I am trying to make a heatmap and in my Fragment shader i want to know the existing color value of the pixel on which the shader will write.

Based on the red color value i want to decide whether to write or not.

currently yellow color is overwriting the already existing red color.

i add points

AddPoint(glm::vec2(0.0f, 0.0f), 0.1f);
AddPoint(glm::vec2(0.2f, -0.5f), 0.2f);
AddPoint(glm::vec2(0.3f, 0.3f), 0.3f);
AddPoint(glm::vec2(0.9f, 1.0f), 1.0f);
AddPoint(glm::vec2(0.7f, 0.9f), 1.0f);
AddPoint(glm::vec2(-0.7f, 0.5f), 0.8f);
AddPoint(glm::vec2(-0.1f, -0.5f), 0.8f);

than render to vertex shader which passes the points to geomtry shader.
The geomerty shader constructs a circle.

This is my Fragment Shader.

 #version 420 core

in float gValue;
in vec2 pointPos; // Receive the original position from the geometry shader

out vec4 FragColor;

uniform sampler1D colormap;  // This has the color range for the Heat Map
uniform float radius;

void main()
{
    // Convert gl_FragCoord to normalized device coordinates (NDC)
    vec2 fragCoordNDC = (gl_FragCoord.xy / vec2(1920.0, 1080.0)) * 2.0 - 1.0;

    float distance = length(fragCoordNDC - pointPos);

    float normalizedDistance = distance / radius;
    float intensity = 1.0 - normalizedDistance;

    float finalIntensity = mix(gValue, gValue - 0.3, normalizedDistance);
      
     
       // Sample color from the colormap
        vec4 newColor = texture(colormap, finalIntensity);

        // Check if the new color's red value is greator
        if (newColor.r > FragColor.r)
        {
            // Update FragColor only if the new alpha is greater
            FragColor = newColor;
        }       
}