Apply PixelShader on Texture2D

@Toemsel wrote:

For some reasons applying pixel shader does not work as excepted.

Shader.fx (Grayscale effect)

sampler TextureSampler : register(s0);

float4 PixelShaderFunction(float2 texCoord : TEXCOORD0) : COLOR0
{
	float4 c = 0;
	c = tex2D(TextureSampler, texCoord);
	c.a = clamp(c.a - 0.05, 0, 1);
	c.r = c.r * c.a;
	c.g = c.g * c.a;
	c.b = c.b * c.a;
	return c;
}

technique Technique1
{
    pass Pass1
    {
		PixelShader = compile ps_4_0_level_9_1 PixelShaderFunction();
    }
}

Draw call

spriteBatch.Begin(SpriteSortMode.Immediate, BlendState.AlphaBlend);
Shader.CurrentTechnique.Passes[0].Apply();
spriteBatch.Draw(...);
spriteBatch.End();

This shader with this draw call, will result in no output. (Transparent or not visible) However, many other .fx also did result in a disappearing texture. Even following code returns nothing:

	float4 c = 0;
	c = tex2D(TextureSampler, texCoord);
	return c;

Am I missing something? Also, using spriteBatch.Begin(SpriteSortMode.Immediate...); kinda grinds my gear. If I interpret the doc correct, after every .draw(); the GPU will receive the texture and instantly draw it. Isn't that very unperformant? Can I use pixel/vertex shader and still draw in batches?

Thanks in advance

Edit:

return float4(1, 0, 0, 0);

works like a charm. (Each pixel same color)

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