Allow indexed color PNGs for Mac icons

optipng sometimes finds that using indexed color yields a smaller PNG
file. Allow that.

Also add a comment to the sRGB code explaining why that value was
chosen.

Bug: 846451
Change-Id: Ia8d6ef7dba59e4a0a0cbbf4d3122f63c501cfabf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2148030
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759421}

docs/mac/icons.md

@@ -26,14 +26,9 @@ scrambling of the icons in display. If the old types work, why mess with them?
 
 ## Source PNG files
 
-Use whatever tools you want to create the PNG files, but please note the
-following:
-
-1. The PNG files must be in full RGB mode (i.e. not indexed-color). This will be
-enforced by the `png_check.py` tool below.
-1. The dimensions of the images in the PNG files must match exactly the size
-indicated by their filename. This will be enforced by the `makeicns2` tool
-below.
+Use whatever tools you want to create the PNG files, but please note that the
+dimensions of the images in the PNG files must match exactly the size indicated
+by their filename. This will be enforced by the `makeicns2` tool below.
 
 ## Construction
 

tools/mac/icons/.style.yapf

 [style]
 based_on_style = chromium
-indent_width = 4
\ No newline at end of file
+indent_width = 4

tools/mac/icons/png_check.py

@@ -16,11 +16,19 @@ def verify(path):
     magic = file.read(8)
     assert magic == b'\x89\x50\x4e\x47\x0d\x0a\x1a\x0a'
 
+    color_type = None
     chunk_types = []
     while True:
         chunk_header = file.read(8)
         (chunk_length, chunk_type) = struct.unpack('>I4s', chunk_header)
-        file.seek(chunk_length, 1)
+
+        if chunk_type == b'IHDR':
+            ihdr = file.read(chunk_length)
+            (width, height, bit_depth, color_type, compression_method,
+             filter_method, interlace_method) = struct.unpack('>2I5b', ihdr)
+        else:
+            file.seek(chunk_length, 1)
+
         chunk_footer = file.read(4)
         (chunk_crc,) = struct.unpack('>I', chunk_footer)
 
@@ -29,10 +37,16 @@ def verify(path):
         if chunk_type == b'IEND':
             break
 
-    assert chunk_types == [b'IHDR', b'sRGB', b'IDAT', b'IEND']
-    # Some images showed up with [b'IHDR', b'sRGB', b'PLTE', b'tRNS', b'IDAT',
-    # b'IEND'] but these wound up being smaller when compressed with
-    # optipng/advpng without being in indexed-color mode.
+    # The only two color types that have transparency and can be used for icons
+    # are types 3 (indexed) and 6 (direct RGBA).
+    assert color_type in (3, 6), "Disallowed color type {}".format(color_type)
+    allowed_chunk_types = [b'IHDR', b'sRGB', b'IDAT', b'IEND']
+    if color_type == 3:
+        allowed_chunk_types.extend([b'PLTE', b'tRNS'])
+
+    for chunk_type in chunk_types:
+        assert (chunk_type in allowed_chunk_types
+               ), "Disallowed chunk of type {}".format(chunk_type)
 
     eof = file.read(1)
     assert len(eof) == 0

tools/mac/icons/png_make_srgb.py

@@ -10,6 +10,7 @@
 # conflict with the sRGB. Obviously, only use this if it's desirable for the
 # png file to be treated as sRGB.
 
+import binascii
 import struct
 import sys
 
@@ -51,8 +52,23 @@ def make_srgb(path):
         remainder = file.read()
         file.seek(after_ihdr)
 
-        # Length, type, data, CRC
-        file.write(b'\x00\x00\x00\x01sRGB\x00\xae\xce\x1c\xe9')
+        # Note that a value of 0 is a perceptual rendering intent (e.g.
+        # photographs) while a value of 1 is a relative colorimetric rendering
+        # intent (e.g. icons). Every macOS icon that has an 'sRGB' chunk uses 0
+        # so that is what is used here. Please forgive us, UX.
+        #
+        # Reference:
+        #   http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.sRGB
+        srgb_chunk_type = struct.pack('>4s', b'sRGB')
+        srgb_chunk_data = struct.pack('>b', 0)  # Perceptual
+        srgb_chunk_length = struct.pack('>I', len(srgb_chunk_data))
+        srgb_chunk_crc = struct.pack(
+            '>I',
+            binascii.crc32(srgb_chunk_type + srgb_chunk_data) & 0xffffffff)
+        srgb_chunk = (
+            srgb_chunk_length + srgb_chunk_type + srgb_chunk_data +
+            srgb_chunk_crc)
+        file.write(srgb_chunk)
 
         file.write(remainder)