Garmin jnx file format hacking

[Bill_clark]

Re: Garmin jnx file format hacking

The device shows only one demo map, and refuses to load all other.

Does it mean that, even in demo mode, we can create one and only one big jnx file? In that case, what is the limit in the size of the jnx file? Did you test that?

Bill.

[WHITEDRAG0N]

Re: Garmin jnx file format hacking

Correct the Demo.JNZ is infact stanstead airport. Nice work.

I think some testing into the size limitation is need. Or we need some way of fooling the garmin into thinking we have a valid subscription. Maybe we could make on up with an expire date set in 2050.

Any ways i am happy to help with anything anyone wants.

[roger47]

Re: Garmin jnx file format hacking

Two pieces of information from looking at that JNX file : The coordinate sets are Maximum Latitude, Maximum Longitude, Minimum Latitude, Minimum Longitude; and the projection is not Mercator.

[roger47]

Re: Garmin jnx file format hacking

lulsgate.zip

This probably won't work but I have to start somewhere. If you need to locate it, try 51.4N, 2.5W

Everything I don't know is filled with zeros (like demo mode). There is only one level.

[Bill_clark]

Re: Garmin jnx file format hacking

[ATTACH]1908[/ATTACH]

This probably won't work but I have to start somewhere. If you need to locate it, try 51.4N, 2.5W

Everything I don't know is filled with zeros (like demo mode). There is only one level.

I transferred the file in the BirdsEye directory on my GPS. When I start the GPS, during the boot process, I have a message : "Invalid BirdsEye File". I click on "OK" and the boot process continue... I check to 51.4N, 2.5W and there is nothing. :-(

Bill.

[roger47]

Re: Garmin jnx file format hacking

.... and I reckon the 'scale' is some measure like 'millimetres per pixel' (but probably not exactly that)!
This imagery is strange. A 256x256 tile has the same delta for Latitude as for Longitude whereas for normal maps the ratio is the cosine of the latitude. So the raw jpg images in the file look squashed.

[Bill_clark]

Re: Garmin jnx file format hacking

I rename the file lulsgate.jnx to Demo.jnx : same result.

Bill.

[roger47]

Re: Garmin jnx file format hacking

3. Map loader info
This part of JNX does not seem to be used for the map rendering

I think this does, in fact, contain some vital information - 300+ bytes of it. My attempt just skipped it. Tomorrow, I will try copying a block from the demo.jnx and see if that is any better.

[Whiter]

Re: Garmin jnx file format hacking

In my format description, I've forgot to mention the end of file marker. Sorry for this.
Each JNX should end with "BirdsEye" text. I've updated the pages on my site.

I've added that marker to your test file, still it didn't work.

[Whiter]

Re: Garmin jnx file format hacking

Roger, I've managed to make your demo JNX loadable. To do this, I had to change its scale, and take some coordinates from Garmin's stock BirdsEye demo file. Also I've cut the header to 4 tiles.

Here's the file: lulsgate1.zip
In this JNX, Bristol resides in Paris (check N 48°51.5', E 2°17.67') - because of the Garmin's demo showed the Effel tower.
Anyway, the images are really displayed in my GPSMAP 62s. So the map loader info really is an optional part of the demo file.

[WHITEDRAG0N]

Re: Garmin jnx file format hacking

Confirm that this file works. all be it a little fuzzy when i zoom in. i suspect that this is due to there being no other images at lower zoom levels

Nice one.

[Bill_clark]

Re: Garmin jnx file format hacking

Roger, you are used to work with coordinates : do you have a formula to calculate the distance between to coordinates? I suspect that the scale factor is in relation with that.... Based on the fact that the map seems to be not mercantor, how can we calculate the distance?

Bill.

[roger47]

Re: Garmin jnx file format hacking

Looking at the demo.zip.... In level 3, the tiles are 256 pixels high. The difference from top to bottom latitude on one tile is .0013733 degrees or 5.35e-06 degrees per pixel. The old definition of a meter is one ten-millionth of the distance from the north pole to the equator (through Paris!). So each degree is 1/90th of this. So there are 111,111,111 metres per degree (north-south) 5.36e-06* 111,111,111,111 (mm) = 596 which is as close to the Garmin scale (597) as (maybe) makes no difference. At higher levels the figures aren't as close but still within 1%. I suspect these figures are 'hard-coded' so 597 is valid but 596 and 598 are not. I will make some experiments.....

[roger47]

Re: Garmin jnx file format hacking

This zip contains 2 maps to try. The only difference is the scale : G1 has 4777, G2 has 4778. I suspect one will work and the other fail.....

[Bill_clark]

Re: Garmin jnx file format hacking

Roger : both don't work....

Bill.