Consider using widget to control some aspects of the rendering. For example, use some sliders to control the shape of the cube. Declare 3 sliders to play with the scaling coordinates of the cube.

struct _GLData {

  Evas_Object *slx;
  Evas_Object *sly;
  Evas_Object *slz;

}

When rendering the GUI, build and show the sliders. Those accept values in a range from 0.0 to 1.5 and control the scaling of each axis of the cube.

// Slider for X-axis scale
gld->slx = elm_slider_add(inner_box);
evas_object_size_hint_align_set(gld->slx, EVAS_HINT_FILL, 0);
elm_slider_horizontal_set(gld->slx, EINA_TRUE);
elm_slider_unit_format_set(gld->slx, "%1.2f units");
elm_slider_indicator_format_set(gld->slx, "%1.2f units");
elm_slider_indicator_show_set(gld->slx, EINA_TRUE);
elm_slider_min_max_set(gld->slx, 0, 1.5);
elm_slider_value_set(gld->slx, 0.75);
evas_object_color_set(gld->slx, 0.0, 0.0, 120, 255);
elm_box_pack_end(inner_box, gld->slx);
evas_object_show(gld->slx);
 
// Slider for Y-axis scale
gld->sly = elm_slider_add(inner_box);
evas_object_size_hint_align_set(gld->sly, EVAS_HINT_FILL, 0);
elm_slider_horizontal_set(gld->sly, EINA_TRUE);
elm_slider_unit_format_set(gld->sly, "%1.2f units");
elm_slider_indicator_format_set(gld->sly, "%1.2f units");
elm_slider_indicator_show_set(gld->sly, EINA_TRUE);
elm_slider_min_max_set(gld->sly, 0, 1.5);
elm_slider_value_set(gld->sly, 0.75);
evas_object_color_set(gld->sly, 0.0, 0.0, 120, 255);
elm_box_pack_end(inner_box, gld->sly);
evas_object_show(gld->sly);
 
// Slider for Z-axis scale
gld->slz = elm_slider_add(inner_box);
evas_object_size_hint_align_set(gld->slz, EVAS_HINT_FILL, 0);
elm_slider_horizontal_set(gld->slz, EINA_TRUE);
elm_slider_unit_format_set(gld->slz, "%1.2f units");
elm_slider_indicator_format_set(gld->slz, "%1.2f units");
elm_slider_indicator_show_set(gld->slz, EINA_TRUE);
elm_slider_min_max_set(gld->slz, 0, 1.5);
elm_slider_value_set(gld->slz, 0.75);
evas_object_color_set(gld->slz, 0.0, 0.0, 120, 255);
elm_box_pack_end(inner_box, gld->slz);
evas_object_show(gld->slz);

Then use the actual the values of sliders and pass them to the scaling function in the rendering loop, _draw_gl:

double scalex = elm_slider_value_get(gld->slx);
double scaley = elm_slider_value_get(gld->sly);
double scalez = elm_slider_value_get(gld->slz);
 
customLoadIdentity(gld->model);
customRotate(gld->model, gld->xangle, gld->yangle, gld->zangle++);
customScale(gld->model, scalex, scaley, scalez);
customMutlMatrix(gld->mvp, gld->view, gld->model);

next page: Implementing Effects