This guide covers the basics for using GRIP and generating vison code. This is not a complete guide for using that vision code with a co-processor or robot. Using GRIP is only one step of a larger series of steps for vision programming. See the Vision with WPILibPi FRC guide for a broader explanation.

To use GRIP you will need a Windows, macOS, or Linux computer. For this guide I am assuming a USB or integrated web camera is connected to your computer or that you have access to an HTTP (web) accessible camera.

What is GRIP?

GRIP is a tool used to help generate vision analysis code. It is a shortcut to prevent manually writing vision detection and analysis code. GRIP is not strictly necessary, but will probably make your life much easier. I do not know how to write object detection code from scratch, and GRIP did in a few minutes what would have probably taken me days otherwise.

FRC's 'Vision with GRIP' guide as well as their Twitch Stream from 2020-12-12 are better guides to GRIP than this article. You should read/watch them to get a deeper understanding.

Get GRIP

Download the latest stable version of GRIP (v1.5.2 at time of writing) to your computer. Under Assets download the exe, dmg, or deb to your developer machine depending on which operating system you use. Install GRIP once downloaded and run it.

GRIP running on Debian
GRIP running on Debian

Select a Camera

In GRIP, click Add Source under Sources. We can select a camera from a variety of sources.

If you have a USB or integrated camera then select Webcam. This is the simplest way to get started.

If you have a web-accessible camera then select IP Camera. In the box that appears type the URL of the camera. For example, I have a streaming webcam on my network at http://192.168.0.109:8087/camera.mjpg.

Regardless of the type of camera, the camera should appear as a source under Sources after it is added.

Click the "open eye" icon next to Image to see a preview from the camera in GRIP.

GRIP with a source camera added
GRIP with a source camera added

Detectable Objects

I am not an expert on vision recognition principles, so I'm winging things a bit here.

At the FIRST Robotics Competition the game pieces are often very brightly colored and reflective. There is a good reason for this. High contrast bright items are easier to detect.

See the Target Info and Retroreflection article from FRC for a better explanation.

A bright orange or yellow ball, or a reflective piece of tape, are easier to identify on an FRC playing field. Image detection works by teaching a program how the target item(s) you want detected are unique and identifiable. Our goal is to train GRIP (and eventually our robot code) how to pick out our desired item among other items.

For this guide I will use a tennis ball as an example.

A tennis ball and other objects visible on camera in GRIP
A tennis ball and other objects visible on camera in GRIP

Pretend this tennis ball is a game piece that we want a robot to "see" and drive towards. After we train it, GRIP will be able to identify where that ball is on screen and will ignore the background and other objects.

Add One Filter

In the right hand column click Blur to add a blur filter step to the processing pipeline. Click the radio button next to Image under the Sources section and drag it over to Input in the Blur step. There should now be a curvy line connecting the two steps.

For my example I changed the Blur Radius to 3 to help smooth the image and make it simpler for other filters to detect whole objects. We'll add other filters in a moment.

Click the "open eye" icon next to Output to see a blurred preview from the camera in GRIP.

The same GRIP output from the camera as before, but with a blur filter
The same GRIP output from the camera as before, but with a blur filter

This is the basic process for taking a source input feed from a camera, and sending it down a "pipeline" for multiple filters to act on it.

Add Additional Filters

We can repeat this same sort of behavior to chain together several processing steps.

In the right hand column click HSV Threshold, Find Contours, and Filter Contours in that order to add those steps to the pipeline.

Drag the Output of Blur to the Input of HSV Threshold. Drag the Output of HSV Threshold to the Input of Find Contours. Drag the Contours of Find Contours to the Contours of Filter Contours.

Now we have a pipeline connecting like so.

[======]   > [====]   > [===]   > [====]   > [======]
|Source|  >  |Blur|  >  |HSV|  >  |Find|  >  |Filter|
[======] >   [====] >   [===] >   [====] >   [======]
A pipeline of filters in GRIP
A pipeline of filters in GRIP

Click the other "open eye" icons to show (or hide) previews for the image in each step of the pipeline. You may need to scroll left and right to various filters if they do not all fit on screen at once. You may also choose to hide the preview images if you are satisified with the output of that filter and do not need to look at it.

The only preview that really matters is the final one as that is the end result of the pipeline.

This is an image processing pipeline. The raw camera input is all the way on the left, and the final result is all the way on the right. Data "flows" down the pipe from left to right.

Adjust HSV Filter

I am using the HSV filter as one possible way to try to distinguish the tennis ball. You may find that RGB, HSL, or some other filter works better for you. Try different filters to see what works best, or consult someone who knows a lot about colors and the science behind them to help figure out which filters work best.

By changing the values of hue, saturation, and value (intensity/brightness) we can (through trial and error) filter out the image so only the tennis ball is white.

HSV filter modified to highlight only the tennis ball
HSV filter modified to highlight only the tennis ball

This looks great. The other objects are filtered out and I found settings that work well for my object.

The values for hue, saturation, and value, work well for this object in this lighting with this camera, but will need to be altered for your situation.

Adjust Filter Contours Filter

The output of Find Contours does not need to be adjusted, but it is informative. In my case, the HSV Filter was well adjusted and so the Find Contours filter showed only one contour (outline). Overall, that is ideal. In that situation GRIP is not confusing the tennis ball with another object on screen.

Do not actually do this, but for the sake of example, I made my HSV filter worse to better illustrate how we might filter contours when multiple are found.

Here I made the HSV filter worse and now my pipeline is confused. It's finding ~15 contours and doesn't know which one I want.

Using the Filer Contours filter we can reliably get to 1 contour.

GRIP filtering the contours down to one
GRIP filtering the contours down to one

The camera image is 320x240 pixels large. By changing the filter contours options I can specify a minimum size, area, and perimeter of a contour to help GRIP understand which contour it should detect.

Ideally, the earlier filters do the work needed. If an earlier filter can be more precise, it should be made more precise. You want as few contours as possible in Find Contours, but Filter Contours is a good pipeline stage to help clean up any noise.

Try moving the object to verify that the contour matches wherever the ball is.

Video showing the filters properly tracking with the ball even if I move it

This is a poor quality example. My office is poorly lit, the test computer is slow, and I am moving the ball with my hand. In reality, an FRC game object is larger, the lighting is better, and the robot moves to the object. That said, it is working (more or less).

Save the Project

Once we are satisfied with the pipeline click File -> Save so that you can open the pipeline later and adjust it if needed.

Generate Code

Click Tools -> Generate Code to generate application code. This application code is taking everything in GRIP and turning it into a file that can be used in a program.

GRIP just did us a huge favor. It took our pipeline and all the settings to identify our specific object and generated code that we can plug in to a vision application! It basically wrote the code for us.

GRIP can generate code in multiple languages. For this example, I am using Java as the language and checking the Implement WPILib VisionPipeline box. Once that file is saved it can be used for a vision tracking application.

At this point we can close GRIP.

Now What?

The code generated by GRIP can be used directly by a robot or by an image co-processor (like a Raspberry Pi) to help tell a robot where to move, how to shoot/use a mechanism, or otherwise what to do when it "sees" an object. But that process is not covered here.

Using GRIP is only one step of a larger series of steps for vision programming. See the Vision with WPILibPi FRC guide for a broader explanation.