Graphics.DrawMesh has wired offset while scaling in Unity, why? - unity3d

Question

I'm implementing after image effect currently and I meet a problem with Graphics.DrawMesh. The code shows below
public class AfterImage3DByCombine : MonoBehaviour
{
public class AfterImange
{
public Mesh mesh;
public Material material;
// public Matrix4x4 matrix;
public float duration;
public float time;
}
protected SkinnedMeshRenderer[] skinRenderers;
protected MeshFilter[] filters;
protected int filtersCount = 0;
public bool IncludeMeshFilter = true;
public Material EffectMaterial;
public float Duration = 5;
public float Interval = 0.2f;
public float FadeoutTime = 1;
private float mTime = 5;
private List<AfterImange> mAfterImageList = new List<AfterImange>();
protected virtual void Awake()
{
skinRenderers = GetComponentsInChildren<SkinnedMeshRenderer>();
if (IncludeMeshFilter)
{
filters = GetComponentsInChildren<MeshFilter>();
filtersCount = filters.Length;
}
}
//call from another place to have after image effect
public void Play()
{
if (skinRenderers.Length + filtersCount <= 0)
{
return;
}
mTime = Duration;
StartCoroutine(AddAfterImage());
}
IEnumerator AddAfterImage()
{
while (mTime > 0)
{
CreateImage();
yield return new WaitForSeconds(Interval);
mTime -= Interval;
}
yield return null;
}
void CreateImage()
{
CombineInstance[] combineInstances = new CombineInstance[skinRenderers.Length + filtersCount];
int index = 0;
for (int i = 0; i < skinRenderers.Length; i++)
{
var render = skinRenderers[i];
var mesh = new Mesh();
render.BakeMesh(mesh);
combineInstances[index] = new CombineInstance
{
mesh = mesh,
transform = render.gameObject.transform.localToWorldMatrix,
subMeshIndex = 0
};
index++;
}
for (int i = 0; i < filtersCount; i++)
{
var render = filters[i];
var temp = (render.sharedMesh != null) ? render.sharedMesh : render.mesh;
var mesh = (Mesh)Instantiate(temp);
combineInstances[index] = new CombineInstance
{
mesh = mesh,
transform = render.gameObject.transform.localToWorldMatrix,
subMeshIndex = 0
};
index++;
}
Mesh combinedMesh = new Mesh();
combinedMesh.CombineMeshes(combineInstances, true, true);
mAfterImageList.Add(new AfterImange
{
mesh = combinedMesh,
material = new Material(EffectMaterial),
time = FadeoutTime,
duration = FadeoutTime,
});
}
void LateUpdate()
{
bool needRemove = false;
foreach (var image in mAfterImageList)
{
image.time -= Time.deltaTime;
if (image.material.HasProperty("_Color"))
{
Color color = Color.red;
color.a = Mathf.Max(0, image.time / image.duration);
image.material.SetColor("_Color", color);
}
Matrix4x4 mat = Matrix4x4.TRS(Vector3.zero, Quaternion.identity, Vector3.one * 2f);
//public static void DrawMesh(Mesh mesh, Matrix4x4 matrix, Material material, int layer, Camera camera, int submeshIndex, MaterialPropertyBlock properties, ShadowCastingMode castShadows);
Graphics.DrawMesh(image.mesh, Matrix4x4.identity, image.material, gameObject.layer, null, 0, null, false);
if (image.time <= 0)
{
needRemove = true;
}
}
if (needRemove)
{
mAfterImageList.RemoveAll(x => x.time <= 0);
}
}
}
Since my prefab has 0.5 times scaling while it's running, then I pass a matrix with two times scaling Matrix4x4 mat = Matrix4x4.TRS(Vector3.zero, Quaternion.identity, Vector3.one * 2f); into
Graphics.DrawMesh.
Then, the mesh created by Graphics.DrawMesh isn't in its original position, there is an offset between original mesh and created mesh.
And if I passed Matrix4x4.Identity into Graphics.DrawMesh, the created mesh will have 0.5 times scaling, which looks smaller than original mesh.
Why there is an offset and how could I eliminate the offset without chaning the prefab's scale?