Linux 帧缓冲区像素位域通用实现

Question

我正在编写一个小型库来与 Linux 的帧缓冲区抽象接口。我的所有显卡都使用相同的像素格式（每个通道一个八位字节，四个通道，BGRA 排序），因此到目前为止，库仅采用这种格式。但是，如果我希望该库在任何 Linux 帧缓冲区上工作，帧缓冲区 API 提供了我必须使用的像素格式数据。你不需要知道帧缓冲区是如何工作的来回答这个问题（我希望），只需要一些我不熟悉的小技巧。这是我的标题中提供的像素格式信息：

/* Interpretation of offset for color fields: All offsets are from the right,
 * inside a "pixel" value, which is exactly 'bits_per_pixel' wide (means: you
 * can use the offset as right argument to <<). A pixel afterwards is a bit
 * stream and is written to video memory as that unmodified.
 *
 * For pseudocolor: offset and length should be the same for all color
 * components. Offset specifies the position of the least significant bit
 * of the pallette index in a pixel value. Length indicates the number
 * of available palette entries (i.e. # of entries = 1 << length).
 */
struct fb_bitfield {
        __u32 offset;                   /* beginning of bitfield        */
        __u32 length;                   /* length of bitfield           */
        __u32 msb_right;                /* != 0 : Most significant bit is */ 
                                        /* right */
};

/* snip */

struct fb_var_screeninfo {
    /* snip */

        __u32 bits_per_pixel;           /* guess what                   */
        __u32 grayscale;                /* 0 = color, 1 = grayscale,    */
                                        /* >1 = FOURCC                  */
        struct fb_bitfield red;         /* bitfield in fb mem if true color, */
        struct fb_bitfield green;       /* else only length is significant */
        struct fb_bitfield blue;
        struct fb_bitfield transp;      /* transparency                 */      

        __u32 nonstd;                   /* != 0 Non standard pixel format */

    /* snip */
};

我需要将使用上述信息格式化的像素写入由四个字符（R、G、B 和 A）组成的字符数组。我为此所做的错误尝试如下所示：

long pxl = 0;
/* downsample each channel to the length (assuming its less than 8 bits) */
/* with a right-shift, then left shift it over into place */
/* haven't done anything with fb_bitfield.msb_right */
pxl |= (r >> (8 - vinfo.red.length)) << vinfo.red.offset;
pxl |= (g >> (8 - vinfo.green.length)) << vinfo.green.offset;
pxl |= (b >> (8 - vinfo.blue.length)) << vinfo.blue.offset;
pxl |= (a >> (8 - vinfo.transp.length)) << vinfo.transp.offset;
fb[xy2off(x, y)] = /* umm... */;
/* little endian, big endian? Can I just assign here? */

xy2off将坐标转换为索引。fb是指向帧缓冲区（内存映射）的指针。

谁能指出我转换和分配这些像素的正确方向？

Answer 1

亲爱的可能不存在的人正在寻找这个问题的答案：这是我现在似乎可以工作的黑客代码：

无效像素（int x，int y，uint8_t r，uint8_t g，uint8_t b，uint8_t a）
{
    整数我；
    长像素 = 0;
    pxl |= (r >> (8 - vinfo.red.length)) << vinfo.red.offset;
    pxl |= (g >> (8 - vinfo.green.length)) << vinfo.green.offset;
    pxl |= (b >> (8 - vinfo.blue.length)) << vinfo.blue.offset;
    pxl |= (a >> (8 - vinfo.transp.length)) << vinfo.transp.offset;
    for (i = 0; i < sizeof(long); i++) {
        fb[xy2off(x, y) + i] = ((char *)(&pxl))[i];
    }
}

缺少字节顺序处理、处理 msb_right 或任何类型的优化。