docs: write journal entry

Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>

journal/2021-08-14.md

@@ -73,6 +73,14 @@ JSDoc annotations and tests).
 - [ ] Implement more parts of the physics engine.
 - [ ] Consider using Nunjucks for interpolation to simplify post-processing.
 
+### Previous in line
+
+See journal entry from [yesterday][yesterday].
+
+### Next in line
+
+See journal entry from [tomorrow][tomorrow].
+
 [cosmic]: https://dribbble.com/shots/15008838-Cosmic
 [diff]: https://jaenis.ch/hobbies/coding/repos/ryuno-ki/js13kgames-2021/compare/711cdbe41b29e66b4a6346b0754f1a38bed7057a...05f095effadbca272b831300498762e87c191781
 [dribbble]: https://dribbble.com/search/space
@@ -88,6 +96,7 @@ JSDoc annotations and tests).
 [sketchbook]: https://www.orangepixel.net/2013/05/18/sketchbook-squad/
 [space]: https://flic.kr/p/85Y8Lj
 [thesaurus]: https://www.thesaurus.com/browse/space
+[tomorrow]: ./2021-08-15.md
 [track]: https://xem.github.io/articles/js13k20.html
 [unsplash]: https://unsplash.com/s/photos/space
 [yesterday]: ./2021-08-13.md

journal/2021-08-15.md

@@ -0,0 +1,49 @@
+# 2021-08-15
+
+Since I could paint my hero onto the next screen, the next step was adding
+gravity. I suceeded at this aspect!
+
+With that in my pocket, I looked into adding boundaries, so I could have a
+first prototype ready. Sadly, this turned out to be trickier than I thought.
+Also, it's prone for me to overthink.
+
+My current strategy (checking for two intersecting line segments) clearly have
+some shortcomings. But now I can add test cases to investigate these more in
+isolation.
+
+Also, I was informed, that geometric algebra isn't that widely known. Once I
+wrapped my head around what's going on here, I can add some pictures and
+explain the logic.
+
+## Today's achievement
+
+I implemented gravity and have first boundary and collision checks in place:
+
+[commit diff 59586dc4684b093e9f68518f114e62100142460b to abe72d67026177be6b028ff21f1b9fd9031cb455][diff]
+
+![screenshot from 2021-08-13][screenshot]
+
+## Next steps
+
+I want to have collision checks working flawlessly. Then, I can refactor the
+code to make gravity be a variable and include a range slider so I can tweak
+the gravity and see the effect on the canvas. It's a first playable prototype
+at that point. Also, it allows me to change the gravity between levels.
+
+Once I get that in, I will add some kind of energy levels. This should split
+up in life support systems as well as transport. It could look a bit like in
+[Sword Art Online][sao]!
+
+### Codewise
+
+- [ ] See ToDos from [yesterday][yesterday].
+- [ ] Investigate, why box and boundary don't collide.
+
+### Previous in line
+
+See journal entry from [yesterday][yesterday].
+
+[diff]: https://jaenis.ch/hobbies/coding/repos/ryuno-ki/js13kgames-2021/compare/59586dc4684b093e9f68518f114e62100142460b...abe72d67026177be6b028ff21f1b9fd9031cb455
+[sao]: https://github.com/SecSamDev/HealthBarSAO
+[screenshot]: ./2021-08-15.png
+[yesterday]: ./2021-08-14.md