OpenGL: Why these codes draws nothing, with interleaved VBO and GLSL - opengl

Question

I've looked many papers and blogs, and I finally reached these code to generate a quad:
program stores in gProgram
vert shader:---------------------
#version 330
layout(location = 0) in vec3 attrib_position;
layout(location = 6) in vec2 attrib_texcoord;
out Varing
{
vec4 pos;
vec2 texcoord;
} VS_StateOutput;
uniform mat4 gtransform;
void main(void)
{
VS_StateOutput.pos = gtransform * vec4(attrib_position,1.0f);
VS_StateOutput.texcoord = attrib_texcoord;
}
frag shader:--------------------
#version 330
uniform sampler2D texUnit;
in Varing
{
vec4 pos;
vec2 texcoord;
} PS_StateInput;
layout(location = 0) out vec4 color0;
void main(void)
{
color0 = texture(texUnit, PS_StateInput.texcoord);
}
Here is my vertex data: stores in gVertexVBO
float data[] =
{
-1.0f, 1.0f, 0.0f, 0.0f, 0.0f,
1.0f, 1.0f, 0.0f, 1.0f, 0.0f,
1.0f, -1.0f, 0.0f, 1.0f, 1.0f,
-1.0f, -1.0f, 0.0f, 0.0f, 1.0f
};
and index data: stores in gIndexVBO
unsigned short idata[] =
{
0, 3, 2, 0, 2, 1
};
in the drawing section, I do like this:
ps: the gtransform is set as an identity matrix
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glUseProgram(gProgram);
glProgramUniformMatrix4fv(gProgram, gtLoc, 1, GL_FALSE, matrix);
glProgramUniform1uiv(gProgram, texLoc, 1, &colorTex);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, gIndexVBO);
glBindBuffer(GL_ARRAY_BUFFER, gVertexVBO);
char* offset = NULL;
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(float)*5, offset);
glEnableVertexAttribArray(0);
glVertexAttribPointer(6, 2, GL_FLOAT, GL_FALSE, sizeof(float)*5, offset+sizeof(float)*3);
glEnableVertexAttribArray(6);
glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, NULL);
glDisableVertexAttribArray(0);
glDisableVertexAttribArray(6);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
I get nothing but the cleared back color, I think the errors located within these codes, but I can't figure out where is it, I appreciate any help.
If you need to check codes in initialization section, I will post them in the reply, thanks!

Answer 1

You're not supplying the built in gl_Position in the vertex shader.
uniform mat4 gtransform;
void main(void)
{
vec4 outPosition = gtransform * vec4(attrib_position,1.0f);
gl_Position = outPosition;
// if you need the position in the fragment shader
VS_StateOutput.pos = outPosition;
VS_StateOutput.texcoord = attrib_texcoord;
}
OpenGL wont know about your VS_StateOutput struct and won't have any idea where to put the new vertices. See here for a quick reference. (Page 7)
This link gives a better detail of the built in structure.
EDIT:
Next I would change the fragment shader main to:
void main(void)
{
color0 = vec4(1.0); //texture(texUnit, PS_StateInput.texcoord);
}
to rule out any texture issues. Sometimes texture problems lead to a blank black texture. If you get a white colour, you then know its a texture problem and can look there instead.