Rotating models with OpenGL using OpenCV cameras, tutorial page 3.

July 12, 2019

This post is an expansion of the previous page, which offered the basic code to get the OpenGL rendering from OpenCV cameras. In this example, I’ll show you how to create a rotating-three-dimensional-model video, which is very effective for showing off your cool reconstructions in a variety of settings (as shown above). This code uses the same repository as the previous set of examples on Github – amy-tabb/OpenCV2OpenGL.

Roadmap.

Back to Tips and Tricks Table of Contents

Writing multiple images, rotating about an axis.

For this example, I altered the existing code in Example1 in the repository, and added another version (1). The structure of this post is to quickly go over running the code and the parameters using the data in the repository, and then go into the details a bit. To call this version, you need the calibration information file like before (but with some more optional parameters), and output directory, and the version number of 1, like:

./OpenCV2OpenGL1 --input /home/username/git/OpenCV2OpenGL/Data2/CaliHorse.txt --output /home/username/git/OpenCV2OpenGL/Data2/HorseFolder  --version 1

The calibration information file can contain the same kind of information as with the version 0 example on the previous page, but with a few additional parameters:

total-degrees: the total number of degrees of rotation.
rotation-vector: the axis of rotation, a 3-element vector.
degrees-per-step: degrees per rotation increment.
first-file-number: this is the number of the first image file written. It may be convenient to string together different rotation axes, and this parameter allows this – shown throughout this post and in the video above.

Examples of these types of files, and results are given in the Data2 folder in the amy-tabb/OpenCV2OpenGL repository. This is CaliHorse1.txt

cols 640
rows 480
K 
1000       0     320
      0 1000     240
      0       0       1
RTworld2cam 
0 1 0 0
0 0 -1 0
1 0 0 5
Model
10 0 0 0
0 10 0 0 
0   0 10  0 
0   0 0  1 
total-degrees 360
rotation-vector 1 0 0
degrees-per-step 5
first-file-number 72
write-camera 1
light-position -10 10 0.0
shininess 0.1
near 1 
far 10
file /home/username/Data2/horse1.ply

Creating a video with multiple rotation axes.

To create the video above, I ran the code three times, with three different files. The files only differ in the rotation-axis and the first-file-number parameter and are all in the Data2 folder. For instance:

./OpenCV2OpenGL1 --input /home/username/git/OpenCV2OpenGL/Data2/CaliHorse.txt --output /home/username/git/OpenCV2OpenGL/Data2/HorseFolder  --version 1

./OpenCV2OpenGL1 --input /home/username/git/OpenCV2OpenGL/Data2/CaliHorse1.txt --output /home/username/git/OpenCV2OpenGL/Data2/HorseFolder  --version 1

./OpenCV2OpenGL1 --input /home/username/git/OpenCV2OpenGL/Data2/CaliHorse2.txt --output /home/username/git/OpenCV2OpenGL/Data2/HorseFolder  --version 1

You’ll notice that the output directory is the same – the result is 216 image files of the horse, \(360\) degrees around three rotation axes, at \(5\) degrees between each image per rotation with the same axis.

To create a video, my choice is ffmpeg. From within the HorseFolder, the command is:

ffmpeg -r 30 -i %d.png -c:v libx264 -pix_fmt yuv420p horse30.mp4

for a .mp4 video at \(30\) frames per second. To create a gif is straightforward given a .mp4:

ffmpeg -i horse30.mp4 horse30.gif

ffmpeg has lots of options; you can tell by the massive numbers of StackOverflow questions and answers on the topic.

Code detail 1 and caution: loop handler.

Recall that in the first version of this code, an OpenGL window is shown, and you can’t really do anything with your computer until you hit escape. That was because there was a while loop that held the OpenGL window open based on the condition below:

	while (!glfwWindowShouldClose(window)){

In this version, the number of images to acquire is computed based on the user-supplied parameters, and this value is number_steps. This loop will hold the OpenGL window until the loop is finished. If you experiment, I encourage creating a loop with only 1 or 2 iterations in the beginning, and double-check the loop bounds. If you get stuck in an infinite loop, you’ll have to reboot or quit the windowing system!

	//while (!glfwWindowShouldClose(window))
	for (int step_counter = 0; step_counter < number_steps; step_counter++){

Code detail 2: applying a rotation to the modelview matrix.

Within the loop described in the previous section, the rotation about an axis is achieved as follows:

	current_angle_rad = float(step_counter)*degrees_per_step*0.0174533;

	modelR = glm::rotate(modelR, current_angle_rad, glm::vec3(rotation_vector(0), rotation_vector(1), rotation_vector(2)));

	model = modelR*model;

model is the modelview matrix from the previous page, in other words, the transformation applied to the object.
current_angle_rad is the angle of rotation in radians.
rotation_vector is the axis of rotation supplied by the user in the calibration information file.
modelR is the rotation matrix, which the glm library will compute for you.

Finally, we create a new modelview matrix by applying the current modelR to the transformation applied to the object for all steps. In the case of the horse model, this is convenient, because I scale it using the model matrix.

Conclusion.

I’m sure you can see there are many more variations you can add to this demo to get interesting visualizations.