adjust custom image format syntax, document, and test

Author: JonasGruenwald

src/ansel/image.gleam

@@ -42,9 +42,11 @@ import snag
 /// own vips binary on the host system to. See the package readme for details.
 /// 
 /// The `Custom` constructor allows for advanced vips save options to be 
-/// used, like `ansel.Custom(".png", "[compression=90,squash=true]")`, refer to the 
-/// [Vix package documentation](https://hexdocs.pm/vix/Vix.Vips.Image.html#write_to_file/2) 
-/// for details.
+/// used as a comma separated list, like `ansel.Custom(".png", "compression=90,squash=true")`,
+/// which is equivalent to the `.png[compression=90,squash=true]` syntax used in vips.
+/// 
+/// You can use the libvips CLI to print a list of all supported options for each image format,
+///  e.g. `vips pngsave`, `vips jpegsave`.
 pub type ImageFormat {
   JPEG(quality: Int, keep_metadata: Bool)
   JPEG2000(quality: Int, keep_metadata: Bool)
@@ -100,7 +102,8 @@ fn image_format_to_string(format: ImageFormat) -> FormatComponents {
     Analyze -> FormatComponents(".analyze", "")
     NIfTI -> FormatComponents(".nii", "")
     DeepZoom -> FormatComponents(".dzi", "")
-    Custom(extension:, format:) -> FormatComponents(extension, format)
+    Custom(extension:, format:) ->
+      FormatComponents(extension, "[" <> format <> "]")
   }
 }
 

test/image_test.gleam

@@ -44,6 +44,17 @@ pub fn write_test() {
   let assert Ok(_) = simplifile.delete(output_path)
 }
 
+pub fn custom_options_test() {
+  let assert Ok(reference_image) = image.new(6, 6, color.Grey)
+  let output =
+    reference_image
+    |> image.to_bit_array(image.JPEG(quality: 50, keep_metadata: True))
+
+  reference_image
+  |> image.to_bit_array(image.Custom(".jpeg", "Q=50,strip=false"))
+  |> should.equal(output)
+}
+
 pub fn new_solid_grey_test() {
   let assert Ok(bin) =
     simplifile.read_bits("test/resources/solid_grey_6x6.avif")