Shadow mapping offset

OpenGL Development
#1

Hello. I’ve noticed that shadows in my game have offsets when I looked closer to the edges. This is how they look like: http://i.imgur.com/LeG60xx.jpg http://i.imgur.com/TPTSnLx.jpg Any suggestions how to solve this problem? Here is the shader code if someone needs (removed all unnecessary codes that is not related to shadow mapping):

Vertex

#version 420

layout(location = 0) in vec3 VertexPosition;
layout(location = 1) in vec3 VertexNormal;

out vec3 Position;
out vec3 Normal;
out vec4 ShadowCoord;

uniform mat4 MVP;
uniform mat4 ModelViewMatrix;
uniform mat3 NormalMatrix;
uniform mat4 ShadowMatrix;

void main()
{
	vec3 pos = vec3(ModelViewMatrix * vec4(VertexPosition, 1.0));
	vec3 norm = normalize(NormalMatrix * VertexNormal);

	Position = pos;
	Normal = norm;
	
	//Shadow mapping
	ShadowCoord = ShadowMatrix * vec4(VertexPosition, 1.0);
	
	gl_Position = MVP * vec4(VertexPosition, 1.0);
}

Fragment:

#version 420

layout(binding = 0) uniform sampler2D Texture;
layout(binding = 1) uniform sampler2DShadow ShadowTexture;

in vec3 Position;
in vec3 Normal;
in vec4 ShadowCoord;

subroutine void RenderPassType();
subroutine uniform RenderPassType RenderPass;

uniform float xPixelOffset;
uniform float yPixelOffset;

layout(location = 0) out vec4 FragColor;

float lookup(vec2 offset)
{
	return textureProj(ShadowTexture, ShadowCoord + 
											vec4(offset.x * xPixelOffset * ShadowCoord.w,
												 offset.y * yPixelOffset * ShadowCoord.w,
												 0.0, 0.0)
				);
}

//SUBROUTINES

subroutine(RenderPassType)
void shadowPass()
{
	vec4 textureColor = texture2D(Texture, TexCoords);

	vec3 ambient, diffuse, specular;
	phongModelDirectional(Position, Normal, ambient, diffuse, specular);
	LightColorDS = diffuse + specular;
	LightColorA = ambient;
	
	float shadow;
	if(ShadowCoord.w > 1.0){
		float x,y;
		for(y=-1.5; y<=1.5; y+=1.0)
			for(x=-1.5; x<=1.5; x+=1.0)
				shadow += lookup(vec2(x,y));
		shadow /= 16.0;
	}
	
	FragColor = textureColor * vec4(LightColorDS * shadow + LightColorA, 1.0);
}

subroutine(RenderPassType)
void depthPass()
{
}

void main()
{
	RenderPass();
}