使用libjpeg的8位灰度源图像,可以做到吗?

我正在使用libjpeg创建jpeg

http://www.ijg.org/

#define COLOR_COMPONENTS    (3)
#define COLOR_SPACE         (JCS_RGB)
JSAMPLE image_buffer[WIDTH*HEIGHT *3] = 
{
0x80, 0x80, 0x80,    0x80, 0x80, 0x80,    0x80, 0x80, 0x80,    0x80, 0x80, 0x80,
0x80, 0x80, 0x80,    0x00, 0x00, 0x00,    0x00, 0x00, 0x00,    0x80, 0x80, 0x80,
0x80, 0x80, 0x80,    0x00, 0x00, 0x00,    0x00, 0x00, 0x00,    0x80, 0x80, 0x80,
0x80, 0x80, 0x80,    0x80, 0x80, 0x80,    0x80, 0x80, 0x80,    0x80, 0x80, 0x80,
};

...
// inside libjpeg I set the values
cinfo.image_width = image_width;    /* image width and height, in pixels */
cinfo.image_height = image_height;
cinfo.input_components = COLOR_COMPONENTS;      /* # of color components per pixel */
cinfo.in_color_space = COLOR_SPACE;     /* colorspace of input image */
/* Now use the library's routine to set default compression parameters.
* (You must set at least cinfo.in_color_space before calling this,
* since the defaults depend on the source color space.)
*/
jpeg_set_defaults(&cinfo);

但是,这不起作用:

#define COLOR_COMPONENTS    (1)
#define COLOR_SPACE         (JCS_GRAYSCALE)
JSAMPLE image_buffer[WIDTH*HEIGHT * 1] =
{
    0x80, 0x80, 0x80, 0x80,
    0x80, 0x00, 0x00, 0x80,
    0x80, 0x00, 0x00, 0x80,
    0x80, 0x80, 0x80, 0x80,
};

cinfo.input_components = ???
cinfo.in_color_space = ???

代码需要在低时钟CPU上运行。因此,我没有多余的周期来转换从灰度图像和RGB。

谢谢您!