diff --git a/.gitignore b/.gitignore
index e929830..8bd652d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 *.zip
 packing
+build.sh
diff --git a/README.md b/README.md
index 94a84f0..233682d 100644
--- a/README.md
+++ b/README.md
@@ -9,3 +9,24 @@ The subject is available here: [Test Algo](./test_algo.pdf)
 ### Question 1
 ### Question 2
 ### Question 3
+
+## PNG support
+
+I chose to support PNG files through the stb files:
+[github.com/nothings/stb](https://github.com/nothings/stb).
+I could have implemented basic image support with the PNM format but I think it
+is nicer to support common image formats with a simple library.
+
+stb also implements a 2D rectangle packer:
+[stb_rect_pack.h](https://github.com/nothings/stb/blob/master/stb_rect_pack.h)
+I added this file in the project in order to compare my implementation to
+another one.
+
+## 3rd party libraries
+
+I use 2 external libraries for better visualization:
+- stb files (for PNG)
+- raylib (for GUI)
+
+I don't rely on them for the algorithm implementation and the core of the
+exercise doesn't rely on external libraries.
diff --git a/justfile b/justfile
index 02765a0..ca30572 100644
--- a/justfile
+++ b/justfile
@@ -9,6 +9,8 @@ run: build
 	{{exe}}
 
 generate-build:
+	git clean -xf src/
+	rm --force {{exe}}
 	tup generate {{build_sh}}
 
 debug: build
diff --git a/src/main_cli.cpp b/src/main_cli.cpp
new file mode 100644
index 0000000..398558b
--- /dev/null
+++ b/src/main_cli.cpp
@@ -0,0 +1,24 @@
+#include <filesystem>
+#include <iostream>
+
+namespace fs = std::filesystem;
+
+int main(int argc, const char* argv[]) {
+    // We take an image path and a list of points (grouped by 4 to define
+    // bounding boxes).
+    if (argc < 6 or (argc - 2) % 4 != 0) {
+        std::cerr
+            << "Usage: " << argv[0] << " path/to/image"
+            << " x1 y1 x2 y2 [...]\n"
+            << "x/y points must be grouped by 4 to define bounding boxes\n";
+        return 1;
+    }
+
+    const fs::path input(argv[1]);
+    if (!fs::exists(input)) {
+        std::cerr << "Input file " << input << " does not exist.\n";
+        return 1;
+    }
+
+    return 0;
+}