fast-quad-rotation

articles/fast-quad-rotation/page.mmd

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+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 };
+
+```