V4L2 Framerate Drops - c

Question

Context
Hello! I've been working on software for a robotics project that will be using vision as one of the primary senses. I've been building the system around the Raspberry Pi 3 running Raspbian Jessie Lite with kernel 4.9.35-v7+. I'm planning to train some ML models on recorded driving data from the camera in addition to other onboard sensors. To retrieve video frames I'm using the v4l2 api with a Minoru 3D USB webcam. The source for my project can be found at https://github.com/mrpossoms/AVC2017.
The Problem
This is a really bizarre issue. That which I described above works fine, with the exception that the frame rate drops significantly from the target 15fps, but only when it is looking at an object over ~1 meter away! I recorded a video here showing the issue. First I place my hand very near the camera, you can see in the terminal that the frame rate starts at the ideal 15fps. However, as I slowly move my hand away the frame rate drops to around 7fps!
Here is my camera code:
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <string.h>
#include <strings.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <assert.h>
#include "structs.h"
#include "cam.h"
#define CAM_FPS 5
cam_t cam_open(const char* path, cam_settings_t* cfg)
{
int fd = open(path, O_RDWR);
int res;
if(fd < 0)
{
fprintf(stderr, "Error opening video device '%s'\n", path);
//exit(-1);
cam_t empty = {};
return empty;
}
struct v4l2_capability cap;
res = ioctl(fd, VIDIOC_QUERYCAP, &cap);
if(res < 0)
{
fprintf(stderr, "Error: %d querying '%s' for capabilities (%d)\n", res, path, errno);
exit(-2);
}
if(!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE))
{
fprintf(stderr, "Error: '%s' lacks V4L2_CAP_VIDEO_CAPTURE capability\n", path);
}
res = cam_config(fd, cfg);
if(res < 0)
{
fprintf(stderr, "Error: %d configuring '%s' (%d)\n", res, path, errno);
exit(-3);
}
// Inform v4l about the buffers we want to receive data through
struct v4l2_requestbuffers bufrequest = {};
bufrequest.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
bufrequest.memory = V4L2_MEMORY_MMAP;
bufrequest.count = 2;
if(ioctl(fd, VIDIOC_REQBUFS, &bufrequest) < 0)
{
fprintf(stderr, "VIDIOC_REQBUFS\n");
exit(-4);
}
if(bufrequest.count < 2)
{
fprintf(stderr, "Not enough memory\n");
exit(-5);
}
struct v4l2_buffer bufferinfo = {};
void** fbs = calloc(sizeof(void*), bufrequest.count);
for(int i = bufrequest.count; i--;)
{
bufferinfo.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
bufferinfo.memory = V4L2_MEMORY_MMAP;
bufferinfo.index = i;
if(ioctl(fd, VIDIOC_QUERYBUF, &bufferinfo) < 0)
{
fprintf(stderr, "VIDIOC_QUERYBUF\n");
exit(-5);
}
fbs[i] = mmap(
NULL,
bufferinfo.length,
PROT_READ | PROT_WRITE,
MAP_SHARED,
fd,
bufferinfo.m.offset
);
if(fbs[i] == MAP_FAILED)
{
fprintf(stderr, "mmap failed\n");
exit(-6);
}
bzero(fbs[i], bufferinfo.length);
ioctl(fd, VIDIOC_QBUF, &bufferinfo);
}
cam_t cam = {
.fd = fd,
.frame_buffers = fbs,
.buffer_info = bufferinfo,
};
cam_wait_frame(&cam);
int type = bufferinfo.type;
if(ioctl(fd, VIDIOC_STREAMON, &type) < 0)
{
fprintf(stderr, "Error starting streaming\n");
exit(-7);
}
return cam;
}
int cam_request_frame(cam_t* cam)
{
cam->buffer_info.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
cam->buffer_info.memory = V4L2_MEMORY_MMAP;
return ioctl(cam->fd, VIDIOC_QBUF, &cam->buffer_info);
}
int cam_wait_frame(cam_t* cam)
{
ioctl(cam->fd, VIDIOC_DQBUF, &cam->buffer_info);
}
int cam_config(int fd, cam_settings_t* cfg)
{
int res = 0;
struct v4l2_format format;
if(!cfg)
{
fprintf(stderr, "Error: null configuration provided\n");
return -1;
}
/*
res = ioctl(fd, VIDIOC_G_FMT, &format);
if(res < 0)
{
fprintf(stderr, "Error: failed retrieving camera settings (%d)\n", errno);
return -2;
}
*/
format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
format.fmt.pix.width = cfg->width;
format.fmt.pix.height = cfg->height;
if(ioctl(fd, VIDIOC_S_FMT, &format) < 0)
{
fprintf(stderr, "Error: failed applying camera settings\n");
return -3;
}
struct v4l2_streamparm parm = {};
parm.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
parm.parm.capture.timeperframe.numerator = 1;
parm.parm.capture.timeperframe.denominator = 15;
res = ioctl(fd, VIDIOC_S_PARM, &parm);
return 0;
}
I can reproduce the problem 100% of the time, and I'm really at a loss as to what could be causing this behavior. Any suggestions, or ideas are greatly appreciated.

Answer 1

As Mark suggested, the problem was the exposure. The driver must have decided that it needed more exposure time per frame in darker situations. Just by bringing the camera outside immediately boosted the frame rate to the target 15fps.