title: Problem with xna shader
body: <p>Trying to make a glow effect in xna but it doesn't show the glow or any change at all. Also my back color is purple instead of black and I can't change that either :</p>
  
  <pre><code>        GraphicsDevice.Clear(Color.Black);
          GraphicsDevice.SetRenderTarget(bulletRenderTarget);
          spriteBatch.Begin();
          foreach (Bullet bullet in bulletList)
          {
              Texture2D bulletTexture = textures[bullet.bulletType];
  
              spriteBatch.Draw(
                  bulletTexture,
                  new Rectangle(
                      (int)bullet.position.X,
                      (int)bullet.position.Y,
                      bulletTexture.Width,
                      bulletTexture.Height
                  ),
                  null,
                  Color.White,
                  MathHelper.ToRadians(bullet.angle),
                  new Vector2(
                      bulletTexture.Width / 2,
                      bulletTexture.Height / 2
                  ),
                  SpriteEffects.None,
                  0
              );
          }
          spriteBatch.End();
  
          GraphicsDevice.SetRenderTarget(null);
          GraphicsDevice.Clear(Color.Black);
          postProcessEffect.CurrentTechnique = postProcessEffect.Techniques["Blur"];
  
          spriteBatch.Begin();
              spriteBatch.Draw(
              bulletRenderTarget,
              new Vector2(0, 0),
              Color.White
          );
          GraphicsDevice.BlendState = BlendState.Additive;
          foreach (EffectPass pass in postProcessEffect.CurrentTechnique.Passes)
          {
              pass.Apply();
              spriteBatch.Draw(
                  bulletRenderTarget,
                  new Vector2(0,0),
                  Color.White
              );
          }
  
          DrawHud();
          foreach (BaseEntity entity in entityList)
          {
              entity.Draw(gameTime);
          }
          spriteBatch.End();
  </code></pre>
  
  <p>I'm only trying to get the bullets to glow.</p>
  
  <p>shader :</p>
  
  <pre><code> float BlurDistance = 0.002f;
   sampler ColorMapSampler : register(s1);
  
   float4 PixelShaderFunction(float2 Tex: TEXCOORD0) : COLOR
   {
    float4 Color;
  
    // Get the texel from ColorMapSampler using a modified texture coordinate. This
    // gets the texels at the neighbour texels and adds it to Color.
    Color  = tex2D( ColorMapSampler, float2(Tex.x+BlurDistance, Tex.y+BlurDistance));
    Color += tex2D( ColorMapSampler, float2(Tex.x-BlurDistance, Tex.y-BlurDistance));
    Color += tex2D( ColorMapSampler, float2(Tex.x+BlurDistance, Tex.y-BlurDistance));
    Color += tex2D( ColorMapSampler, float2(Tex.x-BlurDistance, Tex.y+BlurDistance));
    // We need to devide the color with the amount of times we added
    // a color to it, in this case 4, to get the avg. color
    Color = Color / 4; 
  
    // returned the blurred color
    return Color;
   }
  
   technique Blur
   {
    pass Pass1
    {
     PixelShader = compile ps_2_0 PixelShaderFunction();
    }
   }
  </code></pre>