Linux framebuffer像素位域通用实现

2019年7月21日 219次阅读

我正在编写一个小型库来与
Linux的帧缓冲抽象接口.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),所以到目前为止,库只采用这种格式.但是,framebuffer API提供像素格式数据,如果我希望库在任何Linux帧缓冲区上工作,我必须使用它.你不需要知道帧缓冲如何回答这个问题(我希望),只是有点小提琴我不熟练.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 */
};

我需要将一个使用上述信息格式化的像素写入四个字符(R,G,B和A)的字符数组中.我做过这个误导的尝试看起来像这样：

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是指向帧缓冲区的指针(内存映射).

任何人都可以指出我正确的方向转换和分配这些像素？

最佳答案 亲爱的可能不存在的人正在寻找答案：这是我现在看到的那些似乎现在有用的hackish代码：

void pixel(int x, int y, uint8_t r, uint8_t g, uint8_t b, uint8_t a)
{
    int i;
    long pxl = 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];
    }
}

缺少字节序处理,处理msb_right或任何类型的优化.

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