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doc: document behavior change in image/color in release notes

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Change-Id: I7ad90ab78abb8a39d56c837610d5c311a96b7039
Reviewed-on: https://go-review.googlesource.com/12162
Reviewed-by: Nigel Tao <nigeltao@golang.org>
This commit is contained in:
Rob Pike 2015-07-14 15:20:19 +10:00
parent a74d030557
commit 902345e596
1 changed files with 30 additions and 7 deletions
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@ -931,6 +931,29 @@ the <a href="/pkg/image/color/#CMYKModel"><code>CMYKModel</code></a> color model
needed by some JPEG images.
</li>
<li>
Also in the <a href="/pkg/image/color/"><code>image/color</code></a> package,
the conversion of a <a href="/pkg/image/color/#YCbCr"><code>YCbCr</code></a>
value to <code>RGBA</code> has become more precise.
Previously, the low 8 bits were just an echo of the high 8 bits;
now they contain more accurate information.
Because of the echo property of the old code, the operation
<code>uint8(r)</code> to extract an 8-bit red vaue worked, but is incorrect.
In Go 1.5, that operation may yield a different value.
The correct code is, and always was, to select the high 8 bits:
<code>uint8(r&gt;&gt;8)</code>.
Incidentally, <code>image/draw</code> package
provides better support for such conversions; see
<a href="https://blog.golang.org/go-imagedraw-package">this blog post</a>
for more information.
</li>
<li>
Finally, as of Go 1.5 the closest match check in
<a href="/pkg/image/color/#Palette.Index"><code>Index</code></a>
now honors the alpha channel.
</li>
<li>
The <a href="/pkg/image/gif/"><code>image/gif</code></a> package
includes a couple of generalizations.