Kobo Aura One screensaver cover low-res?

[mgrimace]

Hello,

My issue: After sending a book over, it seems to generate a lower resolution screensaver cover, regardless of the image size in Calibre.

The cover image inside the book on the KA1 is high resolution (i.e., the image that shows up when I start the book on the device), but the screensaver image is jagged and artifact-y, clearly upscaled.

My setup:
High resolution cover images in calibre library
Calibre output settings on 'tablet'
Followed the 'Shelves and collections' wiki to set up Calibre for the KA1
Kobotouchextended driver (output to kepub, my preferred format), conversion to kepub is done on-the-fly when sending to device, options selected:

• Upload covers - selected
• keep cover aspect ratio - selected (also tried not selected, no quality difference other than image stretched to fill screen)
• black and white - not selected (also tried selected, no difference)

Other things I've tried:
- manually upload covers via Kobo Utilities plugin - no difference
- kobotouch driver (output also set to kepub) - same issue with 'upload covers' selected

Seems to be an issue with the kt/kte driver. Maybe a plugin conflict, setting, or something I'm missing, any ideas?

SOLUTION, via Davidfor:
Open the preferences and select "Tweaks" near the bottom of the screen. Select the "maximum_cover_size" tweak in the list on the left to see the value. Hit it the "Reset This Tweak" value to go back to the default.

~M

[jackie_w]

You will get better results if you enable the driver to send the covers as black-and-white.

This is because calibre will do a better job of grayscale conversion than letting the Kobo device make a poor job of it.

[mgrimace]

Originally Posted by jackie_w

You will get better results if you enable the driver to send the covers as black-and-white.

This is because calibre will do a better job of grayscale conversion than letting the Kobo device make a poor job of it.

Thanks jackie_w, I tried the grayscale but I think the issue is that the resolution is dropping whenever I use the 'upload covers' option in the driver, and then upscaling on the device for the screensaver resulting in blocky/jaggy gradients.

I say that because I also just tried the polish books plugin (update the cover in the book files - selected) and resent over the book (with the 'upload covers' option turned off in the driver) and now the screensaver cover looks fine.

Seems like its a problem whenever I select 'upload covers' in the driver, regardless of the other options.

Temp fix is manually using polish books manually before sending, and turning off the upload covers option. Any thoughts about the driver?

[GeoffR]

I have been seeing problems with low-res covers on KePub books synced from the Kobo server for a few months now (edit: problems with the sleep screen image only, the cover inside the book is okay), but no problems at all with sideloaded ePubs.

(I am using Calibre 2.85, KoboTouch 2.3.3, KoboUtilities 2.5.2, to sideload ePubs, sending greyscale covers with original aspect ratio. I'm not using the KoboTouchExtended driver.)

[davidfor]

Originally Posted by GeoffR

I have been seeing problems with low-res covers on KePub books synced from the Kobo server for a few months now (edit: problems with the sleep screen image only, the cover inside the book is okay), but no problems at all with sideloaded ePubs.

I can't say I've noticed a difference recently, but I am usually using the higher resolution devices. Maybe Kobo have changed how they generate them for the lower resolution devices. I'll have a look and see. But, as you are seeing it, can you suggest a book or two that you have seen it with? I can get the preview onto my devices and see if I can tell the difference.

(I am using Calibre 2.85, KoboTouch 2.3.3, KoboUtilities 2.5.2, to sideload ePubs, sending greyscale covers with original aspect ratio. I'm not using the KoboTouchExtended driver.)

It won't be different. The KoboTouch driver implements the cover sending and uses code calibre provides for the conversion. The extended driver and the utilities just use the code in the KoboTouch driver.

[GeoffR]

Originally Posted by davidfor

But, as you are seeing it, can you suggest a book or two that you have seen it with? I can get the preview onto my devices and see if I can tell the difference.

https://www.kobo.com/nz/en/ebook/vic...thout-a-face-3
https://www.kobo.com/nz/en/ebook/the...t-comes-before


Edit: One thing I remember when trying to figure out what as happening with the KePub covers was that some (not all) of the problem books were missing the full-size image file (the one with filename ending in N3_FULL), so the sleep screen must have been using the smaller thumbnail image as a fallback.

[davidfor]

Originally Posted by mgrimace

Thanks jackie_w, I tried the grayscale but I think the issue is that the resolution is dropping whenever I use the 'upload covers' option in the driver, and then upscaling on the device for the screensaver resulting in blocky/jaggy gradients.

That shouldn't be happening. The driver has the sizes for each cover type and device and generates a cover image from the cover in the calibre library. Unless I messed this up in a change earlier this year to clean up the code a bit. I haven't tested this with my Aura ONE, so I'll have a look tonight.

Of course, it is possible that Kobo have improved the image handling here. They have made at least two change in the area this year, but I didn't think they would really affect the cover images.

I say that because I also just tried the polish books plugin (update the cover in the book files - selected) and resent over the book (with the 'upload covers' option turned off in the driver) and now the screensaver cover looks fine.

Seems like its a problem whenever I select 'upload covers' in the driver, regardless of the other options.

Temp fix is manually using polish books manually before sending, and turning off the upload covers option. Any thoughts about the driver?

With the way this works, there shouldn't be a difference between polishing first and relying on calibre to update the cover during the send. They do function differently, but the effect should be the same. During the send, calibre looks for the cover image in the book and replaces it. If it can't find the cover, it will add the image and mark it in the OPF as the cover but it won't be displayed while reading the book. Some of this can be turned off. In older calibre, if calibre couldn't determine which image was the cover, nothing was done. This was changed as it caused problems on some devices, but an option was added to turn it off. With this, if calibre can't replace the cover, then it should mean that the device won't find it either and it will use the default cover (title plus format name)

The Polish does largely the same (looks for the cover and replaces it if it can find it), but makes sure the cover image is displayed as the first page of the book. If the current cover can't be found, it adds the cover and a new first page in the book. This cover could be resized based on the conversion settings you normally use. But, if you have the output profile set to Tablet, the cover should be left full size.

With all that, you should be getting exactly the same cover image in the book in either way. And the device should be using it and resizing it in the same way. And because of how the cover image is extracted from the kepub, sending the covers using the driver has never been as useful as for epubs and other formats.

I'll have a play and see if I can see anything. Finding a book suitable for testing could be the problem.

[mgrimace]

Originally Posted by davidfor

That shouldn't be happening. The driver has the sizes for each cover type and device and generates a cover image from the cover in the calibre library. Unless I messed this up in a change earlier this year to clean up the code a bit. I haven't tested this with my Aura ONE, so I'll have a look tonight.

Hi Davidfor, thanks for looking into it, I've tried a number of different combinations on my KA1. Any 'upload covers' option results in a lower-res screensaver. Specifically, the KT driver, the KTE driver, and/or Kobo Utilities.

Here is an example (kobo link for preview), I used this book because it's easier to see in the gradients below the "P" of the title:
image 1: cover image inside the book (when I start the book for the first time, not the screensaver)

For reference, the cover image is 1000x1500 in Calibre, and the original file in Calibre is epub, converted to kepub on transfer

image 2: screensaver image of the cover using 'upload covers' option

note, this has the 'keep aspect ratio' option unchecked, but the quality is the same for aspect ratio on/off, and greyscale on/off. It is the same overall effect for the KT/KTE drivers, or manual send cover with Kobo Utilities

Originally Posted by davidfor

With the way this works, there shouldn't be a difference between polishing first and relying on calibre to update the cover during the send. They do function differently, but the effect should be the same. During the send, calibre looks for the cover image in the book and replaces it.

When I use polish books on the original epub, then send the book over using the KTE driver, but 'upload covers' off, I get a high quality cover, example:

Image 3: high quality screensaver cover image via polish books


I should clarify, when I use the polish books option, the cover is momentarily blank on the KA1 after sending the book over (≤1s, but noticeable), then it seems to generate the cover.

Originally Posted by davidfor

If it can't find the cover, it will add the image and mark it in the OPF as the cover but it won't be displayed while reading the book. Some of this can be turned off. In older calibre, if calibre couldn't determine which image was the cover, nothing was done. This was changed as it caused problems on some devices, but an option was added to turn it off. With this, if calibre can't replace the cover, then it should mean that the device won't find it either and it will use the default cover (title plus format name)

The Polish does largely the same (looks for the cover and replaces it if it can find it), but makes sure the cover image is displayed as the first page of the book. If the current cover can't be found, it adds the cover and a new first page in the book. This cover could be resized based on the conversion settings you normally use. But, if you have the output profile set to Tablet, the cover should be left full size.

I have no idea how to change those options, but the cover does show when I open/read the book. In fact, the best version of the cover is when I open the book, because it appears to fill the whole screen, in high quality vs the tall aspect ratio with the black bars on the screensaver.

Originally Posted by davidfor

With all that, you should be getting exactly the same cover image in the book in either way. And the device should be using it and resizing it in the same way. And because of how the cover image is extracted from the kepub, sending the covers using the driver has never been as useful as for epubs and other formats.

I'll have a play and see if I can see anything. Finding a book suitable for testing could be the problem.

I get a distinct difference between polish covers + no 'upload covers' option selected in driver (good outcome) vs. 'upload covers' with the driver or koboutilities (bad outcome).

Also, If I select 'upload covers' in the driver or koboutilities after polishing the book I still get the bad outcome.

Thanks for looking into it, is there anything else I can do to help investigate?

edit, additional details:
Calibre 3.7
KTE 2.8.2
Kepub reader/writer 2.7.1
Polish Books 1.0.0
Kobo Utilities 2.5.2

[davidfor]

Originally Posted by GeoffR

https://www.kobo.com/nz/en/ebook/vic...thout-a-face-3
https://www.kobo.com/nz/en/ebook/the...t-comes-before


Edit: One thing I remember when trying to figure out what as happening with the KePub covers was that some (not all) of the problem books were missing the full-size image file (the one with filename ending in N3_FULL), so the sleep screen must have been using the smaller thumbnail image as a fallback.

I had a look at those books on the Aura ONE. The cover for "The Darkness That Comes Before" low resolution and shows it. You can even see it on the store. And gets worse on the device. The other doesn't look to bad. But, the cover sent down is different to that in the book - the background is more detailed and includes birds flying across the title.

On the Glo, they looks worse. But, the second one isn't to bad.

I had a look at a some of the books I've bought, and I don't see any differences.

[JSWolf]

Question, what size is the original cover in the book?

[GeoffR]

Originally Posted by mgrimace

is there anything else I can do to help investigate?

Instead of taking a photograph of the screen, you could look at the actual image file from the device. I.e. after sending the book to the device, plug it back in and make a copy of the image that it generates. Also make a copy of the image that Calibre sends when you use the 'send covers' option. The image used for the sleep screen ends in N3_FULL.parsed.

e.g. the book with filename:

/KOBOeReader/E/Egan, Axiomatic.epub

has the sleep screen image:

/KOBOeReader/.kobo-images/82/151/file____mnt_onboard_E_Egan,_Axiomatic_epub - N3_FULL.parsed

[JSWolf]

I've not had a problem with low-res cover images because I replace low-res with hi-res cover images.

[davidfor]

Originally Posted by mgrimace

Hi Davidfor, thanks for looking into it, I've tried a number of different combinations on my KA1. Any 'upload covers' option results in a lower-res screensaver. Specifically, the KT driver, the KTE driver, and/or Kobo Utilities.

Here is an example (kobo link for preview), I used this book because it's easier to see in the gradients below the "P" of the title:
image 1: cover image inside the book (when I start the book for the first time, not the screensaver)

For reference, the cover image is 1000x1500 in Calibre, and the original file in Calibre is epub, converted to kepub on transfer

image 2: screensaver image of the cover using 'upload covers' option

note, this has the 'keep aspect ratio' option unchecked, but the quality is the same for aspect ratio on/off, and greyscale on/off. It is the same overall effect for the KT/KTE drivers, or manual send cover with Kobo Utilities



When I use polish books on the original epub, then send the book over using the KTE driver, but 'upload covers' off, I get a high quality cover, example:

Image 3: high quality screensaver cover image via polish books


I should clarify, when I use the polish books option, the cover is momentarily blank on the KA1 after sending the book over (≤1s, but noticeable), then it seems to generate the cover.



I have no idea how to change those options, but the cover does show when I open/read the book. In fact, the best version of the cover is when I open the book, because it appears to fill the whole screen, in high quality vs the tall aspect ratio with the black bars on the screensaver.



I get a distinct difference between polish covers + no 'upload covers' option selected in driver (good outcome) vs. 'upload covers' with the driver or koboutilities (bad outcome).

Also, If I select 'upload covers' in the driver or koboutilities after polishing the book I still get the bad outcome.

I did have a play with this last night and I did find a possible cause for the problem. There is a tweak to set the maximum size of the cover in calibre. This affects the generation of the cover when sending it to the Kobo device. I had this set to something like 1200x1600. What that meant was that when I added a cover to calibre that was larger than this it was automatically resize to this size (smaller cover weren't touched). And when the cover was generated by the driver this was the maximum size it would produce. The cover size for the Aura ONE is 1404x1872. A lot of my covers are between 1400 and 1600 in height, so they get resized a little bit to fix this when sent to the device. These look files look good when viewed on the PC.

This size also affects what gets put into the book by a Polish or any of the other actions that change the cover. So, the covers in the books were the same size and looked the same on the PC as the cover generated and sent by the driver.

The difference must be in how the device treats this. When it needs to extract the cover image from the book and generate it, it appears to be doing a good job with any resizing. But, I'd say when doing the cover display, it is a lot simpler. The resizing done at that time shows a lot more artefacts. My guess is that the device expects the stored cover image to be the right size for the screen, it doesn't do as much processing.

The solution for me was to reset the tweak to the default values. These are 1650x2200. Once I did that, the driver generate cover images the same size as the device. In my quick look at them before going to bed last night, I couldn't see any differences between the cover images generated by calibre and the device. That was viewing them on the device.

To check this, the tweak is called "maximum_cover_size". To access them, open the preferences and select "Tweaks" near the bottom of the screen. Select the tweak in the list on the left to see the value. I simple hit the "Reset Tweak" value to go back to the default. I suspect I set this to a smaller size when using the Glo. It made a bit of sense then, but not as much now.

If this doesn't solve it, then I'd like to look at the actual images generated. Also, I don't see what firmware version you are running. There have been at least two changes in the firmware that could affect this. One probably only affects epubs, but the change to 32bpp processing probably has a wider affect.

[mgrimace]

Originally Posted by davidfor

...

To check this, the tweak is called "maximum_cover_size". To access them, open the preferences and select "Tweaks" near the bottom of the screen. Select the tweak in the list on the left to see the value. I simple hit the "Reset Tweak" value to go back to the default. I suspect I set this to a smaller size when using the Glo. It made a bit of sense then, but not as much now.

If this doesn't solve it, then I'd like to look at the actual images generated. Also, I don't see what firmware version you are running. There have been at least two changes in the firmware that could affect this. One probably only affects epubs, but the change to 32bpp processing probably has a wider affect.

Hello Davidfor,

THANK YOU, this fixed my issue, the tweak was set to a lower resolution, and reapplying the defaults fixed it. I'll update the original post with the solution, and I appreciate you going on a hunt, this seemed a bit buried. Thanks again to everyone who helped as well GeoffR, JSWolf.

GeoffR, I won't go on a hunt for the cover images in the file, but I'll keep that in mind in case any other issues crop up (vs. taking photos of the screen). I appreciate the tip, and also where to find the files!

~M