RealSense OpenCV Depth Image Too Dark - c++

Question

Folks,
I have a realsense SR300, but when I display my depth image in a opencv window, it looks too dark. How can I fix this? When I run the realsense examples the images look good, but the examples use OpenGL. But I need OpenCV for my projects. Here is my code:
int main(int argc, char ** argv)
{
// realsense camera setup
rs::log_to_console(rs::log_severity::warn);
// Create a context object. This object owns the handles to all connected realsense devices
rs::context ctx;
if (ctx.get_device_count() == 0)
{
throw std::runtime_error("No device detected. Is it plugged in?");
}
// Access the first available RealSense device
rs::device * dev = ctx.get_device(0);
// Configure depth to run at VGA resolution at 30 frames per second
dev->enable_stream(rs::stream::depth, 640, 480, rs::format::z16, 30);
rs::intrinsics depth_intrin;
rs::format depth_format;
depth_intrin = dev->get_stream_intrinsics(rs::stream::depth);
depth_format = dev->get_stream_format(rs::stream::depth);
cv::namedWindow("Send Display Image", CV_WINDOW_AUTOSIZE);
/* Set callbacks prior to calling start(). */
auto depth_callback = [depth_intrin, depth_format](rs::frame f)
{
cv::Mat image(cv::Size(640, 480), CV_16UC1,
(void*)f.get_data(), cv::Mat::AUTO_STEP);
cv::imshow("Send Display Image", image);
cv::waitKey(1000/80);
};
/* callback to grab depth fream and publish it. */
dev->set_frame_callback(rs::stream::depth, depth_callback);
// Start streaming
dev->start();
While(1)
{
}
return 0;
}
I am not sure why my image is so dark. I want it to look something like the kinect or the Xtion when I run openni_launch from ROS

Answer 1

Edit:
The normalized function below produces some flickering:
I suspect that is due to the maximal depth value flickering.
The minimal depth value is always 0 as this value is used when the depth is invalid and thus the depth range becomes false.
Instead you should use this:
void make_depth_histogram(const Mat &depth, Mat &normalized_depth) {
normalized_depth = Mat(depth.size(), CV_8U);
int width = depth.cols, height = depth.rows;
static uint32_t histogram[0x10000];
memset(histogram, 0, sizeof(histogram));
for(int i = 0; i < height; ++i) {
for (int j = 0; j < width; ++j) {
++histogram[depth.at<ushort>(i,j)];
}
}
for(int i = 2; i < 0x10000; ++i) histogram[i] += histogram[i-1]; // Build a cumulative histogram for the indices in [1,0xFFFF]
for(int i = 0; i < height; ++i) {
for (int j = 0; j < width; ++j) {
if (uint16_t d = depth.at<ushort>(i,j)) {
int f = histogram[d] * 255 / histogram[0xFFFF]; // 0-255 based on histogram location
normalized_depth.at<uchar>(i,j) = static_cast<uchar>(f);
} else {
normalized_depth.at<uchar>(i,j) = 0;
}
}
}
}
What you observe is because the depth stream is coded on 16 bits (rs::stream::z16) whereas when displayed only 8 bits will be used.
You can normalized your depth map:
double min, max;
minMaxLoc(depth, &min, &max);
Mat depth_normalized;
double alpha = 255.0/(max-min);
depth.convertTo(depth_normalized, CV_8U, alpha, -min*alpha);
Or use a kind of colormap to display the depth: make_depth_histogram().
Full demo code:
inline void make_depth_histogram(const Mat &depth, Mat &color_depth) {
color_depth = Mat(depth.size(), CV_8UC3);
int width = depth.cols, height = depth.rows;
static uint32_t histogram[0x10000];
memset(histogram, 0, sizeof(histogram));
for(int i = 0; i < height; ++i) {
for (int j = 0; j < width; ++j) {
++histogram[depth.at<ushort>(i,j)];
}
}
for(int i = 2; i < 0x10000; ++i) histogram[i] += histogram[i-1]; // Build a cumulative histogram for the indices in [1,0xFFFF]
for(int i = 0; i < height; ++i) {
for (int j = 0; j < width; ++j) {
if (uint16_t d = depth.at<ushort>(i,j)) {
int f = histogram[d] * 255 / histogram[0xFFFF]; // 0-255 based on histogram location
color_depth.at<Vec3b>(i,j) = Vec3b(f, 0, 255 - f);
} else {
color_depth.at<Vec3b>(i,j) = Vec3b(0, 5, 20);
}
}
}
}
int main(int argc, char *argv[]) {
// Create a context object. This object owns the handles to all connected realsense devices
rs::context ctx;
// Access the first available RealSense device
rs::device * dev = ctx.get_device(0);
// Configure Infrared stream to run at VGA resolution at 30 frames per second
dev->enable_stream(rs::stream::depth, 640, 480, rs::format::z16, 30);
// Start streaming
dev->start();
// Camera warmup - Dropped several first frames to let auto-exposure stabilize
for(int i = 0; i < 30; i++)
dev->wait_for_frames();
// Creating OpenCV Matrix from a color image
Mat depth(Size(640, 480), CV_16U, (void*)dev->get_frame_data(rs::stream::depth), Mat::AUTO_STEP);
// Create a color depth
Mat color_depth;
make_depth_histogram(depth, color_depth);
// Create a normalized depth
double min, max;
minMaxLoc(depth, &min, &max);
Mat depth_normalized;
double alpha = 255.0/(max-min);
depth.convertTo(depth_normalized, CV_8U, alpha, -min*alpha);
// Display in a GUI
imshow("Display normalized depth", depth_normalized);
imshow("Display color depth", color_depth);
waitKey(0);
return 0;
}

Answer 2

The only solution I found to this problem which gives satisfactory results is the following:
Save the image as a PNG file. (PNG supports saving 16-bit images)
Use matplotlib to view it in a colored map:
#!/usr/bin/python3
import numpy as np
import cv2
import sys
from matplotlib import pyplot as plt
def printCoordinates(event):
x,y = event.xdata,event.ydata
if x != None:
print("X : ",x," Y: ",y," Value = ",img[np.int(y),np.int(x)])
img = cv2.imread(sys.argv[1],cv2.CV_16UC1)
#img = img/65535
fig = plt.figure()
plt.imshow(img,cmap='nipy_spectral')
cid = fig.canvas.mpl_connect('button_press_event',printCoordinates)
plt.colorbar()
plt.show()
The button_press_event is to print the exact pixel value on the pixel clicked.
RGB Image:
Corresponding Depth Image: