1.27.5. 示例

（假设使用视频捕获设备；对于其他设备，请更改 V4L2_BUF_TYPE_VIDEO_CAPTURE；将目标更改为 V4L2_SEL_TGT_COMPOSE_* 系列以配置合成区域）

1.27.5.1. 示例：重置裁剪参数

struct v4l2_selection sel = {
    .type = V4L2_BUF_TYPE_VIDEO_CAPTURE,
    .target = V4L2_SEL_TGT_CROP_DEFAULT,
};
ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
if (ret)
    exit(-1);
sel.target = V4L2_SEL_TGT_CROP;
ret = ioctl(fd, VIDIOC_S_SELECTION, &sel);
if (ret)
    exit(-1);

在显示器中心设置一个大小最多为限制一半的输出合成区域。

1.27.5.2. 示例：简单缩小比例

struct v4l2_selection sel = {
    .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
    .target = V4L2_SEL_TGT_COMPOSE_BOUNDS,
};
struct v4l2_rect r;

ret = ioctl(fd, VIDIOC_G_SELECTION, &sel);
if (ret)
    exit(-1);
/* setting smaller compose rectangle */
r.width = sel.r.width / 2;
r.height = sel.r.height / 2;
r.left = sel.r.width / 4;
r.top = sel.r.height / 4;
sel.r = r;
sel.target = V4L2_SEL_TGT_COMPOSE;
sel.flags = V4L2_SEL_FLAG_LE;
ret = ioctl(fd, VIDIOC_S_SELECTION, &sel);
if (ret)
    exit(-1);

假设使用视频输出设备；对于其他设备，请更改 V4L2_BUF_TYPE_VIDEO_OUTPUT

1.27.5.3. 示例：查询缩放因子

struct v4l2_selection compose = {
    .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
    .target = V4L2_SEL_TGT_COMPOSE,
};
struct v4l2_selection crop = {
    .type = V4L2_BUF_TYPE_VIDEO_OUTPUT,
    .target = V4L2_SEL_TGT_CROP,
};
double hscale, vscale;

ret = ioctl(fd, VIDIOC_G_SELECTION, &compose);
if (ret)
    exit(-1);
ret = ioctl(fd, VIDIOC_G_SELECTION, &crop);
if (ret)
    exit(-1);

/* computing scaling factors */
hscale = (double)compose.r.width / crop.r.width;
vscale = (double)compose.r.height / crop.r.height;