From 7e48c57e1c76d4ef874a2e15bfe9fd43e33f60a8 Mon Sep 17 00:00:00 2001
From: 3gg <3gg@shellblade.net>
Date: Thu, 1 Jan 2026 09:49:59 -0800
Subject: Notes on coordinate systems

---
 include/swgfx.h | 14 ++++++++++----
 src/swgfx.c     |  2 --
 2 files changed, 10 insertions(+), 6 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.
diff --git a/src/swgfx.c b/src/swgfx.c
index 3bec663..7ade051 100644
--- a/src/swgfx.c
+++ b/src/swgfx.c
@@ -272,8 +272,6 @@ void SetDepth(swgfx* gfx, const sgVec2i p, R depth) {
 sgPixel Sample(const sgTexture_t* texture, sgVec2 uv) {
   assert(texture);
   assert(texture->pixels);
-  // TODO: (1/2, 1/2) is the center of the pixel. Do we need to do something
-  //  about it here?
 #define INDEX(X,Y) texture->pixels[(Y) * texture->width + (X)]
   // Doing a nearest sample for now. TODO: Other sampling strategies.
   const int x = (int)(uv.x * (R)texture->width);
-- 
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