From d1fe7a9230f5c4ddead902e63653d0ad55ca2860 Mon Sep 17 00:00:00 2001
From: veclavtalica <veclavtalica@poto.cafe>
Date: Sun, 28 Jul 2024 03:35:32 +0300
Subject: [PATCH] fast-quad-rotation

---
 articles/fast-quad-rotation/page.mmd | 32 ++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)
 create mode 100644 articles/fast-quad-rotation/page.mmd

diff --git a/articles/fast-quad-rotation/page.mmd b/articles/fast-quad-rotation/page.mmd
new file mode 100644
index 0000000..791b656
--- /dev/null
+++ b/articles/fast-quad-rotation/page.mmd
@@ -0,0 +1,32 @@
+Title:  Fast Quad Rotation
+Brief:  A better way to rotate quads around their centers.
+Date:   1722126213
+Tags:   Programming, Optimization, C
+CSS:    /style.css
+
+A similar in essence trick to [by pi rotation](/articles/vector-pi-rotation.html), but with delta calculated
+for some corner which is reused later with negation and coordinate swap.
+
+Scaling by `M_SQRT1_2` is there to retain the quad size (Pythagorean stuffs).
+
+### Code ###
+```c
+const t_fvec2 c = frect_center(sprite.rect);
+const t_fvec2 d = {
+    .x = (cosf(sprite.rotation + (float)M_PI_4) * sprite.rect.w) * (float)M_SQRT1_2,
+    .y = (sinf(sprite.rotation + (float)M_PI_4) * sprite.rect.h) * (float)M_SQRT1_2,
+};
+
+/* upper-left */
+const t_fvec2 v0 = { c.x - d.x, c.y - d.y };
+
+/* bottom-left */
+const t_fvec2 v1 = { c.x - d.y, c.y + d.x };
+
+/* bottom-right */
+const t_fvec2 v2 = { c.x + d.x, c.y + d.y };
+
+/* upper-right */
+const t_fvec2 v3 = { c.x + d.y, c.y - d.x };
+
+```