Book cover extracting enhancements

[vers-one]

Description

EPUB 2 specification doesn't contain explicit requirements on how book cover should be represented in the OPF schema file. Instead it provides only a vague recommendation to use a <guide>/<reference type="cover"> element mentioning the Chicago Manual of Style as the source of the list of applicable <reference> element types.

Most EPUB 2 books use <meta name="cover" content="..." /> element to define the cover, where the value of the content attribute points to a <manifest>/<item> element of the actual cover image. However there are some books that don't follow this pattern, hence all the hacks and heuristics currently present in the BookCoverReader:

EpubReader/Source/VersOne.Epub/Readers/BookCoverReader.cs

Lines 37 to 67 in d160caf

	// For non-standard ebooks, we try several other ways...
	if (null != coverManifestItem) // we have found the item but there was no corresponding image ...
	{
	// some ebooks seem to contain more than one item with Id="cover"
	// thus we test if there is a second item, and whether that is an image....
	coverManifestItem = epubSchema.Package.Manifest.Where(manifestItem => manifestItem.Id.CompareOrdinalIgnoreCase(coverMetaItem.Content)).Skip(1).FirstOrDefault();
	if (null != coverManifestItem?.Href && imageContentRefs.TryGetValue(coverManifestItem.Href, out coverImageContentFileRef))
	{
	return coverImageContentFileRef;
	}
	}
	// we have still not found the item
	// 2019-08-20 Hotfix: if coverManifestItem is not found by its Id, then try it with its Href - some ebooks refer to the image directly!
	coverManifestItem = epubSchema.Package.Manifest.FirstOrDefault(manifestItem => manifestItem.Href.CompareOrdinalIgnoreCase(coverMetaItem.Content));
	if (null != coverManifestItem?.Href && imageContentRefs.TryGetValue(coverManifestItem.Href, out coverImageContentFileRef))
	{
	return coverImageContentFileRef;
	}
	// 2019-08-24 if it is still not found, then try to find an Id named cover
	coverManifestItem = epubSchema.Package.Manifest.FirstOrDefault(manifestItem => manifestItem.Id.CompareOrdinalIgnoreCase(coverMetaItem.Name));
	if (null != coverManifestItem?.Href && imageContentRefs.TryGetValue(coverManifestItem.Href, out coverImageContentFileRef))
	{
	return coverImageContentFileRef;
	}
	// 2019-08-24 if it is still not found, then try to find it in the guide
	var guideItem = epubSchema.Package.Guide.FirstOrDefault(reference => reference.Title.CompareOrdinalIgnoreCase(coverMetaItem.Name));
	if (null != guideItem?.Href && imageContentRefs.TryGetValue(guideItem.Href, out coverImageContentFileRef))
	{
	return coverImageContentFileRef;
	}

EPUB 3 on the other hand does define an explicit requirement for cover images by requesting to specify them via <manifest>/<item properties="cover-image"> elements. EpubReader parses these <manifest>/<item> elements along with their properties attributes correctly but does not currently use this information to obtain a cover image of an EPUB 3 book.

Proposed solution

1. Remove all hacks from BookCoverReader.
2. Replace heuristics with more robust algorithms to search for cover images in EPUB 2 books.
3. Add EPUB 3 cover image support with a fallback to the EPUB 2 cover image extraction implementation if an EPUB 3 cover is not available (to support EPUB 3 books that provide only EPUB 2 covers).

[vers-one]

Implemented in PR #49.

If someone finds a book with a cover that doesn't get extracted by EpubReader, please post a sample file in this issue.