This is how one can interpret YOLO v5 segmentation output!!!

[aravindchakravarti]

Hi All,
There are lot of tutorials online (also discussions in YOLOv5 issues section) about detection output and postprocessing of YOLOv5 detection model.

I was looking something similar for segmentation model from long time. Looks like, there is a very little information online or at least I didn't find much online. Until yesterday, when I understood that YOLOv5 segmentation model is inspired from YOLACT.

So, I am giving some inputs here, so that you know how YOLOv5 processes its segmentation output.

Before we go to YOLOv5 segmentation model, lets see how YOLACT works using simple diagram from its paper.

Basically, YOLOACT produces two outputs.

1. Prototype Generation
   These are prototype masks which gets generated by Fully Connected Convolutional Neural Network. Number of masks generated is determined by last layer which has k channels. Number of masks generated = k

2. Mask Coefficients
   The masks generated above are just prototypes. We need to combine these channels. To do this, YOLOACT also generated mask coefficients.

YOLACT multiplies prototypes with mask coefficients to generate final masks as shown below. They are later cropped and thresholded.

Now, lets come to YOLOv5 segmentation. I am using zidange.jpg for below illustration.

Lets look at below code in segment\predict.py

# Inference
with dt[1]:
     visualize = increment_path(save_dir / Path(path).stem, mkdir=True) if visualize else False
     pred, proto = model(im, augment=augment, visualize=visualize)[:2]

Here model is producing two outputs. One is pred and another is proto.

If you see the output dimension of pred = torch.Size([1, 25200, 117])
and output dimension of proto = torch.Size([1, 32, 160, 160])

Now, for pred. 1 is the batch size, 25200 is the overall predictions including all anchors(same as detection network) and 117 is equal to 85+32. Meaning, there 80 classes, 5 localization information x, y, w, h, conf and last 32 are the mask coefficients

And for proto. 1 is the batch size, 32 is number of prototype masks with each mask being 160x160 pixels. So, how does these 32 prototype look like? Lets see some examples.

So, 32 such prototype masks will be generated by model. We can see, none of the images clearly identifying any object (2 persons and 1 tie) in the image. That is where we need help of mask coefficients.

We know that, model has produced pred with size ([1, 25200, 117]) which contains mask coefficients. Now this pred is passed to NMS.

pred = non_max_suppression(pred, conf_thres, iou_thres, classes, agnostic_nms, max_det=max_det, nm=32)

If we see the dimension of pred then for zidange.jpg it will be torch.Size([3, 38])
Why 3? Because we have 3 objects in input image (2 persons and 1 tie).
Why 38? Because YOLO segmentation also outputs bounding boxes. Hence this 38 is actually 6+32. First 6 will be x, y, w, h, conf, class + 32 co-efficients.

To check if you print pred[0][0] then you will get,

tensor([ 1.82055e+02,  3.81594e+02,  2.48618e+02,  6.38722e+02,  5.47279e-01,  2.70000e+01,  8.05743e-02, -1.95804e-01,  4.02435e-01, -2.21064e-01,  2.24337e-01, -5.27114e-02, -5.52488e-01, -9.28004e-01, -6.14983e-01, -1.40511e-01, -1.34073e-01,  1.70663e-01, -4.58587e-01, -2.04990e-01,  5.66050e-01, -8.52897e-02,
         6.01281e-01, -1.39116e-01, -2.84991e-01, -9.39970e-01,  1.72199e-01, -2.77271e-01,  3.80420e-01, -6.57674e-01, -3.69973e-01, -1.07442e+00, -2.15896e-01,  7.56425e-02, -1.06007e+00,  3.85358e-01,  2.00247e-01, -9.35435e-01], device='cuda:0')

Now, leave first 6 values (because they corresponds to bounding box information). Now take rest 32 values and multiply with 32 prototype channels. You will get the final MASK!!!! One example is below which is indicating tie !!

Hope this is helpful!!

[defend1234]

great!Thank you!

[aravindchakravarti]

@defend1234 Thanks.. you are welcome!

[defend1234]

I am an AI engineer from China.I see your github homepage,feeling you are full of energy!Thank you again!

[hcscysh]

it's very helpful to me. I'm fusing for this when I use yolov5. Thank you!

[aravindchakravarti]

Thanks!!

[trinath3]

It's very helpful ThankYou

[aravindchakravarti]

Hey thanks!

[caspar-frog]

@aravindchakravarti Thank you for the detailed explanation! It's really helpful!

But I still can't figure out how to output masks with a higher resolution (like 640x640), any advice? Thank you!