Opengl texture cylinder trouble

Question

I'm a little bit confused.

I want to texture a "cylinder", so my first approach was this (with n the number of faces and k the number of iteration)

void cylindrer(double r, int n,double h){
    double x[n],y[n];
    for(int k=0; k<n; k++){
        x[k]=r*cos(2*k*M_PI/n);
        y[k]=r*sin(2*k*M_PI/n);
    }


    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, texObject[1]);

    for(int k=0; k<n ;k++){
        int m= (k+1)%n;

        glBegin(GL_POLYGON);
            glTexCoord2f(1.0/n*(k),0.0);   glVertex3f( x[k], y[k], h/2);
            glTexCoord2f(1.0/n*(k),1);   glVertex3f( x[k], y[k], -h/2);
            glTexCoord2f(1.0/n*(k+1),1);   glVertex3f( x[m], y[m], -h/2);
            glTexCoord2f(1.0/n*(k+1),0.0);   glVertex3f( x[m], y[m], h/2);
        glEnd();
    }
    glDisable(GL_TEXTURE_2D);
}

The texture is applied but reverse so I'd changed to

glBegin(GL_POLYGON);
    glTexCoord2f(1.0/n*(n-k), 0.0);   glVertex3f( x[k], y[k], h/2);
    glTexCoord2f(1.0/n*(n-k), 1.0);   glVertex3f( x[k], y[k], -h/2);
    glTexCoord2f(1.0/n*(n-k-1), 1.0);   glVertex3f( x[m], y[m], -h/2);
    glTexCoord2f(1.0/n*(n-k-1), 0.0);   glVertex3f( x[m], y[m], h/2);
glEnd();

And it works and look like this :

when I use this texture :

But now I want to rotate the texture of 90 degrees, so I create a new jpeg file and rotate it.

So the texture now look like this :

But this is the result when I use it :

The texture is twisted around the cylinder and I don't understand why.

Any idea ?

How I load texture :

#include <math.h>
#include <jpeglib.h>
#include <jerror.h>

GLuint texObject[2]; // textures

int main(int argc,char **argv){
    glutInit(&argc,argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
    glutInitWindowPosition(200,200);
    glutInitWindowSize(1366,768);
    glutCreateWindow("");

    glClearColor(0.0,0.0,0.0,0.0);
    //glColor3f(1.0,1.0,1.0);
    glShadeModel(GL_FLAT);
    glPointSize(2.0);
    glEnable(GL_DEPTH_TEST);

    glGenTextures(2, texObject);

    loadJpegImage("./textureCorps5.jpg", 1);

    glutMainLoop();
    return 0;
}


void loadJpegImage(char *fichier, int i)
{
    struct jpeg_decompress_struct cinfo;
    struct jpeg_error_mgr jerr;
    FILE *file;
    unsigned char *ligne;

    cinfo.err = jpeg_std_error(&jerr);
    jpeg_create_decompress(&cinfo);
    #ifdef __WIN32
        if (fopen_s(&file,fichier,"rb") != 0)
        {
            fprintf(stderr,"Error\n");
            exit(1);
        }
    #elif __GNUC__
        if ((file = fopen(fichier,"rb")) == 0)
        {
            fprintf(stderr,"Error\n");
            exit(1);
        }
    #endif
    jpeg_stdio_src(&cinfo, file);
    jpeg_read_header(&cinfo, TRUE);

    unsigned char image[cinfo.image_width*cinfo.image_height*3];

    jpeg_start_decompress(&cinfo);
    ligne=image;
    while (cinfo.output_scanline<cinfo.output_height)
    {
        ligne=image+3*cinfo.image_width*cinfo.output_scanline;
        jpeg_read_scanlines(&cinfo,&ligne,1);
    }
    jpeg_finish_decompress(&cinfo);
    jpeg_destroy_decompress(&cinfo);


    glBindTexture(GL_TEXTURE_2D, texObject[i]);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, cinfo.image_width, cinfo.image_height, 0,
               GL_RGB, GL_UNSIGNED_BYTE, image);
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);
}

Answer 1

GL_UNPACK_ALIGNMENTspecifies the alignment requirements for the start of each pixel row in memory. By default GL_UNPACK_ALIGNMENT is set to 4. This means each row of the texture is supposed to have a lenght of 4*N bytes.

Your texture is an RGB texture, which needs 24 bits or 3 bytes for each texel and you tightly packed the texels and especially the lines of the texture. This means that you may disregard the alignment of 4 for the start of a line of the texture (Except 3 times the width of the texture is divisible by 4 without a remaining).

To deal with that you have to change the alignment to 1. This means you have to set glPixelStorei(GL_UNPACK_ALIGNMENT, 1); before glTexImage2D, for reading a tightly packed texture.

Otherwise you will get an offset per line of 0-3 bytes when reading the texture. This causes a continuously twisted texture.

Instead you can take care of the alignment and the aligned length of a line when you create the turned texture, too:

int bytesPerLine = cinfo.image_width * 3;
bytesPerLine += bytesPerLine % 4; 

unsigned char image[bytesPerLine * cinfo.image_height];

jpeg_start_decompress(&cinfo);
while (cinfo.output_scanline<cinfo.output_height)
{
    ligne = image + bytesPerLine*cinfo.output_scanline;
    jpeg_read_scanlines(&cinfo,&ligne,1);
}
jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);