WebP encoding - Segmentation Fault

Question

So I'm trying to use the webp API to encode images. Right now I'm going to be using openCV to open and manipulate the images, then I want to save them off as webp. Here's the source I'm using:

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <cv.h>
#include <highgui.h>
#include <webp/encode.h>

int main(int argc, char *argv[])
{

    IplImage* img = 0;
    int height,width,step,channels;
    uchar *data;
    int i,j,k;
    if (argc<2) {
        printf("Usage:main <image-file-name>\n\7");
    exit(0);
    }
    // load an image
    img=cvLoadImage(argv[1]);

    if(!img){
        printf("could not load image file: %s\n",argv[1]);
        exit(0);
    }

    // get the image data
    height      = img->height;
    width       = img->width;
    step        = img->widthStep;
    channels    = img->nChannels;
    data        = (uchar *)img->imageData;
    printf("processing a %dx%d image with %d channels \n", width, height, channels);

    // create a window
    cvNamedWindow("mainWin", CV_WINDOW_AUTOSIZE);
    cvMoveWindow("mainWin",100,100);

    // invert the image
    for (i=0;i<height;i++) {
        for (j=0;j<width;j++) {
            for (k=0;k<channels;k++) {
                data[i*step+j*channels+k] = 255-data[i*step+j*channels+k];
            }
        }
    }

    // show the image
    cvShowImage("mainWin", img);

    // wait for a key
    cvWaitKey(0);
    // release the image
    cvReleaseImage(&img);

    float qualityFactor = .9;
    uint8_t** output;
    FILE *opFile;
    size_t datasize;
    printf("encoding image\n");
    datasize = WebPEncodeRGB((uint8_t*)data,width,height,step,qualityFactor,output);

    printf("writing file out\n");
    opFile=fopen("output.webp","w");
    fwrite(output,1,(int)datasize,opFile);
}

When I execute this, I get this:

nato@ubuntu:~/webp/webp_test$ ./helloWorld ~/Pictures/mars_sunrise.jpg 
processing a 2486x1914 image with 3 channels 
encoding image
Segmentation fault

It displays the image just fine, but segfaults on the encoding. My initial guess was that it's because I'm releasing the img before I try to write out the data, but it doesn't seem to matter whether I release it before or after I try the encoding. Is there something else I'm missing that might cause this problem? Do I have to make a copy of the image data or something?

The WebP api docs are... sparse. Here's what the README says about WebPEncodeRGB:

The main encoding functions are available in the header src/webp/encode.h
The ready-to-use ones are:

size_t WebPEncodeRGB(const uint8_t* rgb, int width, int height, 
    int stride, float quality_factor, uint8_t** output);

The docs specifically do not say what the 'stride' is, but I'm assuming that it's the same as the 'step' from opencv. Is that reasonable?

Thanks in advance!

Answer 1

so it looks like the problem was here:

// load an image
img=cvLoadImage(argv[1]);

The function cvLoadImage takes an extra parameter

cvLoadImage(const char* filename, int iscolor=CV_LOAD_IMAGE_COLOR)

and when I changed to

img=cvLoadImage(argv[1],1);

the segfault went away.