读coco数据集的代码接口了解segmentation的处理方法

COCO数据集是微软团队制作的一个数据集，通过这个数据集我们可以训练到神经网络对图像进行detection，classification，segmentation，captioning。具体介绍请祥见官网。

annotation格式介绍
mask存储处理方式简单介绍
相关代码分析
一个实例

annotation格式介绍

//从官网拷贝下来的
{"info": info,"images": [image],"annotations": [annotation],"licenses": [license],
}info{"year": int,"version": str,"description": str,"contributor": str,"url": str,"date_created": datetime,
}image{"id": int,"width": int,"height": int,"file_name": str,"license": int,"flickr_url": str,"coco_url": str,"date_captured": datetime,
}license{"id": int,"name": str,"url": str,
}
----------

Object Instance Annotations

Each instance annotation contains a series of fields, including the category id and segmentation mask of the object. The segmentation format depends on whether the instance represents a single object (iscrowd=0 in which case polygons are used) or a collection of objects (iscrowd=1 in which case RLE is used). Note that a single object (iscrowd=0) may require multiple polygons, for example if occluded. Crowd annotations (iscrowd=1) are used to label large groups of objects (e.g. a crowd of people). In addition, an enclosing bounding box is provided for each object (box coordinates are measured from the top left image corner and are 0-indexed). Finally, the categories field of the annotation structure stores the mapping of category id to category and supercategory names.

中文翻译如下：每个实例注释包含一系列字段，这些字段有category id和segmentation mask。segementation字段的格式取决于实例是代表单个物体（具体来说iscrowd=0，这时候就会用到polygon，也就是多边形）还是目标的集合体（此时iscrowd=1，会用到RLE，后面解释这个的意思）。注意到单个目标可能需要多个多边形来表示，例如在被遮挡的情况下。群体注释是用来标注目标的集合体（例如一群人）。除此之外，每个目标都会有一个封闭的外接矩形框来标记（矩形框的坐标从图像的左上角开始记录，没有索引）。最后，类别字段存储着category id到category和父级category名字的映射。

annotation{"id": int,"image_id": int,"category_id": int,"segmentation": RLE or [polygon],"area": float,"bbox": [x,y,width,height],"iscrowd": 0 or 1,
}categories[{"id": int,"name": str,"supercategory": str,
}]

mask存储处理方式简单介绍

上面提到coco数据集使用了两种方式进行mask存储，一是polygon，一是RLE。polygon比较好理解，就是多边形嘛！RLE是什么呢？

简单点来讲，RLE是一种压缩方法，也是最容易想到的压缩方式。

举个例子：M = [0,0,0,1,1,1,1,1,1,0,0]，则M的RLE编码为[3,6,2]，当然这是针对二进制进行的编码，也是coco里面采用的。RLE远不止这样简单，我们这里并不着重讲RLE，请百度吧。

代码中注释说的

# RLE is a simple yet efficient format for storing binary masks. RLE
# first divides a vector (or vectorized image) into a series of piecewise
# constant regions and then for each piece simply stores the length of
# that piece. For example, given M=[0 0 1 1 1 0 1] the RLE counts would
# be [2 3 1 1], or for M=[1 1 1 1 1 1 0] the counts would be [0 6 1]
# (note that the odd counts are always the numbers of zeros). Instead of
# storing the counts directly, additional compression is achieved with a
# variable bitrate representation based on a common scheme called LEB128.

解释一下就是：RLE将一个二进制向量分成一系列固定长度的片段，对每个片段只存储那个片段的长度。例如M=[0 0 1 1 1 0 1]， RLE就是[2 3 1 1]；M=[1 1 1 1 1 1 0]， RLE为[0 6 1]，注意奇数位始终为0的个数。另外，也使用一个基于LEB128的通用方案的可变比特率来完成额外的压缩。

一个实例

# get all images containing given categories, select one at random
catIds = coco.getCatIds(catNms=['animal']);
imgIds = coco.getImgIds(catIds=catIds );
imgIds = coco.getImgIds(imgIds = [324139])
img = coco.loadImgs(imgIds[np.random.randint(0,len(imgIds))])[0]print imgIds
print img['coco_url']
I = io.imread(img['coco_url'])
plt.axis('off')
plt.imshow(I)
plt.show()  //图一plt.imshow(I); plt.axis('off')
annIds = coco.getAnnIds(imgIds=img['id'], catIds=catIds, iscrowd=None)
anns = coco.loadAnns(annIds)
print len(anns)
masks = []
showonce = True
for ann in anns:if type(ann['segmentation']) == list and showonce:print ann['segmentation']showonce = Falseif type(ann['segmentation']) != list:print ann['segmentation']m = coco.annToMask(ann)masks.append(m)
print len(masks)
coco.showAnns(anns) //图二

15
[[151.06, 113.6, 168.95, 102.49, 182.53, 92.62, 193.64, 80.28, 203.51, 70.4, 208.45, 61.76, 206.6, 53.74, 209.68, 49.42, 220.17, 50.04, 220.79, 55.59, 222.64, 59.3, 222.64, 59.91, 227.58, 66.7, 228.2, 77.19, 228.2, 83.98, 228.2, 87.06, 228.2, 92.0, 227.58, 96.32, 220.79, 101.87, 213.39, 104.96, 205.36, 111.75, 202.9, 113.6, 201.04, 114.22, 202.9, 123.47, 200.43, 129.64, 200.43, 125.94, 198.58, 113.6, 190.55, 111.75, 181.91, 113.6, 168.95, 114.83, 168.95, 114.83, 162.17, 120.39, 157.23, 119.15, 146.74, 117.92, 142.42, 115.45]]
{u’counts’: [113441, 1, 423, 6, 427, 7, 3, 1, 422, 8, 3, 1, 421, 9, 2, 2, 420, 10, 2, 1, 421, 10, 1, 2, 419, 12, 1, 2, 418, 15, 419, 15, 418, 16, 418, 16, 418, 15, 419, 15, 418, 16, 417, 16, 418, 16, 418, 15, 419, 14, 419, 14, 419, 14, 420, 13, 420, 13, 421, 11, 422, 11, 423, 10, 423, 10, 424, 9, 425, 8, 426, 7, 427, 6, 427, 5, 429, 5, 429, 5, 429, 5, 429, 5, 428, 5, 429, 5, 428, 6, 428, 6, 428, 6, 428, 5, 429, 5, 429, 5, 408, 7, 14, 5, 407, 9, 13, 5, 406, 11, 11, 5, 407, 12, 9, 6, 406, 13, 9, 6, 404, 15, 8, 7, 402, 18, 7, 7, 401, 20, 6, 6, 401, 21, 6, 6, 400, 34, 399, 35, 399, 12, 1, 22, 398, 13, 3, 19, 399, 12, 6, 17, 399, 12, 8, 15, 399, 12, 10, 13, 400, 10, 13, 10, 401, 10, 15, 8, 400, 11, 17, 5, 401, 10, 20, 2, 402, 11, 423, 11, 423, 11, 423, 11, 423, 12, 422, 13, 422, 13, 421, 14, 421, 14, 420, 15, 420, 14, 420, 15, 420, 15, 419, 16, 419, 16, 418, 17, 418, 17, 418, 17, 418, 17, 417, 18, 4, 4, 409, 24, 411, 23, 412, 22, 412, 22, 411, 23, 411, 22, 412, 22, 412, 22, 413, 21, 413, 20, 414, 20, 414, 19, 416, 18, 416, 17, 417, 17, 417, 17, 417, 16, 419, 14, 420, 14, 421, 13, 422, 11, 424, 10, 425, 8, 428, 4, 112407, 8, 422, 16, 416, 19, 414, 21, 2, 6, 405, 30, 403, 32, 402, 33, 401, 34, 400, 35, 400, 34, 400, 34, 401, 33, 402, 32, 277], u’size’: [434, 640]}
15

图一

图二