如何用 bit_permute_step 代替手动位操作

发布于 2025-01-13 21:09:59 字数 705 浏览 2 评论 0原文

我怎样才能用 http://programming.sirrida.de/perm_fn 替换以下手动逻辑。 html#bit_permute_step ？

 unsigned int ConvertRGBAToBGRA(unsigned int v) {
    unsigned char r = (v)& 0xFF;
    unsigned char g = (v >> 8) & 0xFF;
    unsigned char b = (v >> 16) & 0xFF;
    unsigned char a = (v >> 24) & 0xFF;
    return (a << 24) | (r << 16) | (g << 8) | b;
};

有没有更好的方法使用 http://programming.sirrida.de/perm_fn 来执行上述操作。 html#bit_permute_step ？

原文

How can I do replace the following manual logic by http://programming.sirrida.de/perm_fn.html#bit_permute_step ?

 unsigned int ConvertRGBAToBGRA(unsigned int v) {
    unsigned char r = (v)& 0xFF;
    unsigned char g = (v >> 8) & 0xFF;
    unsigned char b = (v >> 16) & 0xFF;
    unsigned char a = (v >> 24) & 0xFF;
    return (a << 24) | (r << 16) | (g << 8) | b;
};

Is there a better way to do the above using http://programming.sirrida.de/perm_fn.html#bit_permute_step ?

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我早已燃尽 2025-01-20 21:09:59

是的，即：

return bit_permute_step(v, 0x000000ff, 16);

掩码0x000000ff表示的v的位包含r分量，bit_permute_step将交换它们相应的位 16（距离参数）位于左侧，对应于 v 的 b 组件。因此，bit_permute_step(v, 0x000000ff, 16) 会将 r 与 b 交换，从而将 RGBA 转换为 BGRA（也将 BGRA 转换为 RGBA），因为交换是其自身的逆）。

这也可以通过排列计算器找到：http://programming.sirrida.de/calcperm.php< /a>

使用索引 16 17 18 19 20 21 22 23 8 9 10 11 12 13 14 15 0 1 2 3 4 5 6 7 24 25 26 27 28 29 30 31（源索引）并禁用位组移动。

32 位整数的 bit_permute_step 的 C++ 实现（也可用作 C 代码）可以是：

uint32_t bit_permute_step(uint32_t x, uint32_t m, uint32_t shift) {
    uint32_t t;
    t = ((x >> shift) ^ x) & m;
    x = (x ^ t) ^ (t << shift);
    return x;
}

Yes, namely:

return bit_permute_step(v, 0x000000ff, 16);

The bits of v indicated by the mask 0x000000ff contain the r component, bit_permute_step will exchange them with the corresponding bits 16 (the distance parameter) places to the left, which corresponds to the b component of v. So bit_permute_step(v, 0x000000ff, 16) will swap the r with the b, and hence turn RGBA into BGRA (and also BGRA into RGBA, because a swap is its own inverse).

This could also be found via the permutation calculator: http://programming.sirrida.de/calcperm.php

Use the indices 16 17 18 19 20 21 22 23 8 9 10 11 12 13 14 15 0 1 2 3 4 5 6 7 24 25 26 27 28 29 30 31 (source indices) and disable bit-group moving.

A C++ implementation (also usable as C code) of bit_permute_step for 32-bit integers could be:

uint32_t bit_permute_step(uint32_t x, uint32_t m, uint32_t shift) {
    uint32_t t;
    t = ((x >> shift) ^ x) & m;
    x = (x ^ t) ^ (t << shift);
    return x;
}

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