Models stutter due to client side interpolation

[SamVanheer]

When Half-Life 1 was updated to handle studio model rendering in the client dll with SDK 2.0 a bug started occurring where studio models moved by other entities like func_tracktrain would stutter.

Here's a video that shows the stuttering as well as the expected behavior: video

The most likely cause is that the time value used by model rendering is being overwritten by an older time value used by the client side prediction code. The animation time values jump back and forth from the present to the past in a way that causes interpolation to sometimes interpolate to some point in the future and sometimes to some point in the past, causing the model to "pinball" between the past and future interpolated positions.

The cause of this is somewhere in the engine, not fixable in SDK code as-is. A workaround exists to disable interpolation in these cases to prevent the stuttering effect from occurring, originally provided by Uncle Mike on the HLFX forums: https://hlfx.ru/forum/showthread.php?s=&threadid=15&highlight=%C8%ED%F2%E5%F0%EF%EE%EB%FF%F6%E8%FF

(Unfortunately you need an account to access this thread)

The changes required are as follows:

Change this line:

halflife/dlls/nihilanth.cpp

Line 291 in c7240b9

pev->flags |= FL_MONSTER;

To this:

pev->flags |= FL_MONSTER | FL_FLY;

Change this line:

halflife/dlls/osprey.cpp

Line 156 in c7240b9

pev->flags |= FL_MONSTER;

To this:

pev->flags |= FL_MONSTER | FL_FLY;

(the same change is needed in Opposing Force's Black Ops Osprey code)

After this line:

halflife/dlls/cbase.h

Line 149 in c7240b9

CBaseEntity *m_pLink;// used for temporary link-list operations.

Add this:

/**
*	@brief Entity flags sent to the client in ::AddToFullPack
*/
byte m_EFlags = 0;

After this line:

halflife/dlls/cbase.cpp

Line 586 in c7240b9

DEFINE_FIELD( CBaseEntity, m_pGoalEnt, FIELD_CLASSPTR ),

Add this:

DEFINE_FIELD(CBaseEntity, m_EFlags, FIELD_CHARACTER),

After this line:

halflife/dlls/tentacle.cpp

Line 258 in c7240b9

pev->sequence = 0;

Add this:

//Always interpolate tentacles since they don't actually move.
m_EFlags |= EFLAG_SLERP;

After this line:

halflife/dlls/client.cpp

Line 1097 in c7240b9

int i;

Add this:

auto entity = reinterpret_cast<CBaseEntity*>(GET_PRIVATE(ent));

Change this logic:

halflife/dlls/client.cpp

Lines 1194 to 1201 in c7240b9

	if ( !player &&
	ent->v.animtime &&
	ent->v.velocity[ 0 ] == 0 &&
	ent->v.velocity[ 1 ] == 0 &&
	ent->v.velocity[ 2 ] == 0 )
	{
	state->eflags |= EFLAG_SLERP;
	}

To this:

if ((ent->v.flags & FL_FLY) != 0)
{
	state->eflags |= EFLAG_SLERP;
}
else
{
	state->eflags &= ~EFLAG_SLERP;
}

state->eflags |= entity->m_EFlags;

Change this logic:

halflife/cl_dll/StudioModelRenderer.cpp

Lines 474 to 477 in c7240b9

	for (i = 0; i < 3; i++)
	{
	modelpos[i] += (m_pCurrentEntity->origin[i] - m_pCurrentEntity->latched.prevorigin[i]) * f;
	}

To this:

const auto pseqdesc = (mstudioseqdesc_t*)((byte*)m_pStudioHeader + m_pStudioHeader->seqindex) + m_pCurrentEntity->curstate.sequence;

if ((pseqdesc->motiontype & STUDIO_LX) != 0 || (m_pCurrentEntity->curstate.eflags & EFLAG_SLERP) != 0)
{
	for (i = 0; i < 3; i++)
	{
		modelpos[i] += (m_pCurrentEntity->origin[i] - m_pCurrentEntity->latched.prevorigin[i]) * f;
	}
}

These changes restrict model interpolation to animations that have movement on the X axis and entities that have interpolation enabled, which is enabled for any entity that sets the FL_FLY flag. Entities can also force interpolation by setting the EFLAG_SLERP flag on CBaseEntity::m_EFlags.

Tentacles will not interpolate correctly when loading a save game made before these code changes were applied.

A better solution would be to identify the cause of the bug (likely the incorrect time values) and fixing that, but that would require an investigation and testing to verify that the fix doesn't break anything that may be depending on this buggy behavior somehow.

[di57inct]

Is this somehow related to the bug where if a player would stand on top of another crouching player his camera would stutter vertically?

[SamVanheer]

Is this somehow related to the bug where if a player would stand on top of another crouching player his camera would stutter vertically?

I don't think so, this only affects the model's render position, not the player's camera position.

[di57inct]

Understood. Thanks.

[SamVanheer]

Added additional step to fix monster_osprey not interpolating.

[SamVanheer]

Added additional steps to fix monster_tentacle not interpolating.