doc: Add tee_rendering.md describing how to render a tee

doc/tee_rendering.md

@@ -0,0 +1,52 @@
+This document describes how to render a tee like Teeworlds 0.6 or DDNet, given
+a tee skin image.
+
+A skin image must have an aspect ratio of 2:1 (width:height). We get the
+following body parts by splitting the tee skin image (3/8 means at 3/8 of the
+image's width, 1/4 means at 1/4 of the image's height, etc.):
+
+    body:         (0/8, 0/4) to (3/8, 3/4)
+    body_outline: (3/8, 0/4) to (6/8, 3/4)
+    hand:         (6/8, 0/4) to (7/8, 1/4)
+    hand_outline: (7/8, 0/4) to (8/8, 1/4)
+    foot:         (6/8, 1/4) to (8/8, 2/4)
+    foot_outline: (6/8, 2/4) to (8/8, 3/4)
+    eye_normal:   (2/8, 3/4) to (3/8, 4/4)
+    eye_angry:    (3/8, 3/4) to (4/8, 4/4)
+    eye_pain:     (4/8, 3/4) to (5/8, 4/4)
+    eye_happy:    (5/8, 3/4) to (6/8, 4/4)
+    eye_dead:     (6/8, 3/4) to (7/8, 4/4) -- entirely unused
+    eye_surprise: (7/8, 3/4) to (8/8, 4/4)
+
+This leaves an unused rectangle at `(0/8, 3/4) to (2/8, 4/4)`.
+
+For rendering, the segments need to be scaled like this (relative to body being
+100%):
+
+    body: 100%
+    feet: 150%
+    eyes: 120%
+    hand: 93.75%
+
+The last eye shape `eye_blink` is achieved by scaling `eye_normal` to 45%, but
+only vertically.
+
+Then, the images must be positioned like the following (hands or moving feet
+not handled), relative to 64/64 or 1 being the edge length of the body
+segment).
+
+    body: 4/64 up
+    feet:
+        10/64 down
+        7/64 left/right
+    eyes:
+        0.125 up
+        0.05 left/right
+
+    eye movement:
+        dir = angle of eyes (view angle), right = 0
+        eyes:
+            x: cos(dir) * 0.125
+            y: sin(dir) * 0.1
+        each eye (away from the other):
+            x: abs(cos(dir)) * 0.01