Monster Mass

[Peadly]

Hi, does anyone know what the purpose of a monsters mass is in the game?

[Use]

So they have some kind of weight, naturally. Make it very low and the critter goes flying when you shoot it.

[Peadly]

I figured that much, but does anybody know exactly the formula mass is involved in to calculate how far the monster is pushed back?

[RjY]

Peadly said:

I figured that much, but does anybody know exactly the formula mass is involved in to calculate how far the monster is pushed back?

From P_DamageMobj in p_inter.c

(fixed_t) thrust = damage*(FRACUNIT>>3)*100/target->info->mass;

This is saying that for a damage d imparted to an object of mass m the object receives a thrust of magnitude 100d/8m map units per gametic (masses are typically multiples of 100, FRACUNIT is a conversion factor for Doom's fixed point arithmetic)

So say for example the player (mass 100) gets hit by a baron of hell for 40 points of damage. He will fly backwards initially at (100*40)/(8*100) = 5 map units per tic (1/35 of a second)

The total distance travelled by the attacked object, provided it doesn't go over a cliff or something, will be determined by the friction of the sector it is in. If I'm reading this correctly, according to P_XYMovement in p_mobj.c velocity is typically reduced by multiplying its X and Y components by about 0.91 (ORIG_FRICTION / FRACUNIT) on each tic. This carries on until the object's speed is less than 1/16 map units per tic at which point it is stopped.

So suppose our hypothetical player is at (0,0) and has been hit by our hypothetical baron which was standing at (-128, 0). His x velocity thus becomes +5 units per tic, at time 0. Following through I got this

  0   0.000  5.000
  1   5.000  4.550
  2   9.550  4.141
  3  13.691  3.768
  4  17.458  3.429
  5  20.887  3.120
  6  24.007  2.839
  7  26.847  2.584
  8  29.430  2.351
  9  31.782  2.140
 10  33.921  1.947
 11  35.868  1.772
 12  37.640  1.612
 13  39.253  1.467
 14  40.720  1.335
 15  42.055  1.215
 16  43.270  1.106
 17  44.376  1.006
 18  45.382  0.916
 19  46.298  0.833
 20  47.131  0.758
 21  47.889  0.690
 22  48.579  0.628
 23  49.207  0.571
 24  49.778  0.520
 25  50.298  0.473
 26  50.771  0.431
 27  51.202  0.392
 28  51.594  0.357
 29  51.950  0.324
 30  52.275  0.295
 31  52.570  0.269
 32  52.839  0.245
 33  53.083  0.223
 34  53.306  0.202
 35  53.508  0.184
 36  53.693  0.168
 37  53.860  0.153
 38  54.013  0.139
 39  54.152  0.126
 40  54.278  0.115
 41  54.393  0.105
 42  54.498  0.095
 43  54.593  0.087
 44  54.679  0.079
 45  54.758  0.072
 46  54.830  0.065
 47  54.895  0.059

So he stops moving after 47 tics (about a second and a third), having moved nearly 55 map units.

I hope this made sense. Now someone tell me where I inevitably went wrong :)