VuforiaでImageTargetのサイズでカメラ画像をトリミングする

ImageTrimmer.cs

using UnityEngine;

using Vuforia;

public class ImageTrimmer : MonoBehaviour
{
    private ImageTargetBehaviour imageTargetBehaviour;

    #if UNITY_EDITOR
    private const Image.PIXEL_FORMAT PixcelFormat = Image.PIXEL_FORMAT.RGBA8888;
    private const TextureFormat TexFormat = TextureFormat.RGBA32;
    #else
    private const Image.PIXEL_FORMAT PixcelFormat = Image.PIXEL_FORMAT.RGB888;
    private const TextureFormat TexFormat = TextureFormat.RGB24;
    #endif

    void Start()
    {
        imageTargetBehaviour = GetComponent<ImageTargetBehaviour>();

        CameraDevice.Instance.SetFrameFormat(PixcelFormat, true);
    }

    public Texture2D Trim()
    {
        if(imageTargetBehaviour.CurrentStatus != TrackableBehaviour.Status.TRACKED)
        {
            return null;
        }

        var image = CameraDevice.Instance.GetCameraImage(PixcelFormat);
        if(image == null || !image.IsValid()){ return null; }

        var wholeTexture = new Texture2D(image.Width, image.Height, TexFormat, false);
        wholeTexture.LoadRawTextureData(image.Pixels);
        wholeTexture.Apply();

        var fov = CameraDevice.Instance.GetCameraFieldOfViewRads();
        var markerSize = imageTargetBehaviour.GetSize();
        var position = transform.position; // ARCameraのWorldCenterModeがCAMERA以外の場合は座標変換する必要ありと思われ

        // X方向
        int startX, width;
        GetTrimStartAndLength(fov.x, markerSize.x, position.x, position.z, wholeTexture.width, out startX, out width);

        // Y方向
        int startY, height;
        GetTrimStartAndLength(fov.y, markerSize.y, position.y, position.z, wholeTexture.height, out startY, out height);

        // テクスチャの向き・原点を考慮して切り出し開始位置を指定する必要があり
        var colors = wholeTexture.GetPixels(wholeTexture.width - startX - width, startY, width, height);

        var trimTexture = new Texture2D(width, height, TexFormat, false);
        trimTexture.SetPixels(colors);
        trimTexture.Apply();

        return trimTexture;
    }

    private void GetTrimStartAndLength(float fov, float worldMarkerSize, float worldPosXY, float worldPosZ, int textureSize, out int start, out int length)
    {
        float tan = Mathf.Tan(fov / 2);
        float worldCameraViewSize = 2 * worldPosZ * tan;

        float a = 0.5f - ((worldPosXY + worldMarkerSize * 0.5f) / worldCameraViewSize);
        float b = worldMarkerSize / worldCameraViewSize;
        float c = 0.5f + ((worldPosXY - worldMarkerSize * 0.5f) / worldCameraViewSize);

        start = (int)(textureSize * a);
        length = (int)(textureSize * b);
    }
}