resources/macos: better macos installer background (#1816)

resources/README.md

@@ -12,7 +12,9 @@ This folder contains resources that is used for building or packaging the projec
 - `icons/icon.{icns,ico,inl,rc,svg}`: lite-xl icon in various formats.
 - `linux/com.lite_xl.LiteXL.appdata.xml`: AppStream metadata.
 - `linux/com.lite_xl.LiteXL.desktop`: Desktop file for Linux desktops.
-- `macos/dmg-cover.png`: Background image for packaging macOS DMGs.
+- `macos/background.png`: Background image for packaging macOS DMGs.
+- `macos/background@2x.png`: HiDPI image for packaging macOS DMGs.
+- `macos/background.tiff`: TIFF image for packaging macOS DMGs.
 - `macos/Info.plist.in`: Template for generating `info.plist` on macOS. See `macos/macos-retina-display.md` for details.
 - `macos/lite-xl-dmg.py`: Configuration options for dmgbuild for packaging macOS DMGs.
 - `windows/001-lua-unicode.diff`: Patch for allowing Lua to load files with UTF-8 filenames on Windows.
@@ -21,5 +23,23 @@ This folder contains resources that is used for building or packaging the projec
 
 - `include/lite_xl_plugin_api.h`: Native plugin API header. See the contents of `lite_xl_plugin_api.h` for more details.
 
+### macOS DMG covers
+
+The DMG files uses `macos/background.tiff` as the background, which is created from
+`macos/background.png` and `macos/background@2x.png` with the following command:
+
+```sh
+tiffutil -cathidpicheck macos/background.png macos/background@2x.png -out macos/background.tiff
+```
+
+`macos/background@2x.png` should be double the size of `macos/background.png` with 144 PPI.
+You can set the PPI by running:
+
+```sh
+sips -s dpiWidth 144 -s dpiHeight 144 macos/background@2x.png
+```
+
+`sips` and `tiffutil` are available by default on macOS.
+
 
 [1]: https://mesonbuild.com/Cross-compilation.html