bugfix(physics): Fix diagonal movement speed discrepancy

[xezon]

• Fixes Locomotor makes units move faster diagonally #123

This change fixes the diagonal movement speed discrepancy.

The new 2d and 3d speeds are scaled to the average of the former min and max speeds.

TODO

• Replicate in Generals

[greptile-apps]

Greptile Summary

The PR introduces compatibility-controlled 2D and 3D forward-speed compensation in Zero Hour.

• Adds shared flags for preserving retail and scripted-object movement behavior.
• Replaces the Zero Hour forward-speed calculation with scaled dot products when compatibility is disabled.

Confidence Score: 4/5

The PR is not ready to merge because enabling the correction still leaves the independently built Generals target with the original diagonal-speed discrepancy.

The shared controls imply cross-target behavior, while only the Zero Hour physics implementation consumes them; Generals always executes its unchanged retail calculations.

Files Needing Attention: Core/GameEngine/Include/Common/GameDefines.h and the corresponding Generals PhysicsUpdate.cpp

Important Files Changed

Filename	Overview
Core/GameEngine/Include/Common/GameDefines.h	Adds shared compatibility flags, but the Generals target has no implementation that consumes them.
GeneralsMD/Code/GameEngine/Source/GameLogic/Object/Update/PhysicsUpdate.cpp	Adds compatibility-gated diagonal compensation to the Zero Hour 2D and 3D forward-speed calculations.

Prompt To Fix All With AI

### Issue 1
Core/GameEngine/Include/Common/GameDefines.h:91-98
**Generals ignores correction flags**

When the compatibility flags are disabled to enable the diagonal-speed correction, Generals sees these shared definitions but compiles its separate, unchanged `PhysicsUpdate.cpp`, whose 2D and 3D functions always use the retail square-root calculations. The Generals build therefore retains the diagonal movement speed discrepancy regardless of these flag values, while only Zero Hour receives the corrected branches.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (3): Last reviewed commit: "Polish comments" | Re-trigger Greptile}

[gamezerve]

Why is the horizontal movement speed being increased? In retail, units already move at their locomotor-defined speed when traveling horizontally or vertically. Wouldn't slowing down diagonal movement be the more appropriate solution? Also, changing movement speeds will inevitably alter the timing of scripted in-game cinematics.

[xezon]

Why is the horizontal movement speed being increased? In retail, units already move at their locomotor-defined speed when traveling horizontally or vertically. Wouldn't slowing down diagonal movement be the more appropriate solution?

Because then on average the game unit movements will be around 20% slower than originally, noticably making the game play with less pace.

Also, changing movement speeds will inevitably alter the timing of scripted in-game cinematics.

That is a fair point we probably need to think about.

[Mauller]

This is wrong, the speed is not the dot product, the dot product tells you the difference in direction between the two vectors. If it goes negative then it means your vectors are going in opposite directions.

The speed of a vector is the magnitude of the vector.

The speed for 2D is:
speed = sqrtf( sqr(m_vel.x) + sqr(m_vel.y) ); which you can then scale with a constant

The speed for 3D is:
speed = sqrtf( sqr(m_vel.x) + sqr(m_vel.y) + sqr(m_vel.z) ); then the same can be scaled with a constant.

you still need to check the dot product and negate the speed if the dot product is negative.

[xezon]

dot is correct.

Chat Gippy

Here's a side-by-side comparison using a true velocity magnitude of 100 in each case.

Facing	Moving	dir	vel	Original Function	Dot Product
East	East	(1.000, 0.000)	(100.00, 0.00)	100.00	100.00
North	North	(0.000, 1.000)	(0.00, 100.00)	100.00	100.00
45°	45°	(0.707, 0.707)	(70.71, 70.71)	70.71	100.00
East	Northeast	(1.000, 0.000)	(70.71, 70.71)	70.71	70.71
45°	East	(0.707, 0.707)	(100.00, 0.00)	70.71	70.71
30°	30°	(0.866, 0.500)	(86.60, 50.00)	79.06	100.00
60°	60°	(0.500, 0.866)	(50.00, 86.60)	79.06	100.00
15°	15°	(0.966, 0.259)	(96.59, 25.88)	93.54	100.00
75°	75°	(0.259, 0.966)	(25.88, 96.59)	93.54	100.00

This reveals that the original function is effectively applying a heading-dependent scale factor:

• 0° / 90°: ×1.000

• 15° / 75°: ×0.935

• 30° / 60°: ×0.791

• 45°: ×0.707

So if getForwardSpeed2D() is used in movement logic rather than just for display, the old code was inherently reducing the reported speed whenever the unit faced away from the world axes. That could explain why replacing it with the mathematically correct dot product changed the feel of movement.

[Mauller]

The dot product is not correct for calculating the speed. The only relation the dot product has with the speed is the direction of the movement in relation to the orientation of the model/hull. So whether it is forwards or backwards etc.

in the code, Dir is the direction vector for the objects model/hull to tell which way it is facing and m_vel is the motion vector for the movement of the objects.

the dot product is used to tell the difference in angle between Dir and m_vel so we can determine if the object is moving backwards in relation to the direction it is facing. If the dot product is negative then the two vectors are facing in opposite directions and the speed will be negative relative to the objects orientation.

You have to workout the magnitude of m_vel to determine the speed of the object, which is the equivalent to using Pythagoras theorem to workout the hypotenuse of a triangle.

The flaw in the original code is that they used vy and vx which are intermediate products of calculating the dot product between Dir and m_vel. These should never be used outside of that calculation as they are meaningless outside of that context.

Since these intermediate products are not unit scaled they give the faster motion in the diagonal direction, but they also don't give the true speed either.

[xezon]

Problem is speed = sqrtf( sqr(m_vel.x) + sqr(m_vel.y) ); does not account for object direction. If the object is facing sideways then it will not produce the same direction drag (or lack thereof). What the proposed solution does is eliminate the diagonal speed variance, but otherwise preserve the original average speed.

I tested it in game and it looked right, but I did not do a lab test. Maybe it needs a lab test.

[Mauller]

The m_vel value is already normalised, which means it correctly scales in all directions without the diagonal calculated vector magnitudes being larger than expected. The flaw in the original code is that they don't correctly calculate the speed since they use the intermediate dot product values which are not normalised values.

This function is just returning speed which is only a scalar value, it has no direction information within it apart from forwards and backwards.

Both calculations need to be done, speed = sqrtf( sqr(m_vel.x) + sqr(m_vel.y) ); and the dot product is used to determine if the speed is positive or negative.

Beyond this, to make the speeds, on average, closer to the original flawed speeds you can then scale the calculated speed just by multiplying it with a constant. This will scale in all directions due to m_vel already being normalised.

so finalSpeed = scalingValue * speed * dotProductDirection the dot product direction just being if it's positive or negative.

[Ui-shigure-z]

i think it needs a test on helicopters to see what happened
hover locomotor is the only exception that oriention might be different with movement direction.
eg. 1 comanche and 1 battlemaster which has same speed moves parallelly towards same direction and BM is in comanche range.

[xezon]

I just tested a Comanche Helicopter and it looked ok. What is the problem with Helicopters?

[Ui-shigure-z]

i mean, if comanche not facing the moving direction, but attacking(therefore facing) another direction, in special case like orientation perpendicular to the movement direction, speed vector's projection on orientation vector could go deeply wrong, which is not likely to happen cuz there is nothing obviously wrong with the game.
the fact that nothing obviously wrong probably means orientation on hover units doesn't act like what we thought.
i'm not sure.

as for your previous jets circling test, shouldn't it be like jet's speed always have same direction as its orientation? why projection speed is different from real speed?

[xezon]

Please attach video of your problem.

[Ui-shigure-z]

ok it's a case that never happen in vanilla game.
in mod when helicopters turns to attack another unit the speed looks pretty normal, which means no issue

im not testing with dot product version because i knew even with EA version it's gonna be wrong somewhere
like if orientation vector is (0, 1) speed vector (10, 0) both dot and EA version will got 0 as an answer

still i don't know how exactly it works maybe need more investigation but it looks fine now.

another case which is reverse move that also have orientation doesn't match with speed direction but i saw they use negative speed eventually so it's two different stories

[Skyaero42]

Because then on average the game unit movements will be around 20% slower than originally, noticably making the game play with less pace.

The amount of diagonal and straight movements is highly map dependent. A 2-player game where players start in the corners will have relatively more diagonal movements - therefore the game will be slower than before - than on a map where players start in the (middle) top and bottom - and increases the game speed compared to before.

I feel like this solution is too crude. It is such a major hack that has significant impact on how the game feels.

IF this solution is considered, it will need extensive testing among the community on different maps..

Also, as Pathfinding uses a grid based algorithm, therefore most movements are either horizontal/vertical or diagonal, but rarely any other angle.

[Float1ngFree]

Hell yeah, long time due!
Will need to adjust deliveries of transport payloads for OCLs and some other movement related properties, but this must be done regardless of any naysayers.

[xezon]

The amount of diagonal and straight movements is highly map dependent. A 2-player game where players start in the corners will have relatively more diagonal movements - therefore the game will be slower than before - than on a map where players start in the (middle) top and bottom - and increases the game speed compared to before.

Yes. But the total average of all movements will be right between former min and max speeds.

IF this solution is considered, it will need extensive testing among the community on different maps..

I would like to have this run as a trial.

Also, as Pathfinding uses a grid based algorithm, therefore most movements are either horizontal/vertical or diagonal, but rarely any other angle.

I do not understand this statement. Movements are free into any direction, for both ground and air units.

[xezon]

Also, changing movement speeds will inevitably alter the timing of scripted in-game cinematics.

I have addressed this and confirmed that it works correctly in mission cinematics.

From my POV this change is final right now.

[xezon]

Needs review.

[penfriendz]

Is the scaling factor 1.207 (v(x) = 1+4/pi*(sqrt(2)-1)*x) or 4/pi ~= 1.27 (v(x) = cos(x) + sin(x) if x is positive)?

[xezon]

Here it is (1 + sqrt(2)) / 2