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bugfix(physics): Fix diagonal movement speed discrepancy #3003

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4 changes: 4 additions & 0 deletions Core/GameEngine/Include/Common/GameDefines.h
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Original file line number Diff line number Diff line change
Expand Up @@ -87,6 +87,10 @@
#define PRESERVE_RETAIL_SCRIPTED_CAMERA (1) // Retain scripted camera behavior present in retail Generals 1.08 and Zero Hour 1.04
#endif

#ifndef PRESERVE_RETAIL_PHYSICS_FORWARD_SPEED
#define PRESERVE_RETAIL_PHYSICS_FORWARD_SPEED (1)
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#endif

#ifndef RETAIL_COMPATIBLE_CRC
#define RETAIL_COMPATIBLE_CRC (1) // Game is expected to be CRC compatible with retail Generals 1.08, Zero Hour 1.04
#endif
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25 changes: 21 additions & 4 deletions GeneralsMD/Code/GameEngine/Source/GameLogic/Object/Update/PhysicsUpdate.cpp
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Expand Up @@ -963,15 +963,22 @@ Real PhysicsBehavior::getForwardSpeed2D() const

Real dot = vx + vy;

Real speedSquared = vx*vx + vy*vy;
// DEBUG_ASSERTCRASH( speedSquared != 0, ("zero speedSquared will overflow sqrtf()!") );// lorenzen... sanity check

Real speed = (Real)sqrtf( speedSquared );
#if RETAIL_COMPATIBLE_CRC || PRESERVE_RETAIL_PHYSICS_FORWARD_SPEED
Real speed = (Real)sqrtf( vx*vx + vy*vy );

if (dot >= 0.0f)
return speed;

return -speed;
#else
// Inverse scale len by (1 + sqrt(2)) / 2 to adjust to the average of the former min/max movement speed.
// The inverse looks intuitively wrong, but it is correct, because the value returned by this function is
// used to determine the additional velocity needed to reach the target speed.
constexpr const Real DiagonalCompensation = 1.0f / 1.20710678f;
dot *= DiagonalCompensation;

@Mauller Mauller Jul 22, 2026 •
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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 xezon Jul 22, 2026

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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 Mauller Jul 23, 2026

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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 xezon Jul 23, 2026 •
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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 Mauller Jul 23, 2026 •
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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.

@Mauller Mauller Jul 23, 2026 •
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So similar to what i put in the original issue

This should be the actual fix for this.

Real PhysicsBehavior::getForwardSpeed2D() const
{
	//Determine the direction of the velocity relative to the direction the unit is facing using the dot product
	const Coord3D *dir = getObject()->getUnitDirectionVector2D();
	Real vx = m_vel.x * dir->x;
	Real vy = m_vel.y * dir->y;
	Real dot = vx + vy;

	//Return the speed of the unit - Magnitude of the velocity is the speed
	if (dot >= 0.0f)
		return (Real)sqrtf( sqr(m_vel.x) + sqr(m_vel.y) ) * scalingValue;

	//Negative dot product means the unit is moving in reverse
	return -(Real)sqrtf( sqr(m_vel.x) + sqr(m_vel.y) ) * scalingValue;
}

And for 3D it is the same

Real PhysicsBehavior::getForwardSpeed3D() const
{
	//Determine the direction of the velocity relative to the direction the unit is facing using the dot product
	Vector3 dir = getObject()->getTransformMatrix()->Get_X_Vector();
	Real vx = m_vel.x * dir.X;
	Real vy = m_vel.y * dir.Y;
	Real vz = m_vel.z * dir.Z;
	Real dot = vx + vy + vz;

	//Return the speed of the unit - Magnitude of the velocity is the speed
	if (dot >= 0.0f)
		return (Real)sqrtf( sqr(m_vel.x) + sqr(m_vel.y) + sqr(m_vel.z) ) * scalingValue;

	//Negative dot product means the unit is moving in reverse
	return -(Real)sqrtf( sqr(m_vel.x) + sqr(m_vel.y) + sqr(m_vel.z) ) * scalingValue;
}

@Skyaero42 Skyaero42 Jul 25, 2026

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I agree with @Mauller here, dot in this function is used for direction only, it does not reflect the speed of the object.

@Skyaero42 Skyaero42 Jul 25, 2026

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East
Northeast
(1.000, 0.000)
(70.71, 70.71)
70.71
70.71

Its a bit of a weird situation and don't know if it actually can occur (maybe helicopters?). But this basically says the unit is moving slightly sideways, like it is drifting. The speed with which it moves however is determined by the velocity vector only, hence why the dot product of direction and velocity cannot be used to calculate the speed.


return dot;
#endif
}

//-------------------------------------------------------------------------------------------------
Expand All @@ -989,12 +996,22 @@ Real PhysicsBehavior::getForwardSpeed3D() const

Real dot = vx + vy + vz;

#if RETAIL_COMPATIBLE_CRC || PRESERVE_RETAIL_PHYSICS_FORWARD_SPEED
Real speed = (Real)sqrtf( vx*vx + vy*vy + vz*vz );

if (dot >= 0.0f)
return speed;

return -speed;
#else
// Inverse scale len by (1 + sqrt(3)) / 2 to adjust to the average of the former min/max movement speed.
// The inverse looks intuitively wrong, but it is correct, because the value returned by this function is
// used to determine the additional velocity needed to reach the target speed.
constexpr const Real DiagonalCompensation = 1.0f / 1.36602540f;
dot *= DiagonalCompensation;

return dot;
#endif
}

//-------------------------------------------------------------------------------------------------
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