dpi to cm wrong measure in different skins

[P5music]

This question is related to

https://groups.google.com/g/codenameone-discussions/c/ohobrY3v83s/m/ypQ8fmnvAQAJ

In my code there is the need to have a standard length. The instruction to do so is:

int frameHeight= (int) (Utils.dpi()* 0.393701);

This should be a centimeter. 

(Utils.dpi() is a convenience method to get the dpi from the Display.getInstance().getDeviceDensity() value)

I tested it with two skins:

Android Note5 (dpi=540 xxhdpi -> 212px)

iOS iPad3  (dpi=240 hdpi -> 94px)

The length measure is used in some HTML, displayed in a BrowserComponent.

When running in simulator I get that

the device is different - OK

the font size and the icons size or images size are the same - OK

the calculated pixels value for 1cm has different size, it is bigger in the Note5 skin.

Should I use CN.convertToPixels()?

Thanks in advance

[Shai Almog]

Look at the skin file properties file to see the ratio variable used to calculate the density of the display. Maybe there was a mistake in generating this specific theme file.
You can see the skins here: https://github.com/codenameone/codenameone-skins

[P5music]

It seems to be a skin issue for many device skins.

These skins show big buttons when the size is from the material icon set. They also show that the cm length is big.

The cm length is meant to be what the simulator CEF BrowserComponent shows as a fixed value. It is not a real cm I think but this is another issue of Milestone 7.0.  It is the same whatever device is chosen as skin, it is sometime blatantly wrong because it is in the simulator, not the device.

So let's consider just the differences from right and wrong cm measure from dpi value.

(Other buttons are instead sized according the dpi constant (not the dpi value) with hard coded measure from the Android version of my app, for example 32,48,72 and so on, they happen to be re right, so they will not be considered.)

Some skins with big buttons (material icon set) and big cm length:

Note5, Pixel2XL, Pixel, Pixel2, HTCA9, HTCM8,Nexus5X,Nexus6P, S8

skins with small buttons (material icon set) and big cm length

iPhone6PLus, iPhone7XPlus, iPhone8Plus, X

skins that have big button size (material icon set) and right cm length, for example:

MotoE, MotoG, Nexus4, GrandPrime, S3,iPhone5c

skins with small buttons (material icon set) and right cm length

iPhone6S, iPhone7, iPhone8

[Shai Almog]

I would assume the problem is in the CEF browser and not in the skin. Did you check the values in the properties file and verify that they match the stated DPI of the device?

[P5music]

I cannot find the dpi value in that list.

The problem is with devices sporting the Display.DENSITY_HD=60 device density, that corresponds to XXHDPI devices.

I had dpi=540 as the corresponding value (it is the value used in calculations), I do not remember why I put that value. 

Also with the dpi=480 value (from Android development website) the cm length is too big in the CEF BrowserComponent, although smaller.

These are the devices with that device density HD=60 -> 540/480 dpi

Android

pixel pixel2 pixel2xl htconea9 htconem8 huaweyp8 nexus5x nexus6p note5 s5 s7 s8

iPhone

X 5plus 6splus 7splus 8splus

Maybe the dpi value is wrong but also maybe the device density value is wrong.

Regards

[Shai Almog]

It's calculated based on the pixelRatio value: https://github.com/codenameone/codenameone-skins/blob/master/HTC10/skin.properties#L9

[P5music]

I read on the internet some formulas involving the pixelRatio but the value in the documents you refer to seems to be an order of magnitude larger than typical ratios. What is the correct formula? What that pixelRatio really mean here?

Regards

[Shai Almog]

It's pixel to millimeter ratio not to DPI ratio: https://github.com/codenameone/CodenameOne/blob/master/Ports/JavaSE/src/com/codename1/impl/javase/JavaSEPort.java#L11289-L11294