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The code behind the picture

Follow the lesson’s implementation, one code cell at a time.

Choose a language and read the cells in order. These are small teaching examples; the visual lesson runs separately.

Follow the calculation

Same inputs · Same operation · Same result

3 teaching cells · Python standard library · Read-only

1. Prepare the patch and the Sobel filters
A 3 × 3 patch from the watermelon's lower-right rim (grayscale levels 0–9), and the Sobel x and y kernels. y grows downward.
In [ ]:
```
import json

patch = [[3, 3, 0], [0, 0, 8], [8, 8, 8]]
sobel_x = [[-1, 0, 1], [-2, 0, 2], [-1, 0, 1]]
sobel_y = [[-1, -2, -1], [0, 0, 0], [1, 2, 1]]
```

2. Measure the two components

Each component is the sum of the nine pixel-weight products.

def apply(kernel):
    return sum(
        patch[i][j] * kernel[i][j]
        for i in range(3)
        for j in range(3)
    )

gx = apply(sobel_x)
gy = apply(sobel_y)
squared = gx * gx + gy * gy

3. Inspect the result
Gx, Gy and the squared magnitude; its square root is about 26.4.
In [ ]:
```
print(json.dumps([gx, gy, squared]))
```

Expected result

Running the three cells in order produces this result in either language.