add day 18 of devlog

1 files changed, 22 insertions, 1 deletions

diff --git a/posts/SDL2_devlog.md b/posts/SDL2_devlog.md
index eb11de8..04ac928 100644
--- a/posts/SDL2_devlog.md
+++ b/posts/SDL2_devlog.md
@@ -5,6 +5,27 @@ on the editor. The days are listed in reverse chronological
 order, begin from the bottom, if this is your first time on
 this page.
 
+### Day 18
+
+I added basic support for guides, they can be added and
+activated from the `lisp` REPL. Another long standing
+improvement I wanted to make was reworking the pixmap
+drawing procedure. The old procedure draws a square for each
+pixel in the pixmap, coloured according to its value in the
+pixmap. Naturally, this means, for an **NxN** pixmap, there
+are **N²** calls to SDL! I reworked this procedure to
+compress each line of the pixmap using RLE (run length
+encoding), and call out to SDL for each run in the line.
+This drastically improved drawing speeds on larger grids.
+The following is a comparison between the two procedures,
+the leftmost picture is the rendered image, the middle
+picture is the optimized drawing procedure (draws each run
+instead of pixel), and the right most picture is the
+primitive drawing procedure (draws each pixel):
+
+![Day 18](https://u.peppe.rs/U4B.png)
+
+
 ### Day 17
 
 I decided to give the text-only statusline a touch up, by
@@ -23,7 +44,7 @@ let statusline =
 let mut primary = Container::uninit()
     .width(Size::Absolute(16))
     .height(Size::Absolute(16));
-
+    
 container.place(
     &mut padding_box,
     HorAlign::Right,