I'm using an orthographic projection matrix for the light and it doesn't seem correct unless in the depth vertex shader I set gl_Position.w = 1; (I just noticed in my other project that I had to set a drawing orthographic projection matrix gl_Position.w = 1; to make it pixel-perfect with the dimensions I specified when constructing the matrix, and that the shadow actually moved if I moved the light, which shouldn't happen for a directional/orthographic projection light, so I noticed today that I had to set gl_Position.w = 1; for the light orthographic matrix too.)

attribute vec4 position;

uniform mat4 projection;
uniform mat4 model;
uniform mat4 view;

attribute vec2 texCoordIn0;
varying vec2 texCoordOut0;

void main(void)
{
	gl_Position = projection * (view * (model * position));
	gl_Position.w = 1;	// sun light is orthographic directional light
	texCoordOut0 = texCoordIn0;
}

However that causes the shadow to show up wrong. lpos is the vertex in the light's clip-space on a range [0,1] (scaled by 0.5 and translated 0.5 for texture mapping).

Heightmap vertex shader:

void main(void)
{
	//vec4 vpos = (view * (model * position));
	vec4 vpos = model * position;
	//vec4 vpos = position;
	//vpos.w = 1;
	lpos = lightMatrix * vpos;
	lpos.w = 1;
	gl_Position = projection * (view * (model * position));
	//gl_Position.w = 1;

Setting lpos.w = 1 doesn't help.

Heightmap frag shader:

void main (void)
{
	//....

	vec3 smcoord = lpos.xyz / lpos.w;
	//vec3 smcoord = lpos.xyz;
	float shadow = max(0.6, 
		float(smcoord.z <= texture(shadowmap, smcoord.xy).x));
  	

Here's what I use.

Pretty sure this function is correct, and the problem is with postMultiply.

// http://www.songho.ca/opengl/gl_projectionmatrix.html
// http://www.scratchapixel.com/lessons/3d-advanced-lessons/perspective-and-orthographic-projection-matrix/orthographic-projection/
// http://www.opengl.org/discussion_boards/showthread.php/172280-Constructing-an-orthographic-matrix-for-2D-drawing
Matrix setorthographicmat(float l, float r, float t, float b, float n, float f)
{
    float m[16];
	
#define M(row,col)  m[col*4+row]
    M(0, 0) = 2 / (r - l);
    M(0, 1) = 0;
    M(0, 2) = 0;
    M(0, 3) = 0;
 
    M(1, 0) = 0;
    M(1, 1) = 2 / (t - b);
    M(1, 2) = 0;
    M(1, 3) = 0;
 
    M(2, 0) = 0;
    M(2, 1) = 0;
    M(2, 2) = -1 / (f - n);
    //M(2, 2) = -2 / (f - n);
    M(2, 3) = 0;
 
    M(3, 0) = -(r + l) / (r - l);
    M(3, 1) = -(t + b) / (t - b);
    M(3, 2) = -n / (f - n);
    //M(3, 2) = -(f + n) / (f - n);
    M(3, 3) = 1;
#undef M
	
    Matrix mat;
    mat.set(m);

	return mat;
}

void Matrix::postMultiply( const Matrix& matrix )
{
	float newMatrix[16];

#if 0

	const float *m1 = m_matrix, *m2 = matrix.m_matrix;

	newMatrix[0] = m1[0]*m2[0] + m1[4]*m2[1] + m1[8]*m2[2];
	newMatrix[1] = m1[1]*m2[0] + m1[5]*m2[1] + m1[9]*m2[2];
	newMatrix[2] = m1[2]*m2[0] + m1[6]*m2[1] + m1[10]*m2[2];
	newMatrix[3] = 0;

	newMatrix[4] = m1[0]*m2[4] + m1[4]*m2[5] + m1[8]*m2[6];
	newMatrix[5] = m1[1]*m2[4] + m1[5]*m2[5] + m1[9]*m2[6];
	newMatrix[6] = m1[2]*m2[4] + m1[6]*m2[5] + m1[10]*m2[6];
	newMatrix[7] = 0;

	newMatrix[8] = m1[0]*m2[8] + m1[4]*m2[9] + m1[8]*m2[10];
	newMatrix[9] = m1[1]*m2[8] + m1[5]*m2[9] + m1[9]*m2[10];
	newMatrix[10] = m1[2]*m2[8] + m1[6]*m2[9] + m1[10]*m2[10];
	newMatrix[11] = 0;

	newMatrix[12] = m1[0]*m2[12] + m1[4]*m2[13] + m1[8]*m2[14] + m1[12];
	newMatrix[13] = m1[1]*m2[12] + m1[5]*m2[13] + m1[9]*m2[14] + m1[13];
	newMatrix[14] = m1[2]*m2[12] + m1[6]*m2[13] + m1[10]*m2[14] + m1[14];
	newMatrix[15] = 1;

#else

	const float *a = m_matrix, *b = matrix.m_matrix;

	newMatrix[0]  = a[0] * b[0]  + a[4] * b[1]  + a[8] * b[2]   + a[12] * b[3];
	newMatrix[1]  = a[1] * b[0]  + a[5] * b[1]  + a[9] * b[2]   + a[13] * b[3];
	newMatrix[2]  = a[2] * b[0]  + a[6] * b[1]  + a[10] * b[2]  + a[14] * b[3];
	newMatrix[3]  = a[3] * b[0]  + a[7] * b[1]  + a[11] * b[2]  + a[15] * b[3];

	newMatrix[4]  = a[0] * b[4]  + a[4] * b[5]  + a[8] * b[6]   + a[12] * b[7];
	newMatrix[5]  = a[1] * b[4]  + a[5] * b[5]  + a[9] * b[6]   + a[13] * b[7];
	newMatrix[6]  = a[2] * b[4]  + a[6] * b[5]  + a[10] * b[6]  + a[14] * b[7];
	newMatrix[7]  = a[3] * b[4]  + a[7] * b[5]  + a[11] * b[6]  + a[15] * b[7];

	newMatrix[8]  = a[0] * b[8]  + a[4] * b[9]  + a[8] * b[10]  + a[12] * b[11];
	newMatrix[9]  = a[1] * b[8]  + a[5] * b[9]  + a[9] * b[10]  + a[13] * b[11];
	newMatrix[10] = a[2] * b[8]  + a[6] * b[9]  + a[10] * b[10] + a[14] * b[11];
	newMatrix[11] = a[3] * b[8]  + a[7] * b[9]  + a[11] * b[10] + a[15] * b[11];

	newMatrix[12] = a[0] * b[12] + a[4] * b[13] + a[8] * b[14]  + a[12] * b[15];
	newMatrix[13] = a[1] * b[12] + a[5] * b[13] + a[9] * b[14]  + a[13] * b[15];
	newMatrix[14] = a[2] * b[12] + a[6] * b[13] + a[10] * b[14] + a[14] * b[15];
	newMatrix[15] = a[3] * b[12] + a[7] * b[13] + a[11] * b[14] + a[15] * b[15];

#endif

	set( newMatrix );
}

I made a new function to use just for calculating the light matrix

void Matrix::postMultiply2( const Matrix& matrix )
{
	float newMatrix[16];

#if 1

	const float *m1 = m_matrix, *m2 = matrix.m_matrix;

	newMatrix[0] = m1[0]*m2[0] + m1[4]*m2[1] + m1[8]*m2[2];
	newMatrix[1] = m1[1]*m2[0] + m1[5]*m2[1] + m1[9]*m2[2];
	newMatrix[2] = m1[2]*m2[0] + m1[6]*m2[1] + m1[10]*m2[2];
	newMatrix[3] = 0;

	newMatrix[4] = m1[0]*m2[4] + m1[4]*m2[5] + m1[8]*m2[6];
	newMatrix[5] = m1[1]*m2[4] + m1[5]*m2[5] + m1[9]*m2[6];
	newMatrix[6] = m1[2]*m2[4] + m1[6]*m2[5] + m1[10]*m2[6];
	newMatrix[7] = 0;

	newMatrix[8] = m1[0]*m2[8] + m1[4]*m2[9] + m1[8]*m2[10];
	newMatrix[9] = m1[1]*m2[8] + m1[5]*m2[9] + m1[9]*m2[10];
	newMatrix[10] = m1[2]*m2[8] + m1[6]*m2[9] + m1[10]*m2[10];
	newMatrix[11] = 0;

	newMatrix[12] = m1[0]*m2[12] + m1[4]*m2[13] + m1[8]*m2[14] + m1[12];
	newMatrix[13] = m1[1]*m2[12] + m1[5]*m2[13] + m1[9]*m2[14] + m1[13];
	newMatrix[14] = m1[2]*m2[12] + m1[6]*m2[13] + m1[10]*m2[14] + m1[14];
	newMatrix[15] = 1;

#else

	const float *a = m_matrix, *b = matrix.m_matrix;

	newMatrix[0]  = a[0] * b[0]  + a[4] * b[1]  + a[8] * b[2]   + a[12] * b[3];
	newMatrix[1]  = a[1] * b[0]  + a[5] * b[1]  + a[9] * b[2]   + a[13] * b[3];
	newMatrix[2]  = a[2] * b[0]  + a[6] * b[1]  + a[10] * b[2]  + a[14] * b[3];
	newMatrix[3]  = a[3] * b[0]  + a[7] * b[1]  + a[11] * b[2]  + a[15] * b[3];

	newMatrix[4]  = a[0] * b[4]  + a[4] * b[5]  + a[8] * b[6]   + a[12] * b[7];
	newMatrix[5]  = a[1] * b[4]  + a[5] * b[5]  + a[9] * b[6]   + a[13] * b[7];
	newMatrix[6]  = a[2] * b[4]  + a[6] * b[5]  + a[10] * b[6]  + a[14] * b[7];
	newMatrix[7]  = a[3] * b[4]  + a[7] * b[5]  + a[11] * b[6]  + a[15] * b[7];

	newMatrix[8]  = a[0] * b[8]  + a[4] * b[9]  + a[8] * b[10]  + a[12] * b[11];
	newMatrix[9]  = a[1] * b[8]  + a[5] * b[9]  + a[9] * b[10]  + a[13] * b[11];
	newMatrix[10] = a[2] * b[8]  + a[6] * b[9]  + a[10] * b[10] + a[14] * b[11];
	newMatrix[11] = a[3] * b[8]  + a[7] * b[9]  + a[11] * b[10] + a[15] * b[11];

	newMatrix[12] = a[0] * b[12] + a[4] * b[13] + a[8] * b[14]  + a[12] * b[15];
	newMatrix[13] = a[1] * b[12] + a[5] * b[13] + a[9] * b[14]  + a[13] * b[15];
	newMatrix[14] = a[2] * b[12] + a[6] * b[13] + a[10] * b[14] + a[14] * b[15];
	newMatrix[15] = a[3] * b[12] + a[7] * b[13] + a[11] * b[14] + a[15] * b[15];

#endif

	set( newMatrix );
}

Notice the switch #if is 1 in this one. And it seems to do the trick. Can anybody explain why this one works specifically for orthographic matrixes but not projection?