From 5a4ead9e2494d25c3e07ffd0ed4cc982a7ed2f1b Mon Sep 17 00:00:00 2001 From: Jakob Leben Date: Mon, 28 Nov 2022 23:09:10 -0800 Subject: [PATCH] Specify overview IFD subfile type and IFD order --- spec.md | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/spec.md b/spec.md index e182592..e982aac 100644 --- a/spec.md +++ b/spec.md @@ -11,7 +11,7 @@ of overview metadata, and finally the imagery itself. To make it friendly with s rendering, we recommend starting with the imagery of the smallest overview and finishing with the imagery of the full resolution level. -More formally, the structure of such a file is: +More formally, the layout of such a file is: * TIFF / BigTIFF signature * IFD ([Image File Directory](https://www.awaresystems.be/imaging/tiff/faq.html#q3)) of full resolution image @@ -25,6 +25,10 @@ More formally, the structure of such a file is: * Optional: tile content of first overview level * Tile content of full resolution image. +The IFDs shall be linked by the pointers (the offset of the first IFD in the file header and the offset of the next IFD in each IFD) so that the full resolution image is first, followed by the overviews. + +Each IFD representing an overview shall have the tag NewSubfileType (254) with a value of 1, indicating the "reduced resolution image" subfile type. + ### Unspecified points * size of tile: 256 or 512 pixels are typical however