How to do it properly ? Is it different when ships are oriented perpendicularly ? Who got to decide when attack will be obstructed or not ?

Lines of fire are always the shortest line between two ships, that is still within the relevant firing arc. Usually, that's going to be from corner to corner between attacker and defender, but sometimes (as is demonstrated between the Lambda and Sheathipede in your image) is a line perpendicular to one of the bases.

Occasionally, there could be multiple "shortest lines"; usually when two ships are in the initial joust, and are pointed exactly at one another. In that case, for any given attack, that attacker chooses the precise line of fire , not the defender. Once the line is determined, that's when you check for obstruction, either by obstacles (like usual), ships (if they have Tactical Jammer), or devices (for ships like Captain Nym). Sometimes, an attacker will WANT to use the obstructed line, particularly for cases like Scum Han Solo with Trick Shot. When the other ship gets a turn to attack, it too may choose any line of fire to the defender, provided there is no shorter line that can be chosen.

Yup.

One point in addition: you determine line of fire along the shortest line *that is in the relevant firing arc. So, typically measurements from front and back arcs will follow the line of the arc, because the arc does not exit the base at the corner of the plastic, rather at the corner of the cardboard.

In my case - Sheatipede-class to the rightmost Tie/In, is it obstructed both ways or the attackers can chose for themselves?

14 minutes ago, konbox said:

In my case - Sheatipede-class to the rightmost Tie/In, is it obstructed both ways or the attackers can chose for themselves?

Assuming the Sheathipede is the one firing, you have the take the line pictured, because it is as far to that side of the firing arc as it's possible to draw it and remain perpendicular to the ship base. If both the obstacle and the TIE were further to the left, you might get the ability to choose. On Vassal this is shown by the rangefinder line turning into a blue rectangle. If some of that is on the rock, and some isn't, you can choose.

On 2/3/2019 at 2:23 AM, konbox said:

In my case - Sheatipede-class to the rightmost Tie/In, is it obstructed both ways or the attackers can chose for themselves?

In this particular picture, they don't overlap by enough for an unobstructed shortest line to exist. That obstacle blocks all of them even though there's an overlap where the bases are parallel.

But in general, the attacker gets to pick the exact attack line if multiple are possible.

Just a point, arn’t the template guide nubs counted for measuring too? So that might help decide when ships are parallel and not firing to and from the side.

3 minutes ago, Andyjp said:

Just a point, arn’t the template guide nubs counted for measuring too? So that might help decide when ships are parallel and not firing to and from the side.

No, nubs are only included for bumps, overlapping rocks, and fleeing the map.

Cheers

dont forget they count for bombs!

6 minutes ago, Andyjp said:

Cheers

dont forget they count for bombs!

The nubs on the bombs count, yes.