detect

Detect objects using SSD multibox object detector

Syntax

bboxes = detect(detector,I)

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

[___,labels] = detect(detector,I)

[___] = detect(___,roi)

detectionResults = detect(detector,ds)

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

Description

bboxes = detect(detector,I) detects objects within a single image or an array of images, I, using an single shot multibox detector (SSD). The locations of objects detected are returned as a set of bounding boxes.

When using this function, use of a CUDA^® enabled NVIDIA^® GPU is highly recommended. The GPU reduces computation time significantly. Usage of the GPU requires Parallel Computing Toolbox™. For information about the supported compute capabilities, see GPU Computing Requirements (Parallel Computing Toolbox).

[bboxes,scores] = detect(detector,I) also returns the detection scores for each bounding box.

[___,labels] = detect(detector,I) also returns a categorical array of labels assigned to the bounding boxes, using either of the preceding syntaxes. The labels used for object classes are defined during training using the trainSSDObjectDetector function.

[___] = detect(___,roi) detects objects within the rectangular search region specified by roi.

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

[___] = detect(___,Name,Value) specifies options using one or more Name,Value pair arguments. For example, detect(detector,I,"Threshold",0.75) sets the detection score threshold to 0.75. Any detections with a lower score are removed.

Examples

Detect Vehicles Using SSD Object Detector

Load a pretrained single shot detector (SSD) object to detect vehicles in an image. The detector is trained with images of cars on a highway scene.

vehicleDetector = load("ssdVehicleDetector.mat","detector");
detector = vehicleDetector.detector;

Read a test image into the workspace.

I = imread("highway.png");

Display the test image.

imshow(I)

Run the pretrained SSD object detector by using the detect function. The output contains the bounding boxes, scores, and the labels for vehicles detected in the image. The labels are derived from the ClassNames property of the detector.

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

bboxes = 2×4

   139    78    96    81
    99    67   165   146

scores = 2×1 single column vector

    0.8349
    0.6302

labels = 2×1 categorical
     vehicle 
     vehicle 

Annotate the image with the detection results.

if ~isempty(bboxes)
    detectedI = insertObjectAnnotation(I,"rectangle",bboxes,cellstr(labels));
else
   detectedI = insertText(I,[10 10],"No Detections");
end
   
imshow(detectedI)

Input Arguments

detector — SSD object detector

ssdObjectDetector object

SSD object detector, specified as an ssdObjectDetector object. To create this object, call the trainSSDObjectDetector function with training data as input.

I — Input image

H-by-W-by-C-by-B numeric array of images

Input image, specified as an H-by-W-by-C-by-B numeric array of images. Images must be real, nonsparse, grayscale or RGB image.

• H — Height in pixels.

• W — Width in pixels

• 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.

• B — Number of images in the array.

The detector is sensitive to the range of the input image. Therefore, ensure that the input image range is similar to the range of the images used to train the detector. For example, if the detector was trained on uint8 images, rescale this input image to the range [0, 255] by using the im2uint8 or rescale function. The size of this input image should 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 input image differs from the scale of the objects the detector was trained to identify. Consider whether you used the SmallestImageDimension property during training to modify the size of training images.

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

ds — Datastore

datastore object

Datastore, specified as a datastore object containing a collection of images. Each image must be a grayscale, RGB, or multichannel image. The function processes only the first column of the datastore, which must contain images and must be cell arrays or tables with multiple columns.

roi — Search region of interest

[x y width height] vector

Search region of interest, specified as a four-element vector of the form [x y width height]. 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.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: "SelectStrongest",true

Threshold — Detection threshold

0.5 (default) | scalar

Detection threshold, specified as 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 true or false.

• true — Return the strongest bounding box per object. To select these boxes, detect calls the selectStrongestBboxMulticlass function, which uses nonmaximal suppression to eliminate overlapping bounding boxes based on their confidence scores.

  For example:

   selectStrongestBboxMulticlass(bbox,scores, ...
              "RatioType","Union", ...
              "OverlapThreshold",0.5);

• false — Return all detected bounding boxes. You can then create your own custom operation to eliminate overlapping bounding boxes.

MaxSize — Maximum region size

size(I) (default) | [height width] vector

Maximum region size that contains a detected object, specified as a [height width] vector. Units are in pixels.

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

MinSize — Minimum region size

[1 1] (default) | [height width] vector

Minimum region size that contains a detected object, specified as a [height width] vector. Units are in pixels.

To reduce computation time, set this value to the known minimum region size for the objects being detected in the image. By default, MinSize is set to [1 1].

MiniBatchSize — Minimum batch size

128 (default) | scalar

Minimum batch size, specified as a scalar value. Use the MiniBatchSize to process a large collection of images. Images are grouped into minibatches and processed as a batch, which can improve computation efficiency at the cost of increased memory demand. Decrease the size to use less memory.

ExecutionEnvironment — Hardware resource

"auto" (default) | "gpu" | "cpu"

Hardware resource on which to run the detector, specified as "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 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 Set Up Compiler (GPU Coder).

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

Output Arguments

bboxes — Location of objects detected

M-by-4 matrix | B-by-1 cell array

Location of objects detected within the input image or images, returned as an M-by-4 matrix or a B-by-1 cell array. M is the number of bounding boxes in an image, and B is the number of M-by-4 matrices when the input contains an array of images.

Each row of bboxes contains a four-element vector of the form [x y width height]. This vector specifies the upper left corner and size of that corresponding bounding box in pixels.

scores — Detection scores

M-by-1 vector | B-by-1 cell array

Detection confidence scores, returned as an M-by-1 vector or a B-by-1 cell array. M is the number of bounding boxes in an image, and B is the number of M-by-1 vectors when the input contains an array of images. A higher score indicates higher confidence in the detection.

labels — Labels for bounding boxes

M-by-1 categorical array | B-by-1 cell array

Labels for bounding boxes, returned as an M-by-1 categorical array or a B-by-1 cell array. M is the number of labels in an image, and B is the number of M-by-1 categorical arrays when the input contains an array of images. You define the class names used to label the objects when you train the input detector.

detectionResults — Detection results

3-column table

Detection results, returned as a 3-column table with variable names, Boxes, Scores, and Labels. The Boxes column contains M-by-4 matrices, of M bounding boxes for the objects found in the image. Each row contains a bounding box as a 4-element vector in the format [x,y,width,height]. The format specifies the upper-left corner location and size in pixels of the bounding box in the corresponding 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 codegen constant (coder.const()) and a 1x4 vector.

• Only the Threshold, SelectStrongest, MinSize, MaxSize, and MiniBatchSize Name-Value pairs are supported. All Name-Value pairs must be compile-time constants.

• The channel and batch size of the input image must be fixed size.

• The labels output is returned as a categorical array.

• In the generated code, the input is rescaled to the size of the input layer of the network. But the bounding box that the detect method returns is in reference to the original input size.

GPU Code Generation

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

Usage notes and limitations:

For code generation,

• The roi argument to the detect method must be a codegen constant (coder.const()) and a 1-by-4 vector.

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

• The channel and batch size of the input image must be fixed size.

• The labels output is returned as a categorical array.

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

Version History

Introduced in R2020a