Set GLSL pixels of texture2D - glsl

Question

I want to create a 2D plotter in GLSL (with SFML for window handling). I import an empty texture into the fragment shader via uniform sampler2D texture (which works). Then I try iterating through the gl_TexCoord and set the pixels a colour.
Doing this changes the colour to red
vec4 pixel = texture2D(texture, gl_TexCoord[0].xy);
pixel = vec4(1.0, 0.0, 0.0, 1.0);
gl_FragColor = pixel * gl_Color;
However, this turns the whole thing red as well:
for (int j = 0; j < gl_TexCoord[0].y; j++)
for (int i = 0; i < gl_TexCoord[0].x; i++)
{
vec4 pixel = texture2D(texture, vec2(i, j));
if (i * 2 == j) // y = 2x
{
pixel = vec4(1.0, 0.0, 0.0, 1.0);
}
else
{
pixel = vec4(0.0, 0.0, 0.0, 1.0);
}
gl_FragColor = pixel * gl_Color;
}
This is supposed to only colour the pixels that have coordinates where y = 2x.
I am not very sure whether I understood the idea of texture2D correctly or not. If this is not how to, than how do you change the pixel of an empty texture?

Answer 1

texture2D(texture, vec2(u,v))
texture2D samples texels from the texture bound to sampler at texture coordinate (u,v).
It is an input parameter, you don't write into the texture. you write into the framebuffer.
The fragment shader main task is just to provide a color (RGBA) that will be displayed in the framebuffer (color texture) (the one that you see in the screen by default).
If you want to create a texture using GLSL, you can use a framebuffer with a color(empty) texture. When drawing, you texture will be filled. See details here:
Framebuffers
Other option is to write directly in the texture from the CPU, without GLSL. See glGetTextureSubImage to modify a texture already passed to the GPU.
glTexSubImage?D
In your example, it could be easier to create a 2D array with the vertex colors and use it to fill the Texture with
unsigned char* vertexColors= new char[widthheight4];
// fill vertexColors
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, vertexColors))
glTextImage?D