Re: Transparency in images

Hey Lora,

From a spec perspective, transparency may be supported. The EPUB spec directly references the full PNG spec, and the PNG spec says that transparency may be ignored (it’s considered ancillary information<https://www.w3.org/TR/PNG/#11Ancillary-chunks>). I’m not sure about the gif spec but I would assume it has similar wording (it’s a much older spec and not written in a way I can understand).

On a more practical note: We have found that the way we create transparency is supported by most of the big US retailers, but not all (specifically the big one that doesn’t support epub doesn’t support transparency). If by “transparency layer” you are referring to an “alpha channel”, we didn’t test that since none of our tools create alpha channels. That could be the reason for the discrepancy between our testing and yours.

Hope that helps,
Naomi

From: Lora Friedenthal <lora@scribd.com>
Date: Thursday, May 9, 2019 at 7:05 PM
To: "public-epub3@w3.org" <public-epub3@w3.org>
Subject: Transparency in images
Resent-From: <public-epub3@w3.org>
Resent-Date: Thursday, May 9, 2019 at 6:59 PM

Hello. Can someone tell me whether transparency layers in PNG and GIF are supported in EPUB3? I assume since every viewer shows a white box around images it must not be, but I also can't find anywhere in the spec that says it shouldn't work. I'm trying to determine if we just have to live with ornaments in the text being in white boxes when users change the background color of the reader.



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Lora Friedenthal
Pronoun(s): She/Her/Hers
Content Operations Specialist at Scribd


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