Problem when trying to use simple Shaders + VBOs - graphics

Question

Hello I'm trying to convert the following functions to a VBO based function for learning purposes, it displays a static texture on screen. I'm using OpenGL ES 2.0 with shaders on the iPhone (should be almost the same than regular OpenGL in this case), this is what I got working:
//Works!
- (void) drawAtPoint:(CGPoint)point depth:(CGFloat)depth
{
GLfloat coordinates[] = {
0, 1,
1, 1,
0, 0,
1, 0
};
GLfloat width = (GLfloat)_width * _maxS,
height = (GLfloat)_height * _maxT;
GLfloat vertices[] = {
-width / 2 + point.x, -height / 2 + point.y,
width / 2 + point.x, -height / 2 + point.y,
-width / 2 + point.x, height / 2 + point.y,
width / 2 + point.x, height / 2 + point.y,
};
glBindTexture(GL_TEXTURE_2D, _name);
//Attrib position and attrib_tex coord are handles for the shader attributes
glVertexAttribPointer(ATTRIB_POSITION, 2, GL_FLOAT, GL_FALSE, 0, vertices);
glEnableVertexAttribArray(ATTRIB_POSITION);
glVertexAttribPointer(ATTRIB_TEXCOORD, 2, GL_FLOAT, GL_FALSE, 0, coordinates);
glEnableVertexAttribArray(ATTRIB_TEXCOORD);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}
I tried to do this to convert to a VBO however I don't see anything displaying on-screen with this version:
//Doesn't display anything
- (void) drawAtPoint:(CGPoint)point depth:(CGFloat)depth
{
GLfloat width = (GLfloat)_width * _maxS,
height = (GLfloat)_height * _maxT;
GLfloat position[] = {
-width / 2 + point.x, -height / 2 + point.y,
width / 2 + point.x, -height / 2 + point.y,
-width / 2 + point.x, height / 2 + point.y,
width / 2 + point.x, height / 2 + point.y,
}; //Texture on-screen position ( each vertex is x,y in on-screen coords )
GLfloat coordinates[] = {
0, 1,
1, 1,
0, 0,
1, 0
}; // Texture coords from 0 to 1
glBindVertexArrayOES(vao);
glGenVertexArraysOES(1, &vao);
glGenBuffers(2, vbo);
//Buffer 1
glBindBuffer(GL_ARRAY_BUFFER, vbo[0]);
glBufferData(GL_ARRAY_BUFFER, 8 * sizeof(GLfloat), position, GL_STATIC_DRAW);
glEnableVertexAttribArray(ATTRIB_POSITION);
glVertexAttribPointer(ATTRIB_POSITION, 2, GL_FLOAT, GL_FALSE, 0, position);
//Buffer 2
glBindBuffer(GL_ARRAY_BUFFER, vbo[1]);
glBufferData(GL_ARRAY_BUFFER, 8 * sizeof(GLfloat), coordinates, GL_DYNAMIC_DRAW);
glEnableVertexAttribArray(ATTRIB_TEXCOORD);
glVertexAttribPointer(ATTRIB_TEXCOORD, 2, GL_FLOAT, GL_FALSE, 0, coordinates);
//Draw
glBindVertexArrayOES(vao);
glBindTexture(GL_TEXTURE_2D, _name);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}
In both cases I'm using this simple Vertex Shader
//Vertex Shader
attribute vec2 position;//Bound to ATTRIB_POSITION
attribute vec4 color;
attribute vec2 texcoord;//Bound to ATTRIB_TEXCOORD
varying vec2 texcoordVarying;
uniform mat4 mvp;
void main()
{
//You CAN'T use transpose before in glUniformMatrix4fv so... here it goes.
gl_Position = mvp * vec4(position.x, position.y, 0.0, 1.0);
texcoordVarying = texcoord;
}
The gl_Position is equal to product of mvp * vec4 because I'm simulating glOrthof in 2D with that mvp
And this Fragment Shader
//Fragment Shader
uniform sampler2D sampler;
varying mediump vec2 texcoordVarying;
void main()
{
gl_FragColor = texture2D(sampler, texcoordVarying);
}
I really need help with this, maybe my shaders are wrong for the second case ?
thanks in advance.

Answer 1

Everything is right, except the glVertexAttribPointer call.
When you have a VBO bound, the last parameter o glVertexAttribPointer is used as an offset into the VBO, as a pointer (the pointer value is the offset). So, your data is at the start of the VBO, so the last parameter should be 0 (NULL) for both calls.