The viewport is set at 0,0 corresponding to the bottom left edge of the window, and the height and width of the GL surface. Clear the depth and the color buffers to the values that were selected during initialization. Then call the glUseProgram() function in order to trigger the shader program.

In draw_gl callback :

gl->glViewport(0, 0, w, h);
gl->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
gl->glUseProgram(gld->program);

Also bind the position and color identifiers to the buffers defined above.

gl->glEnableVertexAttribArray(gld->positionLoc);
gl->glBindBuffer(GL_ARRAY_BUFFER, gld->vertexID);
gl->glVertexAttribPointer(gld->positionLoc, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), 0);
 
gl->glEnableVertexAttribArray(gld->colorLoc);
gl->glBindBuffer(GL_ARRAY_BUFFER, gld->colorID);
gl->glVertexAttribPointer(gld->colorLoc, 4, GL_FLOAT, GL_FALSE, 4 * sizeof(float), 0);

After this, initialize and calculate the transformation matrix of the model-view matrix by calling the customRotate() function. This makes the cube view rotate a little for a better perspective. Once the model-view matrix is ready to display, multiply the projection matrix with the model-view matrix.

customLoadIdentity(gld->model);
customRotate(gld->model, gld->xangle, gld->yangle, gld->zangle);
customMutlMatrix(gld->mvp, gld->view, gld->model);

Then load the model-view-projection matrix into the shader and call glDrawArrays() to draw the model.

gl->glUniformMatrix4fv(gld->mvpLoc, 1, GL_FALSE, gld->mvp);
gl->glDrawArrays(GL_TRIANGLES, 0, 36);
gl->glFlush();

You now are the proud owner of a nice cube!

next page: Animating the Cube