Add image band metadata for the 4 channel images

Tests/test_pyarrow.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import json
 from typing import Any, NamedTuple
 
 import pytest
@@ -244,3 +245,29 @@ def test_from_int32array(mode: str, data_tp: DataShape, mask: list[int] | None)
     img = Image.fromarrow(arr, mode, TEST_IMAGE_SIZE)
 
     _test_img_equals_int32_pyarray(img, arr, mask, elts_per_pixel)
+
+
+@pytest.mark.parametrize(
+    "mode, metadata",
+    (
+        ("LA", ["L", "X", "X", "A"]),
+        ("RGB", ["R", "G", "B", "X"]),
+        ("RGBX", ["R", "G", "B", "X"]),
+        ("RGBA", ["R", "G", "B", "A"]),
+        ("CMYK", ["C", "M", "Y", "K"]),
+        ("YCbCr", ["Y", "Cb", "Cr", "X"]),
+        ("HSV", ["H", "S", "V", "X"]),
+    ),
+)
+def test_image_metadata(mode: str, metadata: list[str]) -> None:
+    img = hopper(mode)
+
+    arr = pyarrow.array(img)  # type: ignore[call-overload]
+
+    assert arr.type.field(0).metadata
+    assert arr.type.field(0).metadata[b"image"]
+
+    parsed_metadata = json.loads(arr.type.field(0).metadata[b"image"].decode("utf8"))
+
+    assert "bands" in parsed_metadata
+    assert parsed_metadata["bands"] == metadata

src/libImaging/Arrow.c

@@ -55,6 +55,77 @@ ReleaseExportedSchema(struct ArrowSchema *array) {
     // Mark array released
     array->release = NULL;
 }
+char *
+image_band_json(Imaging im) {
+    char *format = "{\"bands\": [\"%s\", \"%s\", \"%s\", \"%s\"]}";
+    char *json;
+    // Bands can be 4 bands * 2 characters each
+    int len = strlen(format) + 8 + 1;
+    int err;
+
+    json = calloc(1, len);
+
+    if (!json) {
+        return NULL;
+    }
+
+    err = PyOS_snprintf(
+        json,
+        len,
+        format,
+        im->band_names[0],
+        im->band_names[1],
+        im->band_names[2],
+        im->band_names[3]
+    );
+    if (err < 0) {
+        return NULL;
+    }
+    return json;
+}
+
+char *
+assemble_metadata(const char *band_json) {
+    /* format is
+       int32: number of key/value pairs (noted N below)
+       int32: byte length of key 0
+       key 0 (not null-terminated)
+       int32: byte length of value 0
+       value 0 (not null-terminated)
+       ...
+       int32: byte length of key N - 1
+       key N - 1 (not null-terminated)
+       int32: byte length of value N - 1
+       value N - 1 (not null-terminated)
+    */
+    const char *key = "image";
+    INT32 key_len = strlen(key);
+    INT32 band_json_len = strlen(band_json);
+
+    char *buf;
+    INT32 *dest_int;
+    char *dest;
+
+    buf = calloc(1, key_len + band_json_len + 4 + 1 * 8);
+    if (!buf) {
+        return NULL;
+    }
+
+    dest_int = (void *)buf;
+
+    dest_int[0] = 1;
+    dest_int[1] = key_len;
+    dest_int += 2;
+    dest = (void *)dest_int;
+    memcpy(dest, key, key_len);
+    dest += key_len;
+    dest_int = (void *)dest;
+    dest_int[0] = band_json_len;
+    dest_int += 1;
+    memcpy(dest_int, band_json, band_json_len);
+
+    return buf;
+}
 
 int
 export_named_type(struct ArrowSchema *schema, char *format, char *name) {
@@ -95,6 +166,8 @@ export_named_type(struct ArrowSchema *schema, char *format, char *name) {
 int
 export_imaging_schema(Imaging im, struct ArrowSchema *schema) {
     int retval = 0;
+    char *metadata;
+    char *band_json;
 
     if (strcmp(im->arrow_band_format, "") == 0) {
         return IMAGING_ARROW_INCOMPATIBLE_MODE;
@@ -117,13 +190,24 @@ export_imaging_schema(Imaging im, struct ArrowSchema *schema) {
     schema->n_children = 1;
     schema->children = calloc(1, sizeof(struct ArrowSchema *));
     schema->children[0] = (struct ArrowSchema *)calloc(1, sizeof(struct ArrowSchema));
-    retval = export_named_type(schema->children[0], im->arrow_band_format, "pixel");
+    retval = export_named_type(schema->children[0], im->arrow_band_format, im->mode);
     if (retval != 0) {
         free(schema->children[0]);
         free(schema->children);
         schema->release(schema);
         return retval;
     }
+
+    // band related metadata
+    band_json = image_band_json(im);
+    if (band_json) {
+        // adding the metadata to the child array.
+        // Accessible in pyarrow via pa.array(img).type.field(0).metadata
+        // adding it to the top level is not accessible.
+        schema->children[0]->metadata = assemble_metadata(band_json);
+        free(band_json);
+    }
+
     return 0;
 }