Notes on coordinate systems

1 files changed, 10 insertions, 4 deletions

diff --git a/include/swgfx.h b/include/swgfx.h
index 4a98116..d1f56df 100644
--- a/include/swgfx.h
+++ b/include/swgfx.h
@@ -2,10 +2,16 @@
 Software rendering library.
 
 Coordinate systems:
-- Pixel coordinates (i,j) refer to the center of the pixel.
-  Thus, real-valued coordinates (x,y) with no fractional part point at the pixel center.
-- Viewport origin is the top-left corner of the screen.
-  The viewport axes extend down and to the right.
+- The coordinate systems for image addressing and texture sampling are the same
+  except in scale.
+    Origin is in the top-left corner of the image.
+    Axes extend down and to the right.
+- Image addressing:
+    (i,j) integer coordinates refer to the center of the pixel.
+- Texture addressing:
+    (u,v) range in [0,1].
+    (0,0) is the center of the top-left pixel.
+    (1,1) is the center of the bottom-right pixel.
 
 Multi-threading:
 - Internal resources (swgfx context) are externally synchronized.