use backbuffer to render reaction-diffusion not working, any problem with my code?

[zemora]

Hi this is my code for animating the reaction-diffusion simulation, I want to use mouse click to add species B to the screen, I run it in glslEditor, it reports no error but only shows a black background.

Hope you help me check what's wrong with my code

#ifdef GL_ES
precision mediump float;
#endif

uniform vec2 u_resolution;
uniform vec2 u_mouse;
uniform sampler2D u_buffer0;

const vec4 params = vec4(0.16, 0.08, 0.06, 0.062);
const vec4 color1 = vec4(0.0, 0.0, 0.0, 0.3137);
const vec4 color2 = vec4(1.0, 0.1843, 0.5333, 0.3765);
const vec4 color3 = vec4(0.8549, 1.0, 0.5333, 0.3882);
const vec4 color4 = vec4(0.3765, 1.0, 0.4784, 0.3922);
const vec4 color5 = vec4(1.0);

void main()
{
    vec2 uv_texcoord = gl_FragCoord.xy / u_resolution.xy;

#if defined( BUFFER0 )

    if(u_mouse.x < -5.0)
    {
        gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
        return;
    }

    vec2 pixelSize = 1.0 / u_resolution.xy;
    vec2 cen = texture2D(u_buffer0, uv_texcoord).xy;
    vec2 rig = texture2D(u_buffer0, uv_texcoord + vec2( pixelSize.x,          0.0)).xy;
    vec2 top = texture2D(u_buffer0, uv_texcoord + vec2(         0.0,  pixelSize.y)).xy;
    vec2 lef = texture2D(u_buffer0, uv_texcoord + vec2(-pixelSize.x,          0.0)).xy;
    vec2 bot = texture2D(u_buffer0, uv_texcoord + vec2(         0.0, -pixelSize.y)).xy;
    
    float Du   = params.x;
    float Dv   = params.y;
    float feed = params.z;
    float kill = params.w;
    
    vec2 lapl = rig + top + lef + bot - 4.0 * cen;
    float du = Du * lapl.x - cen.x * cen.y * cen.y + feed * (1.0 - cen.x);
    float dv = Dv * lapl.y + cen.x * cen.y * cen.y - (feed + kill) * cen.y;
    vec2 newValue = cen + 0.6 * vec2(du, dv);
    
    if(u_mouse.x > 0.0)
    {
        vec2 diff = (gl_FragCoord.xy - u_mouse.xy);
        float dist = dot(diff, diff);
        if(dist < 1.0)
            newValue.y = 0.9;
    }
    
    gl_FragCoord = vec4(newValue, 0.0, 1.0);
    
#else 
    
    float value = texture2D(u_buffer0, uv_texcoord).y;
    float a;
    vec3 col;
    
    if (value <= color1.w)
    {
        col = color1.xyz;
    }
    
    else if (value > color1.w && value <= color2.w)
    {
        a = (value - color1.w) / (color2.w - color1.w);
        col = mix(color1.xyz, color2.xyz, a);
    }
    else if (value > color2.w && value <= color3.w)
    {
        a = (value - color2.w) / (color3.w - color2.w);
        col = mix(color2.xyz, color3.xyz, a);
    }
    else if (value > color3.w && value <= color4.w)
    {
        a = (value - color3.w) / (color4.w - color3.w);
        col = mix(color3.xyz, color4.xyz, a);
    }
    else if (value > color4.w && value <= color5.w)
    {
        a = (value - color4.w) / (color5.w - color4.w);
        col = mix(color4.xyz, color5.xyz, a);
    }
    
    gl_FragColor = vec4(col.xyz, 1.0);

#endif
}