It's not THAT easy, i'm afraid. First of all, depth of field is a post processing effect. To apply a post processing effect in form of a shader, you have to render the whole scene into a texture and then onto a quad to display it, because neither in the fragment nor in the vertex shader you do have access to the framebuffer's data itself. At the moment, you can't really do this. You would need support for frame buffer objects (fbos) and maybe render the scene into different fbos splitted into color and depth buffer and such. Support for this is on my list, but not implemented yet.
Everything else (i.e. "normal" shader code that isn't used for full screen effects) is possible. However, as i've mentioned in my release-post, it..."lacks some documentation". What i meant with that is, that the engine has to know where to put which data into the shader, or otherwise, you wouldn't be able to access the vertices, normals, lights etc. in your shader. It does this by making you follow a naming convention when you write your shader and i haven't documented this naming yet...i'll do briefly now. The idea is this: If jPCT-AE finds any of the following uniforms or attributes in either shader part (vertex/fragment), it'll put the corresponding engine data in it. If it doesn't find one, it'll simply skip that data. Ok, here we go:
uniform mat4 modelViewMatrix; - The model view matrix, i.e. the matrix that makes the transformation from object into camera space
uniform mat4 modelViewProjectionMatrix; - The model view projection matrix, i.e. the model view matrix * project matrix
uniform mat4 textureMatrix; - The texture matrix
uniform vec4 additionalColor; - An object's additional color (stored in .rgb of the uniform).
uniform vec4 ambientColor; - The world's ambient color (stored in .rgb of the uniform).
uniform float alpha; - The alpha value of the object.
uniform float shininess; - The shininess if specular lighting is being used.
uniform bool useColors; - true, if the object contains additional vertex colors.
uniform float fogStart; - The depth coordinate, at which the fog starts. -1.0 if there is no fog.
uniform float fogEnd; - The depth coordinate of maximum fog.
uniform vec3 fogColor; - The fog color.
uniform int lightCount; - The number of lights that light this object.
uniform vec3 lightPositions[8]; - The light positions in camera space.
uniform vec3 diffuseColors[8]; - The diffuse color of each light source.
uniform vec3 specularColors[8]; - The specular color of each light source.
uniform sampler2D textureUnit0; - The texture sampler for the first texture layer.
uniform sampler2D textureUnit1; - The texture sampler for the second texture layer.
uniform sampler2D textureUnit2; - The texture sampler for the third texture layer.
uniform sampler2D textureUnit3; - The texture sampler for the forth texture layer.
uniform int textureCount; - The number of texture layers.
uniform int blendingMode[4]; - The blending modes between these layers (0==MODULATE, 1==ADD, 2==REPLACE, 3==BLEND).
attribute vec4 position; - The vertex positions in object space.
attribute vec3 normal; - The vertex normals in object space.
attribute vec4 color; - The additional vertex colors (if any).
attribute vec4 tangent; - The tangent vectors. The presence of this attribute in a shader's source code will trigger the tangent vector calculation when calling build().
attribute vec2 texture0; - The texture coordinates for the first stage.
attribute vec2 texture1; - The texture coordinates for the second stage.
attribute vec2 texture2; - The texture coordinates for the third stage.
attribute vec2 texture3; - The texture coordinates for the forth stage.
Any additional uniforms can be given to the shader via the methods in GLSLShader. Currently, it's lacking the option to feed additional vertex attributes into the shader, but for most cases, this shouldn't be needed. If it is, consider to store them in a texture instead.
As you can see, you can't simply copy and paste shaders from the internet, because you have to make them use this naming convention. In addition, the least shaders that can be found are written for OpenGL ES. Most are written for "normal" OpenGL...and "normal" OpenGL is different in a way that it already has access to some pre-defined attributes and uniforms like gl_NormalMatrix, gl_Normal, gl_Vertex, gl_LightSource etc.... If you want to port such a shader, you first have to find out which pre-defined attribute/uniform matches which in jPCT-AE or maybe you have to set it from the outside in your code.
As said, you'll find the default shader set in the jar and an example shader of a different kind in the HelloShader-example.
