Merci Gwenn mais c'est exactement le même.
mais peut être qu'il y a une subtilité autre part?

Voici celui que j'ai:

// Fragment shader for modifying image contrast by
// interpolation and extrapolation
// Author: Randi Rost
//// Copyright (c) 2002: 3Dlabs, Inc.
// See 3Dlabs-License.txt for license information

const vec3 LumCoeff = vec3 (0.2125, 0.7154, 0.0721);

varying vec2 texcoord;
uniform sampler2DRect image;

uniform vec3 avgluma;
uniform float saturation;
uniform float contrast;
uniform float brightness;
uniform float alpha;

void main (void)
{
    vec3 texColor      = texture2DRect(image, texcoord).rgb;
    vec3 intensity     = vec3 (dot(texColor, LumCoeff));
    vec3 color         = mix(intensity, texColor, saturation);
    color              = mix(avgluma, color, contrast);
    color            *= brightness;
    gl_FragColor       = vec4 (color, color.g*alpha);
}

Bon voilà j'ai trouvé:
jai remplacé le shader et ajouter cette ligne dans mon code:

    brcosaShader.setUniform3f("avgluma", 0.62,0.62,0.62);

le shader :
const vec3 LumCoeff = vec3 (0.2125, 0.7154, 0.0721);

varying vec2 texcoord;
uniform sampler2DRect image;

uniform vec3 avgluma;
uniform float saturation;
uniform float contrast;
uniform float brightness;
uniform float alpha;

void main (void)
{
    vec3 texColor      = texture2DRect(image, texcoord).rgb;
    vec3 intensity     = vec3 (dot(texColor, LumCoeff));
    vec3 color         = mix(intensity, texColor, saturation);
    color              = mix(avgluma, color, contrast);
    color            *= brightness;
    gl_FragColor       = vec4 (color, color.g*alpha);

résultat :