CODEWITHSUNDEEP
c++opengl
user2542813
user2542813

Reputation: 333

Unable to draw using glDrawElements correctly

I'm trying to draw a rectangle and apply a texture unto it. The code was copied from a book I'm reading, I just passed to C++(from Java), here it is:

void draw(){
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(2, GL_FLOAT, 4, vertices.data());//vertexSize is 4

glEnableClientState(GL_TEXTURE_COORD_ARRAY);
glTexCoordPointer(2, GL_FLOAT, 2, vertices.data() + 2);

glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices.data());

glDisableClientState(GL_TEXTURE_COORD_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);
}

the vertices is a std::vector of size 16 and the following values: [-0.25,-0.25,0.0,0.25,0.25,-0.25,0.25,0.25,0.25,0.25,0.25,0.0,-0.25,0.25,0.0,0.0] each 4 floats specifies a vertex coord and a texture coord, and the indices is a std::vector of size 6 with the following values: [0,1,2,2,3,0] which specifies two triangles(with textures) in the vertices array. This two arrays are always set before calling draw.

The problem is that the rendered object is not right, instead of a rectangle I have a trapezium rotated. The vectors seems correct, as I'm new to OpenGL, I've no idea of what is going wrong in this code.

Thank you for your attention.

---EDITED--- The opengl matrices are set right before enter the application loop:

glMatrixMode(GL_PROJECTION);
glLoadIdentity();

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

glClearColor(0.0, 1.0, 0.0, 1.0);

glEnable(GL_TEXTURE_2D);
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_REPLACE);

/*application loop, no more matrices use*/

Upvotes: 0

Views: 337

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 474436

glVertexPointer(2, GL_FLOAT, 4, vertices.data());//vertexSize is 4

Yes, there are 4 floats per-vertex. But that's not what the fourth parameter is. It's the number of bytes from one position to the next. That's not 4; it's 4 * sizeof(GLfloat).

Also:

glTexCoordPointer(2, GL_FLOAT, 2, vertices.data() + 2);

The second 2 here should also be 4 * sizeof(GLfloat). The byte stride for your array is the same for positions and texture coordinates.

Upvotes: 4

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