'Prepare ._gts and ._dts for evaluation based on params
:return: None'

def _prepare(self):

    def _toMask(anns, coco):
        for ann in anns:
            rle = coco.annToRLE(ann)
            ann['segmentation'] = rle
    p = self.params
    if p.useCats:
        gts = self.cocoGt.loadAnns(self.cocoGt.getAnnIds(imgIds=p.imgIds, catIds=p.catIds))
        dts = self.cocoDt.loadAnns(self.cocoDt.getAnnIds(imgIds=p.imgIds, catIds=p.catIds))
    else:
        gts = self.cocoGt.loadAnns(self.cocoGt.getAnnIds(imgIds=p.imgIds))
        dts = self.cocoDt.loadAnns(self.cocoDt.getAnnIds(imgIds=p.imgIds))
    if (p.iouType == 'segm'):
        _toMask(gts, self.cocoGt)
        _toMask(dts, self.cocoDt)
    for gt in gts:
        gt['ignore'] = (gt['ignore'] if ('ignore' in gt) else 0)
        gt['ignore'] = (('iscrowd' in gt) and gt['iscrowd'])
        if (p.iouType == 'keypoints'):
            gt['ignore'] = ((gt['num_keypoints'] == 0) or gt['ignore'])
    self._gts = defaultdict(list)
    self._dts = defaultdict(list)
    for gt in gts:
        self._gts[(gt['image_id'], gt['category_id'])].append(gt)
    for dt in dts:
        self._dts[(dt['image_id'], dt['category_id'])].append(dt)
    self.evalImgs = defaultdict(list)
    self.eval = {}
