    img       Image descriptor
  * \param[in]    x         leftmost column
@@ -309,7 +310,7 @@
  * \param[in]    h         height
  * \param[in]    border    A border that is padded on four sides of the image.
  *
- * \return 0 if the requested rectangle is valid, nonzero otherwise.
+ * \return 0 if the requested rectangle is valid, nonzero (-1) otherwise.
  */
 int aom_img_set_rect(aom_image_t *img, unsigned int x, unsigned int y,
                      unsigned int w, unsigned int h, unsigned int border);