ICC profiles written here have their tags 4-byte aligned

moxcms's encode packs tagged elements back to back, so the built-in
sRGB profile embedded in JPEG, TIFF and PSD exports was 634 bytes with
`cprt` at offset 558, where ICC.1 §7.1.2 requires every tagged element
on a 4-byte boundary (stricter readers reject the files). encode now
lays the tag data out again with each element padded, updating the tag
offsets, the size and the profile ID (zeroed, "not computed"); tags
that share data keep sharing it. Reported to moxcms as
awxkee/moxcms#198.

Fixes #1584

crates/color/src/cms/icc.rs

@@ -249,7 +249,7 @@ pub(super) fn builtin_rgb_bytes(name: &str) -> Result<Vec<u8>, CmsError> {
 
 /// Encode `p` as an ICC file. The creation date is fixed so exports are reproducible.
 pub(super) fn encode(p: &ColorProfile) -> Result<Vec<u8>, CmsError> {
-    let mut bytes = p.encode().map_err(|e| CmsError::BadProfile(format!("{e:?}")))?;
+    let mut bytes = aligned(p.encode().map_err(|e| CmsError::BadProfile(format!("{e:?}")))?);
     // Header bytes 24..36: year, month, day, hours, minutes, seconds (u16 each).
     if let Some(date) = bytes.get_mut(24..36) {
         date.copy_from_slice(&[2026u16, 1, 1, 0, 0, 0].map(u16::to_be_bytes).concat());
@@ -257,6 +257,55 @@ pub(super) fn encode(p: &ColorProfile) -> Result<Vec<u8>, CmsError> {
     Ok(bytes)
 }
 
+/// `icc` with every tagged element starting on a 4-byte boundary, padded with zeros, as ICC.1
+/// §7.1.2 requires (moxcms packs them, so a 62-byte `desc` left the next tag unaligned, #1584).
+/// Tags that share data keep sharing it. A profile whose tag table doesn't read is returned as it is.
+fn aligned(icc: Vec<u8>) -> Vec<u8> {
+    const TABLE: usize = 128;
+    let be = |b: &[u8], at: usize| b.get(at..at + 4).map(|v| u32::from_be_bytes([v[0], v[1], v[2], v[3]]) as usize);
+    let Some(n) = be(&icc, TABLE).filter(|n| *n <= 1000) else { return icc };
+    let data_start = TABLE + 4 + 12 * n;
+    let mut entries = Vec::with_capacity(n);
+    for i in 0..n {
+        let e = TABLE + 4 + 12 * i;
+        let (Some(off), Some(size)) = (be(&icc, e + 4), be(&icc, e + 8)) else { return icc };
+        if off < data_start || off.checked_add(size).is_none_or(|end| end > icc.len()) {
+            return icc;
+        }
+        entries.push((off, size));
+    }
+    if entries.iter().all(|(off, _)| off.is_multiple_of(4)) && icc.len().is_multiple_of(4) {
+        return icc;
+    }
+    let mut out = icc.get(..data_start).map(<[u8]>::to_vec).unwrap_or_default();
+    let mut moved: Vec<((usize, usize), usize)> = Vec::new();
+    let mut order: Vec<usize> = (0..n).collect();
+    order.sort_by_key(|&i| entries[i].0);
+    for i in order {
+        let (off, size) = entries[i];
+        let at = match moved.iter().find(|(k, _)| *k == (off, size)) {
+            Some((_, at)) => *at,
+            None => {
+                out.resize(out.len().next_multiple_of(4), 0);
+                let at = out.len();
+                out.extend_from_slice(&icc[off..off + size]);
+                moved.push(((off, size), at));
+                at
+            }
+        };
+        let e = TABLE + 4 + 12 * i + 4;
+        out[e..e + 4].copy_from_slice(&(at as u32).to_be_bytes());
+    }
+    out.resize(out.len().next_multiple_of(4), 0);
+    let len = out.len() as u32;
+    out[0..4].copy_from_slice(&len.to_be_bytes());
+    // The profile ID (an MD5 of the old bytes) no longer matches: zero is "not computed".
+    if let Some(id) = out.get_mut(84..100) {
+        id.fill(0);
+    }
+    out
+}
+
 fn text_tag(s: &str) -> Option<ProfileText> {
     Some(ProfileText::Localizable(vec![LocalizableString::new("en".into(), "US".into(), s.into())]))
 }

crates/color/src/cms/tests_icc.rs

@@ -111,3 +111,26 @@ fn black_point_compensation_maps_the_darkest_ink_to_rgb_black() {
     let abs = |bpc| icc_cmyk(bpc).cmyk_to_rgb(rich, Intent::AbsoluteColorimetric);
     assert_eq!(abs(true), abs(false));
 }
+
+/// #1584: the profiles written here (built-in RGB spaces) have every tag on a 4-byte boundary and
+/// a size that is a multiple of 4 (ICC.1 §7.1.2), and still read back as the same space.
+#[test]
+fn written_profiles_align_their_tags() {
+    for name in [super::SRGB, super::DISPLAY_P3, super::WIDE_GAMUT_RGB, super::PROPHOTO_RGB] {
+        let icc = super::icc_bytes(name).unwrap();
+        let be = |at: usize| u32::from_be_bytes([icc[at], icc[at + 1], icc[at + 2], icc[at + 3]]) as usize;
+        assert_eq!(be(0), icc.len(), "{name}: header size");
+        assert_eq!(icc.len() % 4, 0, "{name}: {} bytes", icc.len());
+        let n = be(128);
+        for i in 0..n {
+            let (off, size) = (be(132 + 12 * i + 4), be(132 + 12 * i + 8));
+            assert_eq!(off % 4, 0, "{name}: tag {i} at {off}");
+            assert!(off + size <= icc.len());
+        }
+        let back = moxcms::ColorProfile::new_from_slice(&icc).unwrap();
+        let orig = moxcms::ColorProfile::new_srgb();
+        if name == super::SRGB {
+            assert_eq!((back.red_colorant, back.green_colorant, back.blue_colorant), (orig.red_colorant, orig.green_colorant, orig.blue_colorant));
+        }
+    }
+}