Introducing ISF Video Generators and FX

ISF video generators and FX loaded into VDMX.

ISF stands for "Interactive Shader Format", and is a file format that describes a GLSL fragment shader, as well as how to execute and interact with it. The goal of this file format is to provide a simple and minimal interface for image filters and generative video sources that allows them to be interacted with and reused in a generic and modular fashion. ISF is nothing more than a [slightly modified] GLSL fragment shader with a JSON blob at the beginning that describes how to interact with the shader (how many inputs/uniform variables it has, what their names are, what kind of inputs/variables they are, that sort of thing). ISF isn't some crazy new groundbreaking technology- it's just a simple and useful combination of two things that have been around for a while to make a minimal- but highly effective- filter format.

ISF is now supported in the newest update to VDMX which you can find here:

Download VDMX installer

To jump right into using ISF, visit the introduction tutorial on making your first GLSL video generator.

The ISF implementation we wrote available as VVISFKit.framework in the open-source repos here: An OpenFrameworks-based ISF loader can be found here:, and the Cycling74's Gen can export to ISF files.

At the time of this writing, there are several other GPU-accelerated image filter formats, which are worth mentioning to touch on the pros and cons of the various formats, and where ISF fits in the mix:

  1. CoreImage can be GPU-accelerated under some circumstances and has a lot of perks (performance degrades gracefully on older machines or super-complicated filters, you only need to write the CI "kernel" once and it will work for both GPU- and CPU-based processing), but has occasional performance or stability issues. Creating new CoreImage filters also generally requires a compiler, a bunch of boilerplate code, and it's not cross-platform (nor is it likely to ever become so). It's a bit unreasonable to expect the average consumer of these filters to have to learn how to create a CIFilter, and correspondingly there are very, very few third-party CI filters- which translates to relatively few options for users.
  2. FreeFrame GL filters are fast and substantially more flexible than ISF (while ISF is limited to GLSL, you can do *anything* with FF/FFGL), they don't have any inherent overhead, and the spec is cross-platform: you'll have to recompile your filters, but they'll be portable across platforms. Unfortunately, the spec is dated, inflexible, and wastes VRAM on the platform we target (OS X). Making filters still requires a compiler and boilerplate code, and this isn't what you'd call an approachable filter format for non-developers. This has had a negative impact on the number of people who make FF/FFGL filters. Interestingly, many FFGL filters consist of little more than a shader, and could be easily re-implmented as ISF files in less time than it took you to read this far.
  3. Quartz Composer isn't really a filter format, per se; it's a visual programming language that, by virtue of its API, can be used as an image filter or generative source (or a great many other things). This is an attractive choice because it's very accessible to users- it comes with os x, it doesn't require a compiler, and as a visual language complete novices can pick it up quickly without prior programming knowledge. Additionally, if you know how to use a compiler you can easily extend the environment further by writing plugins and do some pretty crazy and wonderful things, so there's substantial room for experts to grow. This ease of use has created a rich and enthusiastic community of artists and developers who use QC to create a variety of content. Unfortunately, QC is mac-only, it doesn't appear to be getting any recent active development (though this could certainly change) and has accumulated some bugs over the years, and there's a certain amount of overhead to be expected with the QC runtime- it's not a lot, but it can add up. Nevertheless, there's an important lesson to be learned from QC: the easier you make it to create content, the more content people will create and share.

All of these different filter formats have their pros and cons- this isn't an indictment of any of them. ISF exists because we received a large number of complaints about the performance and stability of CI and QC under many situations (mainly because CI and QC are part of OS X, and comprise the vast majority of filters available on a new user's system), and we wanted to provide alternatives for many of these filters. We probably could've churned out a bunch of FF/FFGL filters, but that didn't seem like a good idea (partially because FFGL doesn't recognized rectangular textures and the replacements would've incurred a performance penalty under a number of circumstances, partially because we didn't want to deal with the boilerplate involved in making FFGL filters). FFGL has its time and place, but we wanted something smaller, simpler- and more approachable to new users- but powerful enough to do some serious work. ISF hits that "sweet spot" for us of being powerful, simple, and extremely easy to work with.

ISF's Disadvantages:

  • You are limited only to what you can accomplish in a GLSL program (vertex + fragment shader)
  • Many of the GLSL frag shaders you'll encounter on the internet may require slight modification before they can be used as ISF files ("uniform" variables you want ISF to interact with can't be declared in your shader, and calls to "texture2D()" and "texture2DRect()" should be replaced).

ISF's Advantages:

  • Fragment shaders are enormously powerful, relatively simple, and there's a burgeoning community of people who are eager to teach, learn, and share knowledge about them. GLSL shaders aren't specific to any one platform / application / purpose- they're used by a lot of different people for a lot of different things, so there's a wide variety of artists and developers who are making them for all sorts of different purposes (for some inspiring and mind-blowing examples, grab a WebGL-enabled browser and take a look at sites like ShaderToy and GLSL Sandbox).
  • Frag shaders are easily recompiled dynamically- experimenting with changes to filters is completely painless. An ISF filter can be as little as a single .fs file, and you only need a text editor to create or edit filters. A sample app is provided that lets you test ISF files (both filters and sources).
  • One of ISF's defining characteristics is the JSON dict at the beginning of your shader which describes it. This is a very loose, open-ended file format: it can easily be extended by recognizing more keys in the future if we think of new things that might be nice to have. For example, here are some of the base features of ISF, all of which were made possible by adding a couple items to the JSON dict:
    • The ISF format allows multiple render passes at different sizes (the same shader can be run several times on a variety of inputs). This is optional- all you have to do if you want multiple passes is add an appropriately-populated array at the "PASSES" key in your JSON dict.
    • The ISF format allows persistent buffers of arbitrary dimensions: you can render into a buffer, and keep that buffer with your filter for later use (like a "backbuffer" or an "image accumulator"). This is also optional- to use it, you just have to add an appropriately-populated array to the "PERSISTENT_BUFFERS" key of the JSON dict.
    • Importing static image resources is equally easy: just add something under the "IMPORTED" key of the JSON dict (details in the spec).
    • Extensible – If you have any ideas or requests, please get in touch!
  • While many GLSL shaders would benefit from slight modification before they're used as ISF files, these modifications are minimal (we didn't trade "boilerplate code" for "lots of modifications"), and they exist first and foremost because they're extremely convenient- they make it a lot easier to write compatible filters the run under a wide variety of circumstances. For example:
    • "uniform" variables that ISF is expected to interact with can't be declared in your shader because the declaration is automatically prepended to your code when your shader is loaded. If you want to change a data type, you only have to change it in one place (in your JSON dict).
    • Replacing "texture2D()" and "texture2DRect()" is convenient because their replacements work with both 2D and RECT samplers, images that aren't full-frame in their textures (like 1080p images in 2048x2048 texture) and images that are flipped vertically in their texture. As a shader author, you no longer have to wonder what kind of sampler you're working with- you can access pixels transparently using either their pixel-based coords (IMG_PIXEL()) or their normalized coords (IMG_NORM_PIXEL()), whichever's more convenient in a given situation. and if you really don't like change, you don't have to do this and your code will still run as written.

Downloads: Sample filters and test app

A simple test applications is available here: 
This app will render and display ISF files which are generative sources, and it can also be used to preview and test ISF video filters (it's capable of receiving a video stream via Syphon, which the filters will be applied to). The test app is interactive- it publishes UI items so you can play with your shaders while testing them- and is extremely useful for shader developers because it displays any GL errors with your shader code as well as the actual text of the vert and frag shaders (after any modifications have been made to your shader code).

Watch a video tutorial on using the ISF Editor.

Here are a bunch of simple test ISF files that demonstrate ISF's basic functionality (these are test filters, and we don't expect them to have significant creative use):
ISF Test/Tutorial filters

And here are a bunch of example ISF files (both sources and filters) which you may find more useful:
Sample ISF image filters (and a couple sources)

Creating ISF files

All you need to create an ISF file is a text editor: an ISF file is nothing more than a text file with a .fs or .frag extension. While ISF files can technically be loaded from anywhere on your drive, the default global repository of ISF files will be at either /Library/Graphics/ISF or ~/Library/Graphics/ISF. The first thing in this text file needs to be a JSON dict inside a comment, which is delineated using "/*" and "*/"- the contents of this JSON dict describe the various attributes of the ISF file.

How ISF works

When you load an ISF file, the code in your shader is modified slightly- some variables are declared, and some code is find-and-replaced. If you haven't explicitly created a vertex shader, one will automatically be generated. Each time you render a frame, the "INPUT" variable values are passed to the shader, and then the shader is rendered (more than once if desired, potentially into image buffers accessible from different rendering passes and across different frames).

Automatic Variables

  • "uniform" variables of the appropriate type are automatically declared for any INPUTS you define. You shouldn't declare uniform variables you want ISF to create UI items for- if you do this, your shader won't work (and the error displayed in the ISF test app will indicate that you've redeclared a variable). "uniform" samplers are also declared for any persistent or temporary buffers (samplers are automatically declared of the appropriate type- the shader will be recompiled behind the scenes if a texture type for any of the textures in use changes).
  • The uniform int "PASSINDEX" is automatically declared, and set to 0 on the first rendering pass. Subsequent passes (defined by the dicts in your "PASSES" array) increment this int.
  • The uniform vec2 "RENDERSIZE" is automatically declared, and is set to the rendering size (in pixels) of the current rendering pass.
  • The uniform vec2 "vv_FragNormCoord" is automatically declared. This is a convenience variable, and repesents the normalized coordinates of the current fragment ([0,0] is the bottom-left, [1,1] is the top-right).
  • The uniform float "TIME" is automatically declared, and is set to the current rendering time (in seconds) of the shader. This variable is updated noce per rendered frame- if a frame requires multiple rendering passes, the variable is only updated once for all the passes.
  • "image"-type inputs automatically declare other additions uniform vars (for passing the image resolution and other associated properties)- if you want to see these, just use the "ISF Tester" app listed above to view the fragment shader source of your filter.

Recognized Input Types

When creating your own custom published variables that can be accessed from the host application such as VDMX, the following types can be declared:

Boolean, float, long (presented as a pop-up button), event (single-frame-on boolean), 2d point (vec2), color (vec4), and image (behind the scenes, images are either sampler2D or sampler2DRect- the shader will be modified automatically).

Additional Functions

<vec4> pixelColor = IMG_PIXEL(<image> imageName, <vec2> pixelCoord); 
<vec4> pixelColor = IMG_NORM_PIXEL(<image> imageName, <vec2> normalizedPixelCoord);

There are really only two functions worth mentioning: IMG_PIXEL() and IMG_NORM_PIXEL(), which fetch the color of a pixel in an image using either pixel-based coords or normalized coords, respectively, and should be used *instead of* texture2D() or texture2DRect(). In both functions, "imageName" refers to the variable name of the image you want to work with. This image can come from a variety of sources- an "image"-type input, a persistent buffer, an imported image, etc: all of these sources of images have a name value associated with them, and this name is what you pass to these functions. For several examples, please check out this .zip of extremely simple shaders that demonstrate the features of ISF, one at a time:
ISF Test/Tutorial filters

Persistent Buffers

ISF files can define persistent buffers. These buffers are images (GL textures) that stay with the ISF file for as long as it exists. This is useful if you want to "build up" an image over time- you can repeatedly query and update the contents of persistent buffers by rendering into them- or if you want to perform calculations across the entire image, storing the results somewhere for later evaluation. Details on exactly how to do this are in the spec (below).

Multiple Rendering Passes, Rendering to Buffers

The ISF file format defines the ability to execute a shader multiple times in the process of rendering a frame for output- each time the shader's executed (each pass), the uniform int variable "PASSINDEX" is incremented. Details on how to accomplish this are described below in the spec, but the basic process involves adding an array of dicts to the "PASSES" key in your top-level JSON dict. each dict in the "PASSES" array describes a different rendering pass (each dict describes the name of the buffer it should render into- you can render into a persistent buffer, or you can make up a new name, and the ISF host will create a new temporary buffer to render into (after you render into it you can read from the temp buffer again in subsequent render passes like any other buffer/input image/imported image, but the buffer will be deleted once ISF has finished rendering a frame).

Converting GLSL shaders

  • You should probably replace any calls in your shader to texture2D() or texture2DRect() with IMG_NORM_PIXEL() or IMG_PIXEL(), respectively. Images in ISF- inputs, persistent buffers, etc- can be accessed by either IMG_NORM_PIXEL() or IMG_PIXEL(), depending on whether you want to use normalized or non-normalized coordinates to access the colors of the image. If your shader isn't using these- if it's using texture2D() or texture2DRect()- it won't compile if the host application tries to send it a different type of texture.
  • Many shaders pass in the resolution of the image being rendered (knowing where the fragment being evaluated is located within the output image is frequently useful). By default, ISF automatically declares a uniform vec2 named "RENDERSIZE" which is passed the dimensions of the image being rendered.
  • If the shader you're converting requires a time value, note that the uniform float "TIME" is declared, and passed the duration (in seconds) which the shader's been running when the shader's rendered.
  • Many shaders don't use (or even acknowledge) the alpha channel of the image being rendered. There's nothing wrong with this- but when the shader's loaded in an application that uses the alpha channel, the output of the shader can look bizarre and unpredictable (though it usually involves something being darker than it should be). If you run into this, try setting gl_FragColor.a to 1.0 at the end of your shader.
  • gl_FragCoord.xy contains the coordinates of the fragment being evaluated. vv_FragNormCoord.xy contains the normalized coordinates of the fragment being evaluated.
  • While ISF files are fragment shaders, and the host environment automatically generates a vertex shader, you can use your own vertex shader if you'd like. If you go this route, your vertex shader should have the same base name as your ISF file (just use the extension .vs), and the first thing you do in your vertex shader's main function is call "vv_verteShaderInit();".
  • If the shader you're converting requires imported graphic resources, note that the ISF format defines the ability to import image files by adding objects to your JSON dict under the "IMPORTED" key. The imported images are accessed via the usual IMG_PIXEL() or IMG_NORM_PIXEL() methods. Details on how to do this are listed below, and examples are included.
  • If your texture doesn't look right, make sure your texture coordinates are ranged properly (textures are typically "clamped" by the host implementation, if you specify an out-of-range texture coordinate it may look funny).

The ISF "specification"

First of all, there are super-simple examples that cover all of this- check out the various "Test____.fs" sample filters located here:
ISF Test/Tutorial filters will probably learn more, faster, from the examples than you'll get by reading this document: each example describes a single aspect of the ISF file format, and they're extremely hand for testing, reference, or as a tutorial (the ISF file format is very small).

Here's the example "Test-Float.fs" for creating a simple FX with a published number input:

    "DESCRIPTION": "demonstrates the use of float-type inputs",
    "CREDIT": "by zoidberg",
        "TEST-GLSL FX"
    "INPUTS": [
            "NAME": "inputImage",
            "TYPE": "image"
            "NAME": "level",
            "TYPE": "float",
            "DEFAULT": 0.5,
            "MIN": 0.0,
            "MAX": 1.0

void main()
    vec4        srcPixel = IMG_THIS_PIXEL(inputImage);
    float        luma = (srcPixel.r+srcPixel.g+srcPixel.b)/3.0;
    vec4        dstPixel = (luma>level) ? srcPixel : vec4(0,0,0,1);
    gl_FragColor = dstPixel;

Now let's look at some of what can be included in the commented out JSON section at the top of this fragment shader:

  • The first thing in your ISF file needs to be a comment (delineated using "/*" and "*/") containing a JSON dict. If the comment doesn't exist- or the JSON dict is malformed or can't be parsed- your ISF file can't be loaded (ISF files can be tested with the test app linked to elsewhere on this page). This JSON dict is referred to as your "top-level dict" throughout the rest of this document. If there's a string in the top-level dict stored at the "DESCRIPTION" key, this string will be displayed as a description associated with this filter in the host app. the use of this key is optional.
  • The "CATEGORIES" key in your top-level dict should store an array of strings. The strings are the category names you want the filter to appear in (assuming the host app displays categories).
  • The "INPUTS" key of your top-level dict should store an array of dictionaries (each dictionary describes a different input- the inputs should appear in the host app in the order they're listed in this array). For each input dictionary:
  • The value stored with the key "NAME" must be a string, and it must not contain any whitespaces. This is the name of the input, and will also be the variable name of the input in your shader.
  • The value stored with the key "TYPE" must be a string. This string describes the type of the input, and must be one of the following: "event", "bool", "long", "float", "point2D", "color", or "image".
  • Where appropriate, "DEFAULT", "MIN", "MAX", and "IDENTITY" may be used to further describe value attributes of the input. Note that "image"-type inputs don't have any of these, and that "color"-type inputs use an array of floats to describe min/max/default colors. Everywhere else values are stored as native JSON values where possible (float as float, bool as bool, etc).
  • The value stored with the key "LABEL" must be a string. This key is optional- the "NAME" of an input is the variable name, and as such it can't contain any spaces/etc. The "LABEL" key provides host sofware with the opportunity to display a more human-readable name. This string is purely for display purposes and isn't used for processing at all.
  • Other notes:
    • An "event" type inputs describe events that do not have an associated value- a momentary click button.
    • The "long" type input is used to implement pop-up buttons/pop-up menus in the host UI. As such, "long"-type input dictionaries have a few extra keys:
      • The "VALUES" key stores an array of integer values. This array may have repeats, and the values correspond to the labels. When you choose an item from the pop-up menu, the corresponding value from this array is sent to your shader.
      • The "LABELS" key stores an array of strings. This array may have repeats, and the strings/labels correspond to the array of values.
  • The "PERSISTENT_BUFFERS" key describes persistent buffers: these are buffers that will stay with your effect until it's deleted. This key is optional: you don't need to include it. The object at this key needs to be either an array or a dictionary:
  • If the item at "PERSISTENT_BUFFERS" is an array, each item in that array must be a string. This string is the name of the persistent buffer in your shader (the variable name). This is the easiest/fastest way to add a persistent dict. If you ask the filter to render a frame at a different resolution, persistent buffers are resized to accommodate.
  • If the item at "PERSISTENT_BUFFERS" is a dictionary, then each key-value pair in that dictionary describes a persistent buffer (the key is the name of the buffer, the value is a dictionary with its properties).
  • If the buffer dictionary has a value for the keys "WIDTH" or "HEIGHT", that value is expected to be a string with an equation describing the width/height of the buffer. This equation may reference variables: the width and height of the image requested from this filter are passed to the equation as "$WIDTH" and "$HEIGHT", and the value of any other inputs declared in "INPUTS" can also be passed to this equation (for example, the value from the float input "blurAmount" would be reqpresented in an equation as "$blurAmount"). This equation is evaluated once per frame, when you initially pass the filter a frame (it's not evaluated multiple times if the ISF file describes multiple rendering passes to produce a sigle frame). For more information (constants, built-in functions, etc) on math expression evaluations, please see the documentation for the excellent DDMathParser by Dave DeLong, which is what we're presently using.
  • The "PASSES" key should store an array of dictionaries. Each dictionary describes a different rendering pass. This key is optional: you don't need to include it, and if it's not present your effect will be assumed to be single-pass.
  • If one of the dicts in "PASSES" has a string stored at the key "TARGET", this string describes the name of the buffer this pass should render into. You can specify that a pass should render into a persistent buffer by putting its name in here. Alternately, you could make up a different name and put it here- the ISF host will automatically create a temporary buffer using this name, and you can read the pixels from this temporary buffer back in your shader in a subsequent rendering pass. Temporary buffers are deleted (or returned to a buffer pool) after the ISF file has finished rendering a frame of output (only persistent buffers last between frames).
  • If one of the dicts in "PASSES" has a string stored at the keys "WIDTH" or "HEIGHT", that string is an equation used to describe the width or height of the target buffer this pass is rendering into. For more info, see the description of the "WIDTH" and "HEIGHT" keys from "PERSISTENT_BUFFERS" (above).
  • The "IMPORTED" key should store an array of dictionaries. Each dictionary describes an image file you want ISF to automatically import. This key is optional. Each dict in the "IMPORTED" array has the following structure:
  • The "NAME" key of an import dict stores the variable name of the imported image. This key is necessary- the "NAME" is what you'd pass to one of the IMG_PIXEL() functions to retrieve the color of a pixel from that image, and is required for operation.
  • The "PATH" key of an import dict stores a string describing the path to the image file relative to the ISF file being evaluated. For example, a file named "asdf.jpg" in the same folder as the ISF file would have the PATH "asdf.jpg", or "./asdf.jpg" (both describe the same location). If the jpg were located in your ISF file's parent directory, its PATH would be "../asdf.jpg", etc.

What's next?

If you're looking to start making some ISF filters of your own, a good place to start is to look at some examples of what's possible with GLSL fragment shaders by visiting online WebGL communities such as ShaderToy and GLSL Sandbox. Then download some sample ISF image filters to get started with, and watch the video tutorial on making your own ISF sources.

For people writing GLSL shaders in Cycling74's Gen, you can now directly export to ISF by following a few simple steps.