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Old 12-25-2020, 04:48 AM   #1
maraskan_user
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KOReader "Screen DPI" setting

The "Screen DPI" setting of KOReader is rather peculiar. I assume its main function is the scaling of the UI buttons/text with content having its separate setting in "Zoom (DPI)", but for some reason it also affects content font size, but specifically only fonts with no absolute size.

I tried to find out how "Screen DPI" is actually changing that font size, so I took a ruler and measured the length of a sample sentence while iterating through the DPI values. I got these values:

120 DPI 3.8 cm
Custom(288) 5.1 cm
320 DPI 5.3 cm
Auto 6.1 cm
480 DPI 6.4 cm
640 DPI 7.4 cm

Doing a linear regression gives approx.
SentenceLength = 0.007*DPI + 2.96
so the used Auto value calculates to ~ 448 DPI.

Now I'm wondering:

- How come changing the DPI doesn't linearly scale the content font size, double DPI, double size?

- Why would 448 DPI be chosen in Auto mode when my tablet is 288 DPI.

- Why isn't the used DPI value shown in the menu like "Auto DPI (448)". It just says "Auto DPI".
The source in "screen_dpi_menu_table.lua" goes like
Code:
text = dpi_auto and T(_("Auto DPI (%1)"), dpi_auto) or _("Auto DPI")
but it shows the DPI value neither on my android nor on my PW4.
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Old 12-25-2020, 04:12 PM   #2
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Screen dpi is hardcoded in each device description. In the case of android it is obtained using the native framework and it should always match your device DPI.

UI elements are not scaled by DPI unless you override your device defaults. They're scaled by the physical resolution of your device.

See https://github.com/koreader/koreader....lua#L371-L383 and https://github.com/koreader/koreader...ze.lua#L31-L80
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Old 12-25-2020, 07:24 PM   #3
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The "custom" entry in that menu will default to your device's actual DPI (once selected), and *will* behave differently than the actual default (Auto) [which doesn't really use DPI at all, hence it not showing any], as the links mentioned above detail. In practice, it should result in slightly tinier UI elements (only really noticeable > 300DPI).

Last edited by NiLuJe; 12-25-2020 at 07:32 PM.
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Old 12-25-2020, 07:29 PM   #4
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Also, define "content" ^^.

The value you select in the menu in ePub documents (w/ CRe) *will* go through that scaling computation, so it'll affect the base font size, yes.

CRe itself has another DPI setting that does... weird things, and that mostly everyone should keep to its defaults at 1/1 .
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Old 12-25-2020, 07:31 PM   #5
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The intent is that, given a similar AR, settings should give consistent results across different devices (remember, KOReader was born on Kindles, which were mostly all 6" and/or using similar AR).
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Old 12-26-2020, 07:24 AM   #6
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What would be the correct resolution on a Kobo Aura One?
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Old 12-26-2020, 07:39 AM   #7
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Quote:
Originally Posted by danielr View Post
What would be the correct resolution on a Kobo Aura One?
The Kobo Aura One is 300DPI.
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Old 12-26-2020, 08:25 AM   #8
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Quote:
Originally Posted by NiLuJe View Post
Also, define "content" ^^.

The value you select in the menu in ePub documents (w/ CRe) *will* go through that scaling computation, so it'll affect the base font size, yes.

CRe itself has another DPI setting that does... weird things, and that mostly everyone should keep to its defaults at 1/1 .
I disagree. Results are overall better at 212 or 300, imo. (E.g., images aren't minuscule.)
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Old 12-26-2020, 11:48 AM   #9
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Quote:
Originally Posted by danielr View Post
What would be the correct resolution on a Kobo Aura One?
Quote:
Originally Posted by NiLuJe View Post
The "custom" entry in that menu will default to your device's actual DPI (once selected)
----

@Frenzie: IIRC, there's a couple style tweaks that achieve more or less the same thing without the potential extra weirdness of that setting ^^.
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Old 12-26-2020, 01:09 PM   #10
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The full width image one isn't relevant to ornaments and formulae. The effect would be significantly more disastrous.

One that says "display the image at 2x or 3x its declared size" would work… but that's basically what that DPI setting does.
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