A management system for OpenGL shader programs and associated buffer objects.

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Michael Fiano <>




A lispy system for defining OpenGL shader programs and associated buffer objects.


Under the hood, Shadow is just a wrapper around the Varjo library used for writing shader programs, and some fluff to allow referencing shader programs by name, querying for basic information about them, modifying uniform variables throughout the lifecycle of an OpenGL application, and managing certain OpenGL buffer object types (UBO, SSBO currently).

The goal of Shadow is to be a simple solution to ease the task of writing and managing OpenGL shader programs and associated buffers.


(ql:quickload :shadow)


Basic Usage

Using Shadow is not very straightforward, mostly due to the borrowing of the "Vari" language used to write shader programs, which does not have much documentation. It does however try to stay familiar and resembles Common Lisp. Additionally, there are several videos of Vari's usage created by its author.

Shader programs are written using a series of DEFUN and DEFSTRUCT forms representing GPU functions and structures respectively. DEFUN and DEFSTRUCT are special CL-like macros that live in the SHADOW.GLSL package. As mentioned, their bodies follow the language rules of "Vari", which is not documented here. All shaders written should be in a dedicated package you create that :USEs :SHADOW.GLSL. This is a special package that takes care of shadowing the CL symbols and exporting all the required symbols for writing shaders. It is not recommended to mix CL code and shader code in the same package, to prevent having to fully-qualify CL:DEFUN everywhere. Please use a dedicated package for writing your shaders which :USEs :SHADOW.GLSL.

Each DEFUN defines a shader stage or auxillary function thereof. It takes in input arguments and uniform variables, and sends its return values to the next stage of the shader pipeline as input arguments. The vertex stage's input arguments correspond to your Vertex Array Object attributes.

A simple OpenGL shader program:

(defun foo-vert ((position :vec3) (uv :vec2) &uniforms (mvp :mat4))
  (values (* mvp (vec4 position 1))
          (vec4 1 0 0 1)))

(defun foo-frag ((color :vec4))
  (values color))

This defines 2 GPU functions, foo-vert and foo-frag that will serve as a very simple program once translated and compiled.

To use this program it first must be translated from the Lisp-like "Vari" language, into GLSL. This is done with the DEFINE-SHADER macro:

(define-shader example-program (:version 330 :primitive :points)
  (:vertex (foo-vert :vec3 :vec2))
  (:fragment (foo-frag :vec4)))

Above, we call DEFINE-SHADER with a name to call our program, EXAMPLE-PROGRAM, the default stage version to use, :version 330, and the OpenGL drawing primitive the vertex stage should use, :primitive :points, followed by a sequence of "stage-specs" of the form: (stage-type function-spec):

stage-type may be one of: :vertex, :tessellation-control, :tessellation-evaluation, :geometry, :fragment, or :compute.

func-spec specifies which DEFUN function to use for this stage, and is a list consisting of the function name followed by the types of all of its input arguments. The types are important because the "Vari" shader language allows the same function name to exist with different signatures, so you must be explicit in which function you want to translate to GLSL.

Issuing the call to DEFINE-SHADER produces a PROGRAM object, which includes some useful information:

The SHADOW:VIEW-SOURCE function can be used to retrieve the translated Varo -> GLSL source for a given program and stage type:

(define-shader ...)

(view-source * :vertex)

"#version 330

layout(location = 0)  in vec3 POSITION;
layout(location = 1)  in vec2 UV;

     out vec4 _VERTEX_STAGE_OUT_1;
} v_out;

uniform mat4 MVP;

void main()
    gl_Position = (MVP * vec4(POSITION,float(1)));
    v_out._VERTEX_STAGE_OUT_1 = vec4(float(1),float(0),float(0),float(1));

(view-source ** :fragment)

"#version 330

     in vec4 _VERTEX_STAGE_OUT_1;
} v_in;

layout(location = 0)  out vec4 _FRAGMENT_STAGE_OUT_0;

void main()

As can be seen by the GLSL source, our vertex stage function is properly making use of the VALUES form. It takes the first value for itself, setting gl_Position, and passes all subsequent values as input arguments to the fragment stage, (vec4 1 0 0 1), which takes that for itself as the final fragment color of the pipeline.

So far, we have only translated the "Vari" shader language into the GLSL language understood by OpenGL. We still have to compile the shader stages and link the final program object on the GPU.

At this point, a valid OpenGL context is needed to continue.

To compile a program's stages and link them into a program, you can use the BUILD-SHADER-PROGRAM function:

(build-shader-program 'example-program)

This will compile all of the stages previously translated to GLSL in our EXAMPLE-PROGRAM program, and link it into a program object on the GPU. This returns a non-zero integer on success.

Alternatively, you can compile and link all GLSL translated programs in one shot, by using the BUILD-SHADER-DICTIONARY function, which takes no arguments and returns a hash table of all program objects keyed by name.

LOAD-SHADERS can be used to automatically build all shader programs in Shadow's registry as well. With LOAD-SHADERS, it takes 1 argument, which is a function. This user-supplied function, which can be a LAMBDA takes a single argument which when called will be the list of shader programs affected when a function is recompiled with C-c C-c. You can use this to automatically update shaders while your program is running to see visual changes instantly. However, this requires some effort on your part, because SLIME (and the Sly fork) recompiles forms in a separate compilation thread when you C-c C-c. This is problematic because you must recompile shader programs on the same thread as your OpenGL context. To get around this, you can do something like the following:

In your project's initialization step, load all shaders passing in a custom function which pushes things to a thread-safe queue:

(defun initialize-my-shaders (my-queue)
  (let ((modify-hook (lambda (queue-enqueue my-queue x))))
    (shadow:load-shaders modify-hook)))

Then periodically, you can dequeue items off of this queue. An item on the queue is a list of programs that need to be recompiled, and the RECOMPILE-SHADERS function expects such a list of program names and does the recompiling:

(defun recompile-my-shaders (my-queue)
  (loop :while (queue-peek my-queue)
        :for programs = (queue-dequeue my-queue)
        :do (shadow:recompile-shaders programs)))

Shadow itself does not do this, nor does it have a thread-safe queue implementation. You'll have to use a library such as queues or lparallel for that. However, it's pretty easy as you can see above, and doing this will be extremely valuable, as you'll be able to see your GPU modifications instantly while your project is running without the downtime of having to restart after every minor change.

After compiling shaders, the only thing left to do, is make use of the shader program to do your rendering. This is done by issuing calls to the various UNIFORM-* functions within the body of the WITH-SHADER macro:

(with-shader 'example-program
  (uniform-mat4 program :mvp *matrix*))

Here, we specify that we want to use EXAMPLE-PROGRAM during rendering, modifying a single 4x4 matrix uniform value. Here *matrix* refers to an imaginary matrix that you should have created for the object you wish to render. There are quite a few UNIFORM-* functions, and the full list can be viewed in the package's exported symbols. Note that each uniform function takes a program object, the name of a uniform variable as a keyword symbol, followed by the value to modify it with.

UBO/SSBO Support

Shadow also includes support for uniform buffer objects (UBO's) and shader storage buffer objects (SSBO's).

A buffer-backed interface block in Shadow is implemented as a struct with DEFSTRUCT. Anytime a particular shader function wishes to read or write to this buffer, it must be specified in that function's signature using the &uniforms part of its lambda list. To do this, you must know the name of the struct, whether you want to access a UBO or SSBO buffer, and the packing layout of that buffer (std140 or std430). For example, this function binds the symbol var using the previously defined struct, foo-block, which will be later filled as an SSBO using the layout rules of the std430 specification:

(defun foo (&uniforms (var foo-block :ssbo :std430)

This special uniform syntax must be present for each function that needs to access a buffer.

Creating block aliases

On the CPU side, we can create aliases for blocks. This is useful, because the same block name can refer to multiple blocks, even in the context of the same shader program. To create a block alias, use CREATE-BLOCK-ALIAS:

(create-block-alias <block-type> <block-id> <program-name> <block-alias>)
  • <block-type>: The keyword symbol :buffer or :uniform, depending if this block is a block which should be used with an SSBO or UBO, respectively.

  • <block-id>: The name of the block. This is always a keyword symbol, derived from the name given

to the struct.

  • <program-name>: A symbol denoting the name of the program where this block can be found, as defined with DEFINE-SHADER.

  • <block-alias>: An identifier to be used to reference this block. May be a symbol, keyword symbol, or a case-sensitive string.

Deleting block aliases

It may be useful to delete a block alias. You can do so using DELETE-BLOCK-ALIAS:

(delete-block-alias <block-alias> &key unbind-block)
  • <block-alias>: An identifier to be used to reference this block. May be a symbol, keyword symbol, or a case-sensitive string.

  • <unbind-buffer>: When non-NIL, also disassociates the block from a binding point.

Referencing Blocks

To find a block object in Shadow's state, you can use FIND-BLOCK:

(find-block <block-alias>)
  • <block-alias>: A symbol, keyword symbol, or case-sensitive string denoting an alias previously defined with CREATE-BLOCK-ALIAS.

Binding Blocks

A block must be bound to a "binding point" for use. A buffer is then bound to this same binding point to associate them with each other. To bind a block to a binding point, use BIND-BLOCK:

(bind-block <block-alias> <binding-point>)
  • <block-alias>: A symbol, keyword symbol, or case-sensitive string denoting an alias previously

defined with CREATE-BLOCK-ALIAS.

  • <binding-point>: An integer to bind the block to. This ranges from 1 to a driver-dependent maximum.

Unbinding Blocks

To disassociate a block from a binding point, use UNBIND-BLOCK:

(unbind-block <block-alias>)
  • <block-alias>: A symbol, keyword symbol, or case-sensitive string denoting an alias previously defined with CREATE-BLOCK-ALIAS.

Creating Buffers

To create a buffer, you first need to create a block alias as per the above instructions. You can then create a buffer which uses the layout of a particular block, using CREATE-BUFFER:

(create-buffer <buffer-name> <block-alias>)
  • <buffer-name>: A symbol that can later be used as a reference to the created buffer.

  • <block-alias>: A symbol, keyword symbol, or case-sensitive string denoting an alias previously defined with CREATE-BLOCK-ALIAS.

Binding Buffers

To bind a buffer to a binding point, use BIND-BUFFER:

(bind-buffer <buffer-name> <binding-point>)
  • <buffer-name>: The name of a buffer that was defined with CREATE-BUFFER.

  • <binding-point>: An integer to bind the buffer to. This ranges from 1 to a driver-dependent maximum.

Unbinding Buffers

To disassociate a buffer from a binding point, use UNBIND-BUFFER:

(unbind-buffer <buffer-name>)
  • <buffer-name>: The name of a buffer that was defined with CREATE-BUFFER.

Deleting Buffers

(delete-buffer <buffer-name>)
  • <buffer-name>: The name of a buffer that was defined with CREATE-BUFFER.


Copyright © 2018-2020 Michael Fiano

Licensed under the MIT License.

Dependencies (6)

  • cffi
  • cl-opengl
  • glsl-packing
  • mfiano-utils
  • static-vectors
  • varjo

Dependents (1)

  • GitHub
  • Quicklisp