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proposal: image: add image.NYCbCrA and color.NYCbCrA types. #12722

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nigeltao opened this issue Sep 23, 2015 · 5 comments
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proposal: image: add image.NYCbCrA and color.NYCbCrA types. #12722

nigeltao opened this issue Sep 23, 2015 · 5 comments
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@nigeltao
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This adds transparency (an Alpha channel) to the existing YCbCr types for JPEG-like images and color. The N stands for non-alpha-premultiplied, the same as the existing NRGBA types.

JPEG-like-with-alpha is used by the WebP format, and there already exists code (see [0]) under golang.org/x to represent such images and colors. The proposal is to move that from golang.org/x/image/webp/nycbcra to image, so that the image/draw package in the standard library can recognize such images, and implement a fast path for drawing them, the way YCbCr images are fast-pathed.

[0] https://go.googlesource.com/image/+/master/webp/nycbcra/nycbcra.go
@minux minux added the Proposal label Sep 23, 2015
@bradfitz
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SGTM. I don't think its inclusion would materially affect the display of https://golang.org/pkg/image/

@robpike
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robpike commented Sep 23, 2015

SGTM

@adg adg added Proposal and removed Proposal labels Sep 25, 2015
@gopherbot
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CL https://golang.org/cl/15671 mentions this issue.

@pierrre
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pierrre commented Dec 20, 2015

@nigeltao I have a question about this change:

In YCbCr.RGBA(), the code is a modified copy of YCbCrToRGB():

    // This code is a copy of the YCbCrToRGB function above, except that it
    // returns values in the range [0, 0xffff] instead of [0, 0xff]. There is a
    // subtle difference between doing this and having YCbCr satisfy the Color
    // interface by first converting to an RGBA. The latter loses some
    // information by going to and from 8 bits per channel.

But in NYCbCrA.RGBA(), YCbCrToRGB() is called, instead of a modified version.
Why don't we use the same code?
I think we lose some information here.

@nigeltao
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This question is now issue #13706.

@golang golang locked and limited conversation to collaborators Dec 29, 2016
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@bradfitz @pierrre @minux @robpike @adg @nigeltao @gopherbot