how can I convert the polygon vertices to rectF(left,top,right,bottom)?-CodePudding

I am using Google Cloud Vision API to detect objects in images. The response is in the following form from google vision API. it returns the array of normalized vertices. But I need the 4 points only for RectF.I have googled it before positing it here but I couldn't get any proper solution.

{
"responses": [
    {
        "localizedObjectAnnotations": [
            {
                "mid": "/m/01c648",
                "name": "Laptop",
                "score": 0.885833,
                "boundingPoly": {
                    "normalizedVertices": [
                        {
                            "x": 0.16581687,
                            "y": 0.5996421
                        },
                        {
                            "x": 0.5108573,
                            "y": 0.5996421
                        },
                        {
                            "x": 0.5108573,
                            "y": 0.9928019
                        },
                        {
                            "x": 0.16581687,
                            "y": 0.9928019
                        }
                    ]
                }
            },
            {
                "mid": "/m/04brg2",
                "name": "Tableware",
                "score": 0.8071477,
                "boundingPoly": {
                    "normalizedVertices": [
                        {
                            "x": 0.61909163,
                            "y": 0.8264213
                        },
                        {
                            "x": 0.7196966,
                            "y": 0.8264213
                        },
                        {
                            "x": 0.7196966,
                            "y": 0.9963302
                        },
                        {
                            "x": 0.61909163,
                            "y": 0.9963302
                        }
                    ]
                }
            },
            {
                "mid": "/j/984ysm",
                "name": "Table top",
                "score": 0.66904813,
                "boundingPoly": {
                    "normalizedVertices": [
                        {
                            "y": 0.8069201
                        },
                        {
                            "x": 0.86148286,
                            "y": 0.8069201
                        },
                        {
                            "x": 0.86148286,
                            "y": 0.99502665
                        },
                        {
                            "y": 0.99502665
                        }
                    ]
                }
            },
            {
                "mid": "/m/0d4v4",
                "name": "Window",
                "score": 0.5146187,
                "boundingPoly": {
                    "normalizedVertices": [
                        {
                            "x": 0.004114019,
                            "y": 0.00019616824
                        },
                        {
                            "x": 0.3921472,
                            "y": 0.00019616824
                        },
                        {
                            "x": 0.3921472,
                            "y": 0.25323766
                        },
                        {
                            "x": 0.004114019,
                            "y": 0.25323766
                        }
                    ]
                }
            }
        ]
    }
]}

I want to draw rectangle around the detected object but I am not sure how can I get the rectangle points from the Polygon vertices. What is the algorithm for converting the polygon into a rectangle.

CodePudding user response：

The API provides you with four points, which are the four corner of an axis-aligned rectangle. The four corners can be referred as:

The topleft corner;
The topright corner;
The bottomright corner;
The bottomleft corner.

Each corner is a point which has two coordinates; for instance, the two coordinates of the topleft corner are (x=left, y=top), and the two coordinates of the bottomright corner are (x=right, y=bottom).

Identify which point is the topleft corner, and which point is the bottomright corner, and that will give you the four values you seek:

left   = topleft.x
top    = topleft.y
right  = bottomright.x
bottom = bottomright.y

As an additional note, these values are extremely easy to identify if you know how to take the minimum or the maximum, since for instance:

right = max(left, right)
left  = min(left, right)

Whether top = max(top, bottom) or top = min(top, bottom) depends on the orientation of the coordinate system, so you'll have to figure that one out for yourself. For instance, in a mathematical plot we almost always use top = max(top, bottom), but when describing pixel coordinates on a screen, we more often use top = min(top, bottom).