Adjusting contrast of a texture in OpenGL ES
public void onDrawFrame(GL10 gl) {
    if(cameraFrame == null) return;

    gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);
    // modelview matrix
    gl.glLoadIdentity();
    // posunme TODO
    gl.glTranslatef(0.0f, 0.0f, -5.0f);

    if(context.getResources().getBoolean(pda.lupa.R.bool.GL_view))
        bindCameraTexture(gl);
    else
        gl.glColor4f(0f, 0f, 0.0f, 0f);

    gl.glNormal3f(0,0,1);
    gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glEnableClientState(GL10.GL_TEXTURE_COORD_ARRAY);

    gl.glFrontFace(GL10.GL_CW);

    gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexBuffer);
    gl.glTexCoordPointer(2, GL10.GL_FLOAT, 0, textureBuffer);

    gl.glDrawArrays(GL10.GL_TRIANGLE_STRIP, 0, vertices.length / 3);

    gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glDisableClientState(GL10.GL_TEXTURE_COORD_ARRAY);
}

gl.glGenTextures(1, cameraTexture, 0);
int tex = cameraTexture[0];
gl.glBindTexture(GL10.GL_TEXTURE_2D, tex);
gl.glTexImage2D(GL10.GL_TEXTURE_2D, 0, GL10.GL_LUMINANCE,
    this.prevX, this.prevY, 0, GL10.GL_LUMINANCE,
    GL10.GL_UNSIGNED_BYTE, ByteBuffer.wrap(this.cameraFrame));

gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_MIN_FILTER, GL10.GL_LINEAR);