Issue getting gradient square in glsl es 2.0, Game

Question

I made a triangle list with 4 triangles, having the middle point a different color. And then aim to combine the triangles to get a nice gradient. But the edges of the triangles create unwanted lines, I don't want these lines I want it to be smooth al the way. How can I get the desired result?

Images:

Shader Code:

    // Simple passthrough vertex shader
    //
    attribute vec3 in_Position;                  // (x,y,z)
    attribute vec4 in_Colour;                    // (r,g,b,a)
    attribute vec2 in_TextureCoord;              // (u,v)

    varying vec2 v_texcoord;
    varying vec4 v_colour;

    void main()
    {
        vec4 object_space_pos = vec4( in_Position.x, in_Position.y,         in_Position.z, 1.0);
        gl_Position = gm_Matrices[MATRIX_WORLD_VIEW_PROJECTION] * object_space_pos;

        v_colour = in_Colour;
        v_texcoord = in_TextureCoord;
    }

    //
    // Simple passthrough fragment shader
    //
    varying vec2 v_texcoord;
    varying vec4 v_colour;

    void main()
    {
        gl_FragColor = v_colour;
    }

Gamemaker code: Create event:

    //Build vertices list


    vertex_format_begin();
    vertex_format_add_position();
    vertex_format_add_colour();
    vertex_format_add_textcoord();
    v_format = vertex_format_end();
    v_buff = vertex_create_buffer();
    vertex_begin(v_buff, v_format);

    //triangle 0
    vertex_position(v_buff, 200, 100);
    vertex_colour(v_buff, c_black, 1);
    vertex_texcoord(v_buff, 0.0, 0.0);

    vertex_position(v_buff, 600, 100);
    vertex_colour(v_buff, c_black, 1);
    vertex_texcoord(v_buff, 1.0, 0.0);

    vertex_position(v_buff,  400, 300);
    vertex_colour(v_buff, c_red, 1);
    vertex_texcoord(v_buff, 0.5, 0.5);

    //triangle 1
    vertex_position(v_buff, 200, 100);
    vertex_colour(v_buff, c_black, 1);
    vertex_texcoord(v_buff, 0.0, 0.0);

    vertex_position(v_buff, 200, 500);
    vertex_colour(v_buff, c_black, 1);
    vertex_texcoord(v_buff, 0.0, 1.0);

    vertex_position(v_buff,  400, 300);
    vertex_colour(v_buff, c_red, 1);
    vertex_texcoord(v_buff, 0.5, 0.5);

    //triangle 2
    vertex_position(v_buff, 600, 100);
    vertex_colour(v_buff, c_black, 1);
    vertex_texcoord(v_buff, 1.0, 0.0);

    vertex_position(v_buff, 600, 500);
    vertex_colour(v_buff, c_black, 1);
    vertex_texcoord(v_buff, 1.0, 1.0);

    vertex_position(v_buff,  400, 300);
    vertex_colour(v_buff, c_red, 1);
    vertex_texcoord(v_buff, 0.5, 0.5);

    //triangle 3
    vertex_position(v_buff, 200, 500);
    vertex_colour(v_buff, c_black, 1);
    vertex_texcoord(v_buff, 0.0, 1.0);

    vertex_position(v_buff, 600, 500);
    vertex_colour(v_buff, c_black, 1);
    vertex_texcoord(v_buff, 1.0, 1.0);

    vertex_position(v_buff,  400, 300);
    vertex_colour(v_buff, c_red, 1);
    vertex_texcoord(v_buff, 0.5, 0.5);

    vertex_end(v_buff);
    tex = sprite_get_texture(sprite_index, 0);

Draw event:

    shader_set(shd_prim);
    shader_set_uniform_f(uni_radius, var_radius);
    vertex_submit(v_buff, pr_trianglelist, tex);
    shader_reset();

Answer 1

The effect that you can see is optical illusion. You can make this visible by grading the colors. Use the following fragment shader for this:

varying vec2 v_texcoord;
varying vec4 v_colour;

void main()
{
    float steps   = 4.0;
    //float steps   = 8.0;
    //float steps   = 16.0;
    //float steps   = 32.0;

    vec3 gradColor = floor(v_colour.rgb * steps) / steps;
    gl_FragColor   = vec4(gradColor, 1.0);
}

4 colors:

8 colors:

16 colors:

32 colors:

To achieve a better result, you have to do the color calculated in the fragment shader. The following shader smoothly change the gradient, from a circular gradient in the middle of the the view, to a square gradient at the borders of the view. The fragment color is interpolated form color1 to color2, using the GLSL mix function.

varying vec2 v_texcoord;
varying vec4 v_colour;

void main()
{
    vec4 color1 = vec4(1.0, 0.0, 0.0, 1.0);
    vec4 color2 = vec4(0.0, 0.0, 0.0, 1.0);

    vec2 distV     = v_texcoord * 2.0 - 1.0;
    float maxDist  = max(abs(distV.x), abs(distV.y));
    float circular = length(distV);
    float square   = maxDist;

    gl_FragColor = mix(color1, color2, mix(circular,square,maxDist));
}

Preview: