Check if sampler2D is empty

I'm designing a sprite class, and I would like to display only a color if no texture is loaded.

Here are my vertex shader

#version 330 core
layout (location = 0) in vec4 vertex; // <vec2 pos, vec2 tex>

out vec2 vs_tex_coords;

uniform mat4 model;
uniform mat4 projection;

void main()
{
    vs_tex_coords = vertex.zw;
    gl_Position = projection * model * vec4(vertex.xy, 0.0, 1.0);
}

And the fragment shader :

#version 330 core

in vec2 vs_tex_coords;

out vec4 fs_color;

uniform sampler2D image;
uniform vec3 sprite_color;

void main()
{
    fs_color = vec4(sprite_color, 1.0) * texture(image, vs_tex_coords);
}

My problem is that if I don't bind a texture, it displays only a black sprite. I think the problem is that the texture function in my fragment shader returns a 0, and screw all the formula.

Is there a way to know if the sampler2D is not initialized or null, and just return the sprite_color?

Solution

A sampler cannot be "empty". A valid texture must be bound to the texture units referenced by each sampler in order for rendering to have well-defined behavior.

But that doesn't mean you have to read from the texture that's bound there. It's perfectly valid to use a uniform value to tell the shader whether to read from the texture or not.

But you still have to bind a simple, 1x1 texture there. Indeed, you can use textureSize on the sampler; if it is a 1x1 texture, then don't bother to read from it. Note that this might be slower than using a uniform.