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image: YCbCr lacks Set #9941

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josharian opened this issue Feb 20, 2015 · 2 comments
Closed

image: YCbCr lacks Set #9941

josharian opened this issue Feb 20, 2015 · 2 comments

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@josharian
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@josharian josharian commented Feb 20, 2015

This means that is not usable out of the box with the image/draw package. Is this an oversight, or is there a particular reason for the omission?

/cc @nigeltao

@nigeltao
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@nigeltao nigeltao commented Feb 20, 2015

Quoting myself from https://www.mattcutts.com/blog/photo-mosaic-effect-with-go/#comment-102923


You can’t set the pixels of an image.YCbCr (i.e. a JPEG image) directly, because the underlying data doesn’t map 1:1 to pixels, due to chroma sub-sampling. If you know what you’re doing, and are aware that setting one Chroma sample can affect multiple pixels, then the image.YCbCr struct exposes enough to let you set pixels. But if you don’t know what you’re doing, then we figured it’d be less dangerous to disallow behavior (setting pixels) than have potentially surprising behavior.

http://en.wikipedia.org/wiki/Chroma_subsampling

In general, work in RGBA as long as you can. The jpeg Encoder converts to YCbCr just before writing bits on the wire, but the intention is that you draw on your RGBA and pass your RGBA to jpeg.Encode.

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@nigeltao nigeltao closed this Feb 20, 2015
@nigeltao
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@nigeltao nigeltao commented Feb 20, 2015

By "disallow behavior", I should have said "not easily expose behavior".

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