opengl 2.1 and textures - c++

Question

I'm trying to show a texture(yes it is a pot) with opengl 2.1 and glsl 120, but i'm not sure on how to do it, all i can get is a black quad, i've been following this tutorials: A Textured Cube, OpenGl - Textures and what i have understood is that i need to:
Specify the texture coordinates to attach to each vertex(in my case are 6 vertices, a cube without indexing)
Load the texture and bind it in a texture unit(default is 0)
call glDrawArrays
Inside the shaders i need to:
Receive the texture coords in an attribute in the vertex shader and pass it to the fragment shader through a varying variable
In the fragment shader use a sampler object to sample a pixel, in the position specified by the varying variable, from the texture.
Is it all correct?
Here is how i create the texture VBO and load the texture:
void Application::onStart(){
unsigned int format;
SDL_Surface* img;
float quadCoords[] = {
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 0.0f,
0.5f, 0.5f, 0.0f,
0.5f, 0.5f, 0.0f,
-0.5f, 0.5f, 0.0f,
-0.5f, -0.5f, 0.0f};
const float texCoords[] = {
0.0f, 0.0f,
1.0f, 0.0f,
1.0f, 1.0f,
1.0f, 1.0f,
0.0f, 1.0f,
0.0f, 0.0f};
//shader loading omitted ...
sprogram.bind(); // call glUseProgram(programId)
//set the sampler value to 0 -> use texture unit 0
sprogram.loadValue(sprogram.getUniformLocation(SAMPLER), 0);
//quad
glGenBuffers(1, &quadBuffer);
glBindBuffer(GL_ARRAY_BUFFER, quadBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(float)*18, quadCoords, GL_STATIC_DRAW);
//texture
glGenBuffers(1, &textureBuffer);
glBindBuffer(GL_ARRAY_BUFFER, textureBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(float)*12, texCoords, GL_STATIC_DRAW);
//load texture
img = IMG_Load("resources/images/crate.jpg");
if(img == nullptr)
throw std::runtime_error(SDL_GetError());
glGenTextures(1, &this->texture);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, this->texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, img->w, img->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, img->pixels);
SDL_FreeSurface(img);
}
rendering phase:
glClear(GL_COLOR_BUFFER_BIT);
glEnableVertexAttribArray(COORDS);
glBindBuffer(GL_ARRAY_BUFFER, quadBuffer);
glVertexAttribPointer(COORDS, 3, GL_FLOAT, GL_FALSE, 0, nullptr);
glEnableVertexAttribArray(TEX_COORDS);
glBindBuffer(GL_ARRAY_BUFFER, textureBuffer);
glVertexAttribPointer(TEX_COORDS, 2, GL_FLOAT, GL_FALSE, 0, nullptr);
//draw the vertices
glDrawArrays(GL_TRIANGLES, 0, 6);
vertex shader:
#version 120
attribute vec3 coord;
attribute vec2 texCoord;
varying vec2 UV;
void main(){
gl_Position = vec4(coord.x, coord.y, coord.z, 1.0);
UV = texCoord;
}
fragment shader:
#version 120
uniform sampler2D tex;
varying vec2 UV;
void main(){
gl_FragColor.rgb = texture2D(tex, UV).rgb;
gl_FragColor.a = 1.0;
}
I know that the tutorials use out instead of varying so i tried to "convert" the code, also there is this tutorial: Simple Texture - LightHouse that explain the gl_MultiTexCoord0 attribute and gl_TexCoord array wich are built in, but this is almost the same thing i'm doing. I want to know if 'm doing it all right and if not, i would like to know how to show a simple 2d texture in the screen with opengl 2.1 and glsl 120

Answer 1

Do you have a particular reason to use opengl 2.1 with glsl version 1.2 ? If not stick to the openGl 3.0 because its easier to understand imho.
My guess is you have 2 big problems :
First of all getting a black quad: If its size occupies your hole app then its the background color. That means it doesn't draw anything at all .
I think(by testing this) OpenGL has a default program which will activate and even if you have already set a vertex array/buffer object on the gpu.It should render as a white quad in your window... So that might be ur 1st problem . I dont know if opengl 2.1 has vertex buffer arrays but opengl 3.0 has and you should definetly make use of that!
Second : you don't use your shader program in the rendering phase;
Call this function before drawing your quad:
glUseProgram(myProgram); // The myProgram variable is your compiled shader program
If by any chance you would like me to explain how to draw your quad using OpegGL 3.0 ++ let me know :) ...It is not far from what you already wrote in your code .