Projector-Camera Calibration - the "easy" way

First let me open by saying projector-camera calibration is NOT EASY. But it's technically not complicated too.

It is however, an amalgamation of optimizations that accrue and accumulate error with each step, so that the end product is not far from a random guess.
So 3D reconstructions I was able to get from my calibrated pro-cam were just a distorted mess of points.

Nevertheless, here come the deets.
Continue reading "Projector-Camera Calibration - the "easy" way"


Revisiting graph-cut segmentation with SLIC and color histograms [w/Python]

As part of the computer vision class I'm teaching at SBU I asked students to implement a segmentation method based on SLIC superpixels. Here is my boilerplate implementation.

This follows the work I've done a very long time ago (2010) on the same subject.

For graph-cut I've used PyMaxflow:, which is very easily installed by just pip install PyMaxflow

The method is simple:

  • Calculate SLIC superpixels (the SKImage implementation)
  • Use markings to determine the foreground and background color histograms (from the superpixels under the markings)
  • Setup a graph with a straightforward energy model: Smoothness term = K-L-Div between superpix histogram and neighbor superpix histogram, and Match term = inf if marked as BG or FG, or K-L-Div between SuperPix histogram and FG and BG.
  • To find neighbors I've used Delaunay tessellation (from scipy.spatial), for simplicity. But a full neighbor finding could be implemented by looking at all the neighbors on the superpix's boundary.
  • Color histograms are 2D over H-S (from the HSV)



WTH OpenGL 4? Rendering elements arrays with VAOs and VBOs in a QGLWidget

I spent an entire day getting OpenGL 4 to display data from a VAO with VBOs so I thought I'd share the results with you guys, save you some pain.

I'm using the excellent GL wrappers from Qt, and in particular QGLShaderProgram.
This is pretty straightforward, but the thing to remember is that OpenGL is looking for the vertices/other elements (color? tex coords?) to come from some bound GL buffer or from the host. So if your app is not working and nothing appears on screen, just make sure GL has a bound buffer and the shader locations match up and consistent (see the const int I have on the class here).


Quickly: How to render a PDF to an image in C++?

Using Poppler, of course!
Poppler is a very useful tool for handling PDF, so I've discovered lately. Having tried both muPDF and ImageMagick's Magick++ and failed, Poppler stepped up to the challenge and paid off.

So here's a small example of how work the API (with OpenCV, naturally):

#include <iostream>
#include <fstream>
#include <sstream>
#include <opencv2/opencv.hpp>
#include <poppler-document.h>
#include <poppler-page.h>
#include <poppler-page-renderer.h>
#include <poppler-image.h>

using namespace cv;
using namespace std;
using namespace poppler;

Mat readPDFtoCV(const string& filename,int DPI) {
    document* mypdf = document::load_from_file(filename);
    if(mypdf == NULL) {
        cerr << "couldn't read pdf\n";
        return Mat();
    cout << "pdf has " << mypdf->pages() << " pages\n";
    page* mypage = mypdf->create_page(0);

    page_renderer renderer;
    image myimage = renderer.render_page(mypage,DPI,DPI);
    cout << "created image of  " << myimage.width() << "x"<< myimage.height() << "\n";

    Mat cvimg;
    if(myimage.format() == image::format_rgb24) {
    } else if(myimage.format() == image::format_argb32) {
    } else {
        cerr << "PDF format no good\n";
        return Mat();
    return cvimg;

All you have to do is give it the DPI (say you want to render in 100 DPI) and a filename.
Keep in mind it only renders the first page, but getting the other pages is just as easy.

That's it, enjoy!