normal-grain-merge 0.0.2

Fused normal and grain merge C extension

normal_grain_merge

This implements a combined version of the blend modes normal and grain merge. Grain merge is performed on s and t with the result normal-merged with b. Subscripts indicate channels, with alpha (α) channels broadcast to three channels.

$$ (((\mathrm{t_{rgb}} + \mathrm{s_{rgb}} - 0.5) * \mathrm{t_\alpha} + \mathrm{t_{rgb}} * (1 - \mathrm{t_\alpha})) * (1 - 0.3) + \mathrm{s_{rgb}} * 0.3) * \mathrm{t_\alpha} + \mathrm{b_{rgb}} * (1 - \mathrm{t_\alpha}) $$

Installation

pip install normal-grain-merge

Usage

import numpy as np
from normal_grain_merge import normal_grain_merge, KernelKind


# Example arrays
base = np.zeros((100, 100, 3), dtype=np.uint8)
texture = np.zeros((100, 100, 3), dtype=np.uint8)
skin = np.zeros((100, 100, 4), dtype=np.uint8)
im_alpha = np.zeros((100, 100), dtype=np.uint8)

result_scalar = normal_grain_merge(base, texture, skin, im_alpha, KernelKind.KERNEL_SCALAR.value)
print(result_scalar.shape, result_scalar.dtype)

There are three kernels implemented in this module as defined in KernelKind.

• KERNEL_AUTO: Automatically chooses the kernel, preferring AVX2
• KERNEL_SCALAR: Portable scalar implementation.
• KERNEL_SSE42: SSE4.2 intrinsics kernel. Likely better on AMD CPUs.
• KERNEL_AVX2: AVX2 intrinsics kernel. Likely better on Intel CPUs.

Parameters

All input matrices should have the same height and width.

base

RGB or RGBA, dropping the alpha channel if it exists. The base image for application.

texture

RGB or RGBA, applying the alpha if it exists. This is the texture to be applied.

skin

RGBA, the segmented portion of base to texture. The "skin" of the object the texture is to be applied to.

im_alpha

The alpha of parameter skin. This is mostly a holdover from the Python implementation to deal with NumPy.

kernel

One of KernelKind.

Performance

The entire reason for me writing this was NumPy being slow when this operation is in the hot path. So, I decided to write a SIMD version that does the type casting outside NumPy with only the intermediate values being in FP32.

How much of a speedup is this? All numbers are from a Ryzen 7 4800H running Windows 11 and Python 3.12.4.

Method/Kernel	Average Iteration Time
C scalar kernel	0.019565s
C SSE4.2 kernel	0.013705s
C AVX2 kernel	0.016842s
NumPy version	0.228098s
Old NumPy version	0.350554s

Method Comparison	Speedup
NumPy -> scalar	91.4227%
NumPy -> SSE4.2	93.9915%
NumPy -> AVX2	92.6165%
Old np -> SSE4.2	96.0904%
C scalar -> SSE4.2	29.9487%
C scalar -> AVX2	13.9183%