Explain how CPDF_Page::GetDisplayMatrix() inverts y.

Change-Id: Ia6e9fe790065d0bfd288d425c10d1dd9e0cf8c31
Reviewed-on: https://pdfium-review.googlesource.com/28850
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/core/fpdfapi/page/cpdf_page.cpp b/core/fpdfapi/page/cpdf_page.cpp
index 0c8d63de71..6518d80aa3 100644
--- a/core/fpdfapi/page/cpdf_page.cpp
+++ b/core/fpdfapi/page/cpdf_page.cpp
@@ -133,6 +133,12 @@ CFX_Matrix CPDF_Page::GetDisplayMatrix(int xPos,
   float x2 = 0;
   float y2 = 0;
   iRotate %= 4;
+  // This code implicitly inverts the y-axis to account for page coordinates
+  // pointing up and bitmap coordinates pointing down. (x0, y0) is the base
+  // point, (x1, y1) is that point translated on y and (x2, y2) is the point
+  // translated on x. On iRotate = 0, y0 is (yPos + ySize) and the translation
+  // to get y1 is performed as negative. This results in the desired
+  // transformation.
   switch (iRotate) {
     case 0:
       x0 = xPos;