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This is what I did:
1. In GUI, set options for generated cover Silver and Banner. Option "Save these settings as the defaults for future use" checked.
2. Downloaded from Project Gutenberg txt and epub files of book Under Orders by Kirk Munroe (1890).
Obtained Under_Orders.txt and Under_Orders.epub
txt
3. Converted Under_Orders.txt->Under_Orders(txt).epub. Obtained file with expected generated cover
azw3
4. Converted txt->azw3. Obtained Under_Orders(txt).azw3 without any cover
5. Converted Under_Orders(txt).azw3->Under_Orders(azw3)epub. Obtained file with expected generated cover
fb2
6. Converted txt->fb2. Obtained Under_Orders(txt).fb2 without any cover
7. Converted Under_Orders(txt).fb2->Under_Orders(fb2).epub. Obtained file with expected generated cover
epub (only cover file deleted)
8. Using Calibre editor, deleted cover in file Under_Orders.epub and saved it as file Under_Orders_deleted_cover.epub
9. Converted Under_Orders_deleted_cover.epub->Under_Orders_deleted_cover_blank_cover.epub. Obtained file with blank calibre_raster_cover.jpg
epub (all refs to cover erased)
8. Using Calibre editor, deleted cover in file Under_Orders.epub, searched and erased all cover refrences and saved it as file Under_Orders_deleted_cover_all_cover_refs_erased.e pub
9. Converted Under_Orders_deleted_cover_all_cover_refs_erased.e pub->Under_Orders_deleted_cover_all_cover_refs_erased_ auto_cover.epub. Obtained file with expected generated cover.
Bottom line:
10. Three conversions, namely txt->epub, fb2->epub, and azw3->epub succeeded to obtain expected generated cover in cover absence in input files.
11. Conversion epub->epub provides expected generated cover in output epub only if cover references in input epub are missing as well cover itself.
12. Conversion epub->epub embeds blank calibre_raster_cover.jpg in output file if cover itself is missing from input but cover is still referenced.
My problem was with item 12: if input epub somehow on the road lost cover (this may happen) and references are still in place than this blank cover gets into the output epub and from there to azw3 (I'm running two conversions epub->epub and epub->azw3 in batch file). My hope was to obtain nice Calibre generated cover even in this situation.
All files mentioned above are attached.
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