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Old 09-06-2014, 06:31 PM   #1
il_mix
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Best image resolution for cbz/cbr

I would like to resize the images of my cbz/cbr on PC so that they are optimized for Kobo Touch. This means they won't be resized by the device, because the width or height are set to the maximum available value.

I've read this thread, where it is said that the resolution is 600x800.

I made a test to verify this. I've created an image with a black and white checkboard with 1px square size, and saved it in PNG (grayscale and RGB).
The image was than cropped to create:
- images with fixed height (200px) and increasing width (from 591px to 610px) to test the horizontal resolution
- images with fixed width (200px) and increasing height (from 791px to 810px) to test the vertical resolution. The images was archived in the attached file (rename as .cbz to test), so that:
- images 1 to 20 are grayscale images with increasing width
- images 21 to 40 are grayscale images with increasing height
- images 41 to 60 are RGB images with increasing width
- images 61 to 80 are RGB images with increasing height

Reading the cbz image on the Kobo, with refresh on every page turn, I've seen that:
- almost all images creates visual artifacts, due to resize operations by the device. The cleaner images are the ones with width=600px and the ones with height=800px
- the images are rendered in different ways if I turn the pages forward or backwards (?!?!?)
- grayscale and RGB images are displayed in the same way (regarding the resolution)

Sooooo, it seems that 600x800 images won't be resized by the Kobo Touch. So images will be displayed as-is (not resized) if they have width=600 and height<=800, or height=800 and width<=600.

What do you think about it?
Can you test the attached file and share your opinion?
!BEWARE! The Kobo automatically turn the first image, that is an horizontal image, by 90° (to maximize its size on the display). Remember to turn it again in it's original position to make the test correctly! After that, the other images are displayed correctly.


NOTE: I will remark that I'm talking about CBZ/CBR archives, NOT ePub books (where images maximum size may vary due to margins and other settings)
Attached Files
File Type: zip KoboSizeTest1pxWRGB.zip (31.1 KB, 20 views)

Last edited by il_mix; 09-06-2014 at 07:03 PM.
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Old 09-07-2014, 08:22 PM   #2
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Hi, il_mix!

I tried to see zip file. I think we should change the extension of this file from zip to cbz to watch these images in a cbz file. And I think you should make the images sorted by numbers, like 01, 02, 03....

Then we can see the images in cbz file.
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Old 09-08-2014, 03:40 AM   #3
il_mix
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The file in .zip format because .cbz can't be attached on forum messages. Just rename the file with .cbz and it can be used.
The images are already sorted by name, but they are named given the horizontal/vertical size, not 001, 002, ...
As said, the images are ordered this way:
- images 1 to 20 are grayscale images with increasing width (591px to 610px)
- images 21 to 40 are grayscale images with increasing height (791px to 810px)
- images 41 to 60 are RGB images with increasing width (591px to 610px)
- images 61 to 80 are RGB images with increasing height (791px to 810px)
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Old 09-08-2014, 04:05 AM   #4
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Quote:
Originally Posted by il_mix View Post
- the images are rendered in different ways if I turn the pages forward or backwards (?!?!?)
Couldn't it just be ghosting from the next or previous image?

By the way, I personly prefer be to use large images, and then let Kobo do the downscaling. This enables you to zoom in while reading, and will also be compitible with ereaders with larger screens.
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Old 09-08-2014, 04:56 AM   #5
il_mix
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Quote:
Originally Posted by Kasper Hviid View Post
Couldn't it just be ghosting from the next or previous image?
I've set refresh on every page, so I think there is no ghosting.

EDIT: well, it seems that the different behavior when turning pages forward or backward is really connected to ghosting. This leads to a new question: why is there still ghosting even when setting refresh every page turn? Looks like the page is not really completly refreshed...

Last edited by il_mix; 09-08-2014 at 07:56 AM.
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Old 09-08-2014, 08:39 AM   #6
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Smile The results are very interesting

Hi il_mix!

Sorry for my misunderstanding. I see what you mean now. Kobo automatically turned the direction of cbz file and I've got lost. I made new test file from your file and uploaded.

The instruction for this file is as follows.
1) Download it
2) Change the file extension from zip to cbz
3) Transfer it in your kobo
4) Open it and check and change the direction on the first image which indicates the upside.
5) Go through the cbz files which has marks on each 600x800 or 800x600 files.

The results were very interesting. Exactly 600x800 or 800x600 files are the finest and they change the direction of artifacts from holizontal to vertical around 600x800 or 800x600 files.

I think we can get finest image by exactly 600x800 or 800x600 files on cbz images.
Attached Files
File Type: zip SizeTest.zip (32.7 KB, 15 views)
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Old 09-09-2014, 03:48 AM   #7
il_mix
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Yeah, I think that 600x800 images are actually rendered as-is (no resizing) on Kobo Touch, so that are the best image dimensions for cbz/cbr books.
The fact that in this test a 600px with image (or 800px height) looks "better rendered" than the others instead of "as-is rendered" is due to ghosting. Again, it's kinda upsetting that there is still ghosting when refreshing every page...
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