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The first step would be to get encoders ticks -> wheel velocity for the left and right wheel. Besides the tick count/motor revolutions this obviously depends on the wheel diameter.

Next, convert left/right velocity to translational and rotational velocity. Google should give you some simple formulas using the wheel base.

Given the measured time between two odometry readings, you now get a delta in translation and orientation that you can add to your last send odometry.

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