Introduction

In this post, we’ll cover how to create a square crop of an image in Unity. The approach used in this tutorial can be adapted to crop other sections of an image as well.

Create a 2D Unity Project

Open the Unity Hub and create a new 2D project. I’m using Unity 2019.4.20f1, but you should be fine using other versions.

unity-hub-create-new-project

Create Crop Script

In Unity, right-click an empty space in the Assets folder and select C# Script in the Create submenu. Name the new script, Crop and open it in your code editor.

unity-create-crop-script

Define Variables

Create a public GameObject variable called screen. We’ll be using this screen to confirm our script is correctly cropping the test images. Add a public bool variable called cropImage as well. This will let us toggle whether to crop the image during runtime. Lastly, we’ll create a private RenderTexture called image to store a copy of the original test image.

crop-script-public-variables

Define Start() Method

In the Start() method, we’ll store a copy the original test image in the image RenderTexture. We can do so by getting a reference to the Texture attached to the screen and using the Graphics.Blit() method. We’ll also adjust the camera so that we can see the entire image.

crop-script-start-method

Define Update() Method

First, we need to make another copy of the original image so that we can edit it. We’ll store this copy in a temporary RenderTexture called rTex that will get released at the end of the method.

We can’t change the dimensions of a RenderTexture after it’s been created. Instead, we’ll create a cropped image by copying part of rTex to another temporary RenderTexture called tempTex that will be square. We can copy the square image to rTex after we release the current RenderTexture assigned to rTex and make a new square one.

The size of tempTex will depend on whether the original image is wider or taller. We want to use the smallest side of the original image.

We’ll determine what part of rTex we need to copy by calculating either (image.width - image.height) / 2f or (image.height - image.width) / 2f depending on whether the image is wider or taller.

We can copy part of rTex to tempTex using the Graphics.CopyTexture() method. We need to specify several parameters in order to use this method to crop images.

  1. src: The original image
  2. srcElement: The source texture element, set to 0
    • Not relevant for our use case
  3. srcMip: The mipmap level for the image RenderTexture, set to 0
    • Not relevant for our use case
  4. srcX: The X coordinate of the top left corner of the center square of the original image
  5. srcY: The Y coordinate of the top left corner of the center square of the original image
  6. srcWidth: Width of the new square image
  7. srcHeight: Height of the new square image
  8. dst: An empty square RenderTexture
  9. dstElement: The destination texture element, set to 0
    • Not relevant for our use case
  10. dstMip: The mipmap level for destination texture, set to 0
    • Not relevant for our use case
  11. dstX: The X coordinate of the top left corner of the new square image
  12. dstY: The Y coordinate of the top left corner of the new square image

After we copy tempTex back to rTex we’ll update the Texture for the screen with the new square image and adjust the shape of the screen to fit the new image.

crop-script-update-method

Create Screen GameObject

Back in Unity, right-click an empty space in the Hierarchy tab and select Quad from the 3D Object submenu. Name the new object Screen. The size will be updated automatically by the Crop.cs script.

unity-create-screen-object

Create ImageCropper

Right-click an empty space in the Hierarchy tab and select Create Empty from the pop-up menu. Name the empty object ImageCropper

unity-create-image-cropper-object

With the ImageCropper selected drag and drop the Crop.cs script into the Inspector tab.

unity-attach-crop-script

Drag and drop the Screen object from the Hierarchy tab onto the Screen parameter in the Inspector tab.

unity-inspector-tab-assign-screen

Test it Out

We’ll need some test images to try out the ImageCropper. You can use your own or download the ones I used for this tutorial.

Drag and drop the test images into the Assets folder. Select one of the images and drag it onto the Screen in the Scene.

unity-import-images

Next, we need to set our Screen to use an Unlit shader. Otherwise it will be a bit dim. With the Screen object selected, open the Shader drop-down menu in the Inspector tab and select Unlit.

unity-inspector-tab-shader-drop-down

Select Texture from the Unlit submenu.

unity-inspector-tab-unlit-texture

Now we can click the Play button and toggle the Crop Image checkbox to confirm our script is working properly. If you check the performance stats, you should see that there is basically no performance hit from cropping the image.

crop_image_on_gpu_unity_1

crop_image_on_gpu_unity_2

Conclusion

That is one method to efficiently crop images on the GPU in Unity. As mentioned earlier, this method can be adapted to crop different parts of the image. You can do so by changing the values for the Graphics.CopyTexture() method to adjust what part of the source image gets copied and where in the target image it gets copied to.