detect

Detect objects using YOLO v3 object detector

Since R2021a

Syntax

bboxes = detect(detector,I)

[bboxes,scores] = detect(detector,I)

[bboxes,scores,labels] = detect(detector,I)

[bboxes,scores,labels,info] = detect(detector,I)

detectionResults = detect(detector,ds)

[___] = detect(___,roi)

[___] = detect(___,Name=Value)

Description

bboxes = detect(detector,I) detects objects within a single image or an array of images, I, using a you only look once version 3 (YOLO v3) object detector, detector. The input size of the image must be greater than or equal to the network input size of the pretrained detector. The locations of objects detected are returned as a set of bounding boxes.

Note

To use the pretrained YOLO v3 object detection networks trained on COCO dataset, you must install the Computer Vision Toolbox™ Model for YOLO v3 Object Detection. You can download and install the Computer Vision Toolbox Model for YOLO v3 Object Detection from Add-On Explorer. For more information about installing add-ons, see Get and Manage Add-Ons. To run this function, you will require the Deep Learning Toolbox™.

[bboxes,scores] = detect(detector,I) also returns the class-specific confidence scores for each bounding box.

[bboxes,scores,labels] = detect(detector,I) returns a categorical array of labels assigned to the bounding boxes. The labels for object classes are defined during training.

[bboxes,scores,labels,info] = detect(detector,I) also returns information about the class probabilities and objectness scores for each detection.

detectionResults = detect(detector,ds) detects objects within all the images returned by the read function of the input datastore ds.

[___] = detect(___,roi) detects objects within the rectangular search region roi, in addition to any combination of arguments from previous syntaxes.

[___] = detect(___,Name=Value) specifies options using one or more name-value arguments, in addition to any combination of arguments from previous syntaxes.

Examples

Detect Objects Using YOLO v3 Object Detector

Load a pretrained YOLO v3 object detector.

detector = yolov3ObjectDetector('tiny-yolov3-coco');

Read a test image and preprocess the test image by using the preprocess function.

img = imread('sherlock.jpg');
img = preprocess(detector,img);

Detect objects in the test image.

[bboxes,scores,labels] = detect(detector,img);

Display the detection results.

results = table(bboxes,labels,scores)

results=1×3 table
             bboxes             labels    scores 
    ________________________    ______    _______

    133     67    283    278     dog      0.51771

detectedImg = insertObjectAnnotation(img,'Rectangle',bboxes,labels);
figure
imshow(detectedImg)

Detect Objects from Images Stored in Image Datastore

Load a pretrained YOLOv3 object detector.

detector = yolov3ObjectDetector('tiny-yolov3-coco');

Read the test data and store as an image datastore object.

location = fullfile(matlabroot,'toolbox','vision','visiondata','vehicles');
imds = imageDatastore(location);

Detect objects in the test dataset. Set the Threshold parameter value to 0.3 and MiniBatchSize parameter value to 32.

detectionResults = detect(detector,imds,'Threshold',0.3,'MiniBatchSize',32);

Read an image from the test dataset and extract the corresponding detection results.

num = 10;
I = readimage(imds,num);
bboxes = detectionResults.Boxes{num};
labels = detectionResults.Labels{num};
scores = detectionResults.Scores{num};

Perform non-maximal suppression to select strongest bounding boxes from the overlapping clusters. Set the OverlapThreshold parameter value to 0.2.

[bboxes,scores,labels] = selectStrongestBboxMulticlass(bboxes,scores,labels,'OverlapThreshold',0.2);

Display the detection results.

results = table(bboxes,labels,scores)

results=3×3 table
             bboxes             labels    scores 
    ________________________    ______    _______

     14     71     52     27     car      0.93352
     74     73      7      5     car      0.65369
    102     73     15     10     car      0.85313

detectedImg = insertObjectAnnotation(I,'Rectangle',bboxes,labels);
figure
imshow(detectedImg)

Detect Objects Within ROI

Load a pretrained YOLO v3 object detector.

detector = yolov3ObjectDetector('tiny-yolov3-coco');

Read a test image.

img = imread('highway.png');

Specify a region of interest (ROI) within the test image.

roiBox = [70 40 100 100];

Detect objects within the specified ROI.

[bboxes,scores,labels] = detect(detector,img,roiBox);

Display the ROI and the detection results.

img = insertObjectAnnotation(img,'Rectangle',roiBox,'ROI','AnnotationColor',"blue");
detectedImg = insertObjectAnnotation(img,'Rectangle',bboxes,labels);
figure
imshow(detectedImg)

Input Arguments

detector — YOLO v3 object detector

yolov3ObjectDetector object

YOLO v3 object detector, specified as a yolov3ObjectDetector object.

I — Test images

numeric array

Test images, specified as a numeric array of size H-by-W-byC or H-by-W-byC-by-T. Images must be real, nonsparse, grayscale or RGB image.

• H: Height

• W: Width

• C: The channel size in each image must be equal to the network's input channel size. For example, for grayscale images, C must be equal to 1. For RGB color images, it must be equal to 3.

• T: Number of test images in the array. The function computes the object detection results for each test image in the array.

The intensity range of the test image must be similar to the intensity range of the images used to train the detector. For example, if you train the detector on uint8 images, rescale the test image to the range [0, 255] by using the im2uint8 or rescale function. The size of the test image must be comparable to the sizes of the images used in training. If these sizes are very different, the detector has difficulty detecting objects because the scale of the objects in the test image differs from the scale of the objects the detector was trained to identify.

Data Types: uint8 | uint16 | int16 | double | single

ds — Test images

ImageDatastore object | CombinedDatastore object | TransformedDatastore object

Test images, specified as a ImageDatastore object, CombinedDatastore object, or TransformedDatastore object containing full filenames of the test images. The images in the datastore must be grayscale, or RGB images.

roi — Search region of interest

[x y width height] vector

Search region of interest, specified as an [x y width height] vector. The vector specifies the upper left corner and size of a region in pixels.

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Example: detect(detector,I,Threshold=0.25)

Threshold — Detection threshold

0.5 (default) | scalar in the range [0, 1]

Detection threshold, specified as a comma-separated pair consisting of 'Threshold' and a scalar in the range [0, 1]. Detections that have scores less than this threshold value are removed. To reduce false positives, increase this value.

SelectStrongest — Select strongest bounding box

true (default) | false

Select the strongest bounding box for each detected object, specified as the comma-separated pair consisting of 'SelectStrongest' and either true or false.

• true — Returns the strongest bounding box per object. The method calls the selectStrongestBboxMulticlass function, which uses nonmaximal suppression to eliminate overlapping bounding boxes based on their confidence scores.

  By default, the selectStrongestBboxMulticlass function is called as follows

   selectStrongestBboxMulticlass(bboxes,scores,...
                                 'RatioType','Union',...
                                 'OverlapThreshold',0.5);

• false — Return all the detected bounding boxes. You can then write your own custom method to eliminate overlapping bounding boxes.

MinSize — Minimum region size

[1 1] (default) | vector of the form [height width]

Minimum region size, specified as the comma-separated pair consisting of 'MinSize' and a vector of the form [height width]. Units are in pixels. The minimum region size defines the size of the smallest region containing the object.

By default, MinSize is 1-by-1.

MaxSize — Maximum region size

size(I) (default) | vector of the form [height width]

Maximum region size, specified as the comma-separated pair consisting of 'MaxSize' and a vector of the form [height width]. Units are in pixels. The maximum region size defines the size of the largest region containing the object.

By default, 'MaxSize' is set to the height and width of the input image, I. To reduce computation time, set this value to the known maximum region size for the objects that can be detected in the input test image.

MiniBatchSize — Minimum batch size

128 (default) | scalar

Minimum batch size, specified as the comma-separated pair consisting of 'MiniBatchSize' and a scalar value. Use the MiniBatchSize to process a large collection of image. Images are grouped into minibatches and processed as a batch to improve computation efficiency. Increase the minibatch size to decrease processing time. Decrease the size to use less memory.

ExecutionEnvironment — Hardware resource

'auto' (default) | 'gpu' | 'cpu'

Hardware resource on which to run the detector, specified as the comma-separated pair consisting of 'ExecutionEnvironment' and 'auto', 'gpu', or 'cpu'.

• 'auto' — Use a GPU if it is available. Otherwise, use the CPU.

• 'gpu' — Use the GPU. To use a GPU, you must have Parallel Computing Toolbox™ and a CUDA^®-enabled NVIDIA^® GPU. If a suitable GPU is not available, the function returns an error. For information about the supported compute capabilities, see GPU Computing Requirements (Parallel Computing Toolbox).

• 'cpu' — Use the CPU.

Acceleration — Performance optimization

'auto' (default) | 'mex' | 'none'

Performance optimization, specified as the comma-separated pair consisting of 'Acceleration' and one of the following:

• 'auto' — Automatically apply a number of optimizations suitable for the input network and hardware resource.

• 'mex' — Compile and execute a MEX function. This option is available when using a GPU only. Using a GPU requires Parallel Computing Toolbox and a CUDA enabled NVIDIA GPU. If Parallel Computing Toolbox or a suitable GPU is not available, then the function returns an error. For information about the supported compute capabilities, see GPU Computing Requirements (Parallel Computing Toolbox).

• 'none' — Disable all acceleration.

The default option is 'auto'. If 'auto' is specified, MATLAB^® applies a number of compatible optimizations. If you use the 'auto' option, MATLAB does not ever generate a MEX function.

Using the 'Acceleration' options 'auto' and 'mex' can offer performance benefits, but at the expense of an increased initial run time. Subsequent calls with compatible parameters are faster. Use performance optimization when you plan to call the function multiple times using new input data.

The 'mex' option generates and executes a MEX function based on the network and parameters used in the function call. You can have several MEX functions associated with a single network at one time. Clearing the network variable also clears any MEX functions associated with that network.

The 'mex' option is only available for input data specified as a numeric array, cell array of numeric arrays, table, or image datastore. No other types of datastore support the 'mex' option.

The 'mex' option is only available when you are using a GPU. You must also have a C/C++ compiler installed. For setup instructions, see MEX Setup (GPU Coder).

'mex' acceleration does not support all layers. For a list of supported layers, see Supported Layers (GPU Coder).

DetectionPreprocessing — Option to preprocess test images

'auto' (default) | 'none'

Option to preprocess the test images before performing object detection, specified as the comma-separated pair consisting of 'DetectionPreprocessing' and one of these values:

• 'auto' — To preprocess the test image before performing object detection. The detect function calls the preprocess function that perform these operations:

  • Rescales the intensity values of the training images to the range [0, 1].

  • Resizes the training images to one of the nearest network input sizes and updates the bounding box coordinate values for accurate training. The function preserves the original aspect ratio of the training data.

• 'none' — To perform object detection without preprocessing the test image. If you choose this option, the datatype of the test image must be either single or double.

Data Types: char | string

Output Arguments

bboxes — Location of objects detected

M-by-4 matrix | M-by-5 matrix | T-by-1 cell array

Location of objects detected within the input image or images, returned as a

• M-by-4 matrix or an M-by-5 matrix if the input is a single test image.

• T-by-1 cell array if the input is an array of test images. T is the number of test images in the array. M is the number of bounding boxes in an image

The table describes the format of bounding boxes.

Bounding Box	Description
rectangle	Defined in spatial coordinates as an M-by-4 numeric matrix with rows of the form [x y w h], where: M is the number of axis-aligned rectangles. x and y specify the upper-left corner of the rectangle. w specifies the width of the rectangle, which is its length along the x-axis. h specifies the height of the rectangle, which is its length along the y-axis.
rotated-rectangle	Defined in spatial coordinates as an M-by-5 numeric matrix with rows of the form [xctr yctr xlen ylen yaw], where: M is the number of rotated rectangles. xctr and yctr specify the center of the rectangle. xlen specifies the width of the rectangle, which is its length along the x-axis before rotation. ylen specifies the height of the rectangle, which is its length along the y-axis before rotation. yaw specifies the rotation angle in degrees. The rotation is clockwise-positive around the center of the bounding box.

scores — Detection scores

M-by-1 numeric vector | T-by-1 cell array

Detection confidence scores for each bounding box, returned as one of these options:

• M-by-1 numeric vector — The input is a single test image. M is the number of bounding boxes detected in the image.

• B-by-1 cell array — The input is a batch of test images, where B is the number of test images in the batch. Each cell in the array contains an M-element row vector, where each element indicates the detection score for a bounding box in the corresponding image.

A higher score indicates higher confidence in the detection. The confidence score for each detection is a product of the corresponding objectness score and maximum class probability. The objectness score is the probability that the object in the bounding box belongs to a class in the image. The maximum class probability is the largest probability that a detected object in the bounding box belongs to a particular class.

labels — Labels for bounding boxes

M-by-1 categorical vector | T-by-1 cell array

Labels for bounding boxes, returned as one of these options:

• M-by-1 categorical vector if the input is a single test image.

• T-by-1 cell array if the input is an array of test images. T is the number of test images in the array. Each cell in the array contains a M-by-1 categorical vector containing the names of the object classes.

M is the number of bounding boxes detected in an image.

detectionResults — Detection results

3-column table

Detection results, returned as a 3-column table with variable names, Boxes, Scores, and Labels. The Boxes column can contain rectangles or rotated rectangle bounding boxes of the form :

• rectangle — M-by-4 matrices, of M bounding boxes for the objects found in the image. Each row specifies a rectangle as a 4-element vector of the form [x,y,width,height], where (x,y) specifies the upper-left corner location and (width, height) specifies the size in pixels

• rotated rectangle — M-by-5 matrices of M bounding boxes for the objects found in the image. Each row specifies a rotated rectangle as a 5-element vector of the form [xctr,yctr,width, height,yaw], where (xctr,yctr) specifies the center, (width,height) specifies the size, and yaw specifies the rotated angle.

info — Class probabilities and objectness scores

structure array

Class probabilities and objectness scores of the detections, returned as a structure array with these fields.

• ClassProbabilities — Class probabilities for each of the detections, returned as a B-by-1 cell array. B is the number of images in the input batch of images, I. Each cell in the array contains the class probabilities as an M-by-N numeric matrix. M is the number of bounding boxes and N is the number of classes. Each class probability is a numeric scalar, indicating the probability that the detected object in the bounding box belongs to a class in the image.

• ObjectnessScores — Objectness scores for each of the detections, returned as a B-by-1 cell array. B is the number of images in the input batch of images, I. Each cell in the array contains the objectness score for each bounding box as an M-by-1 numeric vector. M is the number of bounding boxes. Each objectness score is a numeric scalar, indicating the probability that the bounding box contains an object belonging to one of the classes in the image.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• The roi argument to the detect method must be a code generation constant (coder.const()) and a 1x4 vector.

• Only the Threshold, SelectStrongest, MinSize, and MaxSize name-value pairs for detect are supported.

GPU Code Generation

Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Usage notes and limitations:

• The roi argument to the detect method must be a codegen constant (coder.const()) and a 1x4 vector.

• Only the Threshold, SelectStrongest, MinSize, and MaxSize name-value pairs are supported.

• The height, width, and channel of the input image must be fixed size.

GPU Arrays

Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

Usage notes and limitations:

• GPU Arrays is not supported for rotated rectangle bounding box.

Version History

Introduced in R2021a

R2024b: Code generation support for rotated rectangle bounding boxes

Starting in R2024b, bboxes output argument returned as rotated rectangle bounding box predictions, supports both CPU and GPU code generation.

R2024a: Rotated rectangle bounding box support

The output argument bboxes now support rotated rectangle bounding box predictions.

R2024a: Option to return class probabilities and objectness scores

Specify the info output argument to return information about the class probability and objectness score for each detection.