OpenGL ES 2.0 glDrawElements odd behavior - c

Question

In my application (running on Mali-400 GPU) I am using OpenGL ES 2.0 to draw UI. For that I setup orthogonal projection matrix and use 2D vectors to describe the geometry (only x, y attributes).
So my vertex structure looks something like this:
struct my_vertext {
struct vec2 pos;
struct unsigned int color;
}
later then I prepare GL context, setup shaders:
vertex:
uniform mat4 matrix;
attribute vec2 pos;
attribute vec4 color;
varying vec4 frag_color;
void main() {
frag_color = color;
gl_Position = matrix * vec4(pos.xy, 0, 1);
};
fragment:
precision mediump float;
varying vec4 frag_color;
void main() {
gl_FragColor = frag_color;
};
and bind them with attribute arrays:
GLuint prog = glCreateProgram(); CHECK_GL;
// create shaders, load source and compile them
...
/// binding with attributes
GLuint attrib_pos, attrib_col, vertex_index = 0;
glBindAttribLocation(prog, vertex_index, "pos"); CHECK_GL;
attrib_pos = vertex_index++;
glBindAttribLocation(prog, vertex_index, "color"); CHECK_GL;
attrib_col = vertex_index++;
// link program
glLinkProgram(prog); CHECK_GL;
glUseProgram(prog); CHECK_GL;
When I render my geometry (for simplicity I am drawing 2 rectangles one behind another) - only the first call to glDrawElements produces an image on the screen (dark gray rectangle), the second (red) - doesn`t.
For rendering I am using Vertex Array Object with 2 bound buffers - one for geometry (GL_ARRAY_BUFFER) and the second one for indices (GL_ELEMENT_ARRAY_BUFFER). All the geometry is placed to this buffers and later is drawn with glDrawElements calls providing needed offsets.
My drawing code:
glBindVertexArray(vao); CHECK_GL;
...
GLuint *offset = 0;
for each UI object:
{
glDrawElements(GL_TRIANGLES, (GLsizei)ui_object->elem_count,
GL_UNSIGNED_SHORT, offset); CHECK_GL;
offset += ui_object->elem_count;
}
This puzzles me a lot since I check each and every return code of glXXX functions and all of them return GL_NO_ERROR. Additionally, I ran my program in Mali Graphics Debugger and the latter hasn`t revealed any problems/errors.
Geometry and indices for both calls (obtained from Mali Graphics Debugger):
First rectangle geometry (which is shown on screen):
0 Position=[30.5, 30.5] Color=[45, 45, 45, 255]
1 Position=[1250.5, 30.5] Color=[45, 45, 45, 255]
2 Position=[1250.5, 690.5] Color=[45, 45, 45, 255]
3 Position=[30.5, 690.5] Color=[45, 45, 45, 255]
Indices: [0, 1, 2, 0, 2, 3]
Second rectangle geometry (which isnt` shown on screen):
4 Position=[130.5, 130.5] Color=[255, 0, 0, 255]
5 Position=[230.5, 130.5] Color=[255, 0, 0, 255]
6 Position=[230.5, 230.5] Color=[255, 0, 0, 255]
7 Position=[130.5, 230.5] Color=[255, 0, 0, 255]
Indices: [4, 5, 6, 4, 6, 7]
P.S.: On my desktop everything works perfectly. I suppose it has something to do with limitations/peculiarities of embedded Open GL.
On my desktop:
$ inxi -F
$ ....
$ GLX Renderer: Mesa DRI Intel Ivybridge Desktop GLX Version: 3.0 Mesa 10.1.3

Answer 1

I know very little of OpenGL ES, but this part looks wrong:
glBindVertexArray(vao); CHECK_GL;
...
GLuint *offset = 0;
for each UI object:
{
glDrawElements(GL_TRIANGLES, (GLsizei)ui_object->elem_count,
GL_UNSIGNED_SHORT, offset); CHECK_GL;
offset += ui_object->elem_count;
}
compared to
glBindVertexArray(vao); CHECK_GL;
...
GLuint offset = 0;
for each UI object:
{
glDrawElements(GL_TRIANGLES, (GLsizei)ui_object->elem_count,
GL_UNSIGNED_SHORT, &offset); CHECK_GL;
offset += ui_object->elem_count;
}