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OpenGL Alpha values having no effect in shader

I have a simple shader to make scanlines in my 2d game which works fine and is as follows:
#ifdef GL_ES
precision mediump float;
#endif
varying vec4 v_color;
varying vec2 v_texCoords;
uniform sampler2D u_texture;
void main() {
vec2 p = vec2(floor(gl_FragCoord.x), floor(gl_FragCoord.y));
if (mod(p.y, 6.0)==0.0)
gl_FragColor = vec4(0.0,0.0,0.0, 0.1);
else
gl_FragColor = v_color * texture2D(u_texture, v_texCoords);
}
But if I change the alpha value in the vec4 for the fragment colour, it has no effect, the scanlines are equally black at 0.1 as they are at 1.0. I saw some other questions regarding this which advised to enable blending, but I tried that to no avail.
This is my render method with blending enabled.
#Override
public void render(float delta) {
Gdx.gl.glClearColor(0.0f, 0.0f, 0.0f, 0);
Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
Gdx.gl.glEnable(GL20.GL_BLEND);
Gdx.gl.glBlendFunc(GL20.GL_SRC_ALPHA, GL20.GL_ONE_MINUS_SRC_ALPHA);
// batch.enableBlending(); tried this too but no effect
batch.setProjectionMatrix(cam.combined);
batch.setShader(shaderProgram);
batch.begin();
rendup.renderAll(batch);//renders all sprites, tiles etc
batch.setShader(null);
batch.end();
}

This isn't a blending problem. The issue is your shader only draws either a translucent black pixel, or the color of whatever texture region is being drawn. These black pixels are just getting blended with what's already on the screen (in this case the black clear color).
I assume what you actually want here is for the scan lines not to be pure black. So you should be drawing the color of the texture region everywhere, and just darken it slightly where the scan lines are. You don't want to be modifying the alpha that the shader outputs, or when you have overlapping sprites, the scanline will appear darker in that area.
So change your shader like this:
#ifdef GL_ES
precision mediump float;
#endif
varying vec4 v_color;
varying vec2 v_texCoords;
uniform sampler2D u_texture;
const float DARK_LINE_BRIGHTNESS = 0.9;
void main() {
vec4 color = v_color * texture2D(u_texture, v_texCoords);
vec2 p = vec2(floor(gl_FragCoord.x), floor(gl_FragCoord.y));
if (mod(p.y, 6.0)==0.0)
gl_FragColor = vec4(color.rgb * DARK_LINE_BRIGHTNESS, color.a);
else
gl_FragColor = color;
}
However, if/else should be avoided in fragment shaders because it performs significantly worse in most cases. (Exception is if the if statement evaluates to the same value for many pixels in a row, such as with if (u_someUniformBoolean).) You could rewrite it like this:
void main() {
vec4 color = v_color * texture2D(u_texture, v_texCoords);
vec2 p = vec2(floor(gl_FragCoord.x), floor(gl_FragCoord.y));
gl_FragColor = (step(0.01, mod(p.y, 6.0)) * (1.0 - DARK_LINE_BRIGHTNESS) + DARK_LINE_BRIGHTNESS) * color;
}

libgdx - how to set sprite batch to blend/shade like this?

I currently have a default sprite batch:
batch = new SpriteBatch();
I have read about using shaders to modify how the batch draws each sprite. In my game, I am try to create a 'nighttime' effect - I want every pixel on screen to be black, except for pixels that are already white. For the white pixels, I want a shade of blue. Obviously I am new to libgdx and openGL - can anyone who is familiar with blending or shaders help me out with this? What should I do to my spritebatch to achieve the effect that I am describing?

Effect which you would like to achieve could be done with something like this.
Vertex Shader
attribute vec4 a_position;
attribute vec4 a_color;
attribute vec2 a_texCoord;
attribute vec2 a_texCoord0;
uniform mat4 u_projTrans;
varying vec4 v_color;
varying vec2 v_texCoords;
void main()
{
v_color = a_color;
v_texCoords = a_texCoord0;
gl_Position = u_projTrans * a_position;
}
Fragment shader
precision mediump float;
varying vec4 v_color;
varying vec2 v_texCoords;
uniform sampler2D u_texture;
uniform mat4 u_projTrans;
bool equals(float a, float b) {
return abs(a-b) < 0.0001;
}
bool isWhiteShade(vec4 color) {
return equals(color.r, color.g) && equals(color.r, color.b);
}
void main() {
vec4 color = texture2D (u_texture, v_texCoords) * v_color;
if(isWhiteShade(color)) {
color *= vec4(0, 0, 1, 1);
}
else {
color *= vec4(0, 0, 0, 1);
}
gl_FragColor = color;
}
Add them to assets folder and then pass as arguments while creating instance of ShaderProgram and of course don't forget to apply this shader program to your SpriteBatch.

To be honest it has (almost) nothing common with SpriteBatch - all you have to do with it is just apply created shader
batch.setShader(ShaderProgram shader)
The shaders topic is very very wide (and to be honest independent of Libgdx) and there is no simple answer to your question without informations of what it should do. Also - to be honest - try at first to read something about shaders and come back on SO when you will have some troubles with this.
You can start right here:
https://github.com/libgdx/libgdx/wiki/Shaders

OpenGL ES 2.0 Android Clipping Color

i'm using my fragment shader to clip objects in OpenGL ES 2.0. Everything is working well, however the colour of the clipped surface is all black... I can not figure out how to change the colour (well ideally I'd want to make a similar texture to the rest of the object. I have included the code for my fragment shader below.
precision mediump float;
varying vec2 texCoord;
varying vec3 v_Normal;
varying vec3 v_Position;
varying vec4 originalPosition;
uniform sampler2D texSampler2D;
uniform vec3 lightPosition;
uniform vec4 lightColor;
void main()
{
vec3 L = normalize(lightPosition - v_Position);
vec3 N = normalize(v_Normal);
float NdotL = max(dot(N,L),0.0);
if(originalPosition.y >= 2.0){
discard;
}else{
gl_FragColor = NdotL * lightColor * texture2D(texSampler2D, texCoord);
}
}

Using discard in a fragment shader doesn't render anything to the pixel, it just leaves it exactly as it was before, so the black color is probably your clear color or whatever you had rendered previously. If you want to render a particular color to the pixels you are currently discarding, add another uniform to your shader for the clip color, like this:
uniform vec4 clipColor;
Set it in the same way you set the lightColor, then instead of discarding the pixel when clipping, you can set the pixel to the clip color:
if(originalPosition.y >= 2.0) {
gl_FragColor = clipColor;
} else {
gl_FragColor = NdotL * lightColor * texture2D(texSampler2D, texCoord);
}

2D glsl shader transformation

I would like to create a shader to simulate a pseudo 3D water surface on a 2D scene build with libgdx. The idea is to recreate the following effect:
http://forums.tigsource.com/index.php?topic=40539.msg1104986#msg1104986
But I am stuck at creating the trapezoid shape, I think I didn't understand how texture coordinate are calculated on opengl shaders. May I modify the vertex in the vertex shader or may I displace the texture on the fragment shader?
here is a test I have done but that don't work as expected.
vertex shader:
attribute vec4 a_position;
attribute vec4 a_color;
attribute vec2 a_texCoord0;
uniform mat4 u_projTrans;
varying vec4 v_color;
varying vec2 v_texCoord0;
void main()
{
v_color = a_color;
float factor = (a_texCoord0.x - 0.5) * (a_texCoord0.y);
v_texCoord0 = a_texCoord0;
v_texCoord0.x += factor;
gl_Position = u_projTrans * a_position;
}
the fragment shader is just a passthrough
varying vec4 v_color;
varying vec2 v_texCoord0;
uniform sampler2D u_texture;
void main()
{
gl_FragColor = v_color * texture2D(u_texture, v_texCoord0);
}
and the result image
I am sure that my approach is too naive.

Finally, I decided to use only the fragment shader to achieve the effect.
Fake depth calculation is done by applying X displacement in regards of the Y axis.
Here is the snipet:
// simulate 3D surface
vec2 new_coord = v_texCoord0;
// calculate the stretching factor
float factor = ((0.5 - new_coord.x) * new_coord.y);
// apply the factor and stretch the image
new_coord.x += factor;
gl_FragColor = texture2D(u_texture, new_coord);

GLSL renders textures wrong

I am trying to make a lighting system, the program changes the texture(block) brightness depends on the light it gets, and the program does it for every block(texture) that is visible to the player.
However, the lighting system works perfectly, but when it comes to rendering with shaders everything gets destroyed.
This code is in the render loop -
float light = Lighting.checkLight(mapPosY, mapPosX, this); // Returns the light the current block gets
map[mapPos].light = light; // Applies the light to the block
Shaders.block.setUniformf("lightBlock", light); // Sets the light value to the shader's uniform, to change the brightness of the current block / texture.
batch.draw(map[mapPos].TEXTURE, (mapPosX * Block.WIDTH), (mapPosY * Block.HEIGHT), Block.WIDTH, Block.HEIGHT); // Renders the block / texture to the screen.
The result is pretty random..
As i said the first two lines work perfectly, the problem is probably is in the third line or in the shader itself.
The shader:
Vertex shader -
attribute vec4 a_color;
attribute vec3 a_position;
attribute vec2 a_texCoord0;
uniform mat4 u_projTrans;
varying vec4 vColor;
varying vec2 vTexCoord;
void main() {
vColor = a_color;
vTexCoord = a_texCoord0;
gl_Position = u_projTrans * vec4(a_position, 1.0f);
}
Fragment shader -
varying vec4 vColor;
varying vec2 vTexCoord;
uniform vec2 screenSize;
uniform sampler2D tex;
uniform float lightBlock;
const float outerRadius = .65, innerRadius = .4, intensity = .6;
const float SOFTNESS = 0.6;
void main() {
vec4 texColor = texture2D(tex, vTexCoord) * vColor;
vec2 relativePosition = gl_FragCoord.xy / screenSize - .5;
float len = length(relativePosition);
float vignette = smoothstep(outerRadius, innerRadius, len);
texColor.rgb = mix(texColor.rgb, texColor.rgb * vignette * lightBlock, intensity);
gl_FragColor = texColor;
}

I fixed the problem.. but i dont have any idea why it fixed it.
Thanks to Pedro, i focused more on the render loop instead of the shader itself.
Before the loop i added those 2 lines -
List<Integer> lBlocks = new ArrayList<Integer>();
Shaders.block.setUniformf("lightBlock", 0.3f);
Basically I created an array to store the bright blocks later on.
I set the uniform of the shader to be 0.3f which means pretty dark. the value should be 0-1f.
Now in the render loop, inside the for -
float light = Lighting.checkLight(mapPosY, mapPosX, this);
map[mapPos].light = light;
if( light == 1.0f) {
lBlocks.add(mapPos);
} else {
batch.draw(map[mapPos].TEXTURE, (mapPosX * Block.WIDTH), (mapPosY * Block.HEIGHT), Block.WIDTH, Block.HEIGHT);
}
As you can see, the bright blocks i add to the array and the dark ones i render, i set the uniform to 0.3f before the render loop as you can in the first code sample.
After the render loop i loop again through the bright blocks.. because we didn't render them.
Shaders.block.setUniformf("lightBlock", 1.0f);
for(int i = 0; i < lBlocks.size(); i++) {
batch.draw(map[lBlocks.get(i)].TEXTURE, ((lBlocks.get(i) % width) * Block.WIDTH), ((lBlocks.get(i) / width) * Block.HEIGHT), Block.WIDTH, Block.HEIGHT);
}
Now i rendered the bright blocks and it works.. the result was good.
But I don't have any idea why its like that, its like cutting the render loop to two, one for dark blocks and one for the bright ones.
Thanks :)