Gradient Computation

Overview

The gradient module provides Sobel-based image gradient computation, which is the first step in nearly every radsym pipeline. All proposal and refinement algorithms operate on the precomputed gradient field rather than the raw image.

Sobel 3x3 operator

Two functions are provided:

• sobel_gradient – accepts an ImageView<u8> (8-bit grayscale).
• sobel_gradient_f32 – accepts an ImageView<f32> (floating-point).

Both apply the standard 3x3 Sobel kernels:

$$G_x = \begin{bmatrix} -1 & 0 & +1 \ -2 & 0 & +2 \ -1 & 0 & +1 \end{bmatrix}, \qquad G_y = \begin{bmatrix} -1 & -2 & -1 \ 0 & 0 & 0 \ +1 & +2 & +1 \end{bmatrix},$$

producing horizontal ($\partial I / \partial x$) and vertical ($\partial I / \partial y$) derivatives respectively.

GradientField

The result is stored in a GradientField struct with two OwnedImage<f32> buffers:

• gx – horizontal gradient. Positive values indicate intensity increasing rightward.
• gy – vertical gradient. Positive values indicate intensity increasing downward.

Both buffers have the same dimensions as the input image. Accessor methods gx() and gy() return borrowed ImageView slices for zero-copy downstream use.

Border handling

Border pixels (the outermost 1-pixel ring) are set to zero, since the 3x3 kernel requires a 1-pixel neighborhood that is not available at the boundary. This is consistent across both u8 and f32 variants.