Custom Shader Effects

Naming Effects

The name of an effect is determined by the following properties:

• category — The type of effect.
• group — The group of effect. If not provided, Corona will assume custom.
• name — The name within a given category.

When you set an effect on a display object, you must provide a fully-qualified string by concatenating the above values and separating each by a . as follows:

local effectName = "[category].[group].[name]"

Defining Kernels

Corona packages snippets of shader code in the form of kernels. By structuring specific vertex and fragment processing tasks in kernels, the creation of custom effects is dramatically simplified.

Essentially, a kernel is shader code which the main shader program relies upon to handle specific processing tasks. Corona supports both vertex kernels and fragment kernels. You must specify at least one kernel type in your effect (or both). If a vertex/fragment kernel is not specified, Corona inserts the default vertex/fragment kernel respectively.

Time

The following time uniforms can be accessed by the vertex kernel:

• P_DEFAULT float CoronaTotalTime — Running time of the app in seconds.
• P_DEFAULT float CoronaDeltaTime — Time in seconds since previous frame.

If you use these variables in your kernel's shader code, your kernel is implicitly time-dependent. In other words, your kernel will output different results and evolve as time progresses.

When using these variables, you need to tell Corona that your shader requires the GPU to re-render the scene, even if there are no other changes to the display objects in the scene. You can do this by setting the kernel.isTimeDependent property in your kernel definition as indicated below. Note that you should only set this if your shader code is truly time-dependent, since it effectively forces the GPU to re-render every frame.

kernel.isTimeDependent = true

Size

The following size uniforms can be accessed by the vertex kernel:

• P_POSITION vec2 CoronaContentScale — The number of content pixels per screen pixels along the x and y axes. Content pixels refer to Corona's coordinate system and are determined by the content scaling settings for your project.

• P_UV vec4 CoronaTexelSize — These values help you understand normalized texture pixels (texels) as they relate to actual pixels. This is useful because texture coordinates are normalized (0 to 1) and normally you only have information about proportion (the percentage of the width or height of the texture). Effectively, these values help you create effects based on actual screen/content pixel distances.

Coordinate

• P_UV vec2 CoronaTexCoord — The texture coordinate for the vertex.

Example

The following example causes the bottom edge of an image to wobble by a fixed amplitude:

local kernel = {}

-- "filter.custom.myWobble"
kernel.category = "filter"
kernel.name = "myWobble"

-- Shader code uses time environment variable CoronaTotalTime
kernel.isTimeDependent = true

kernel.vertex =
[[
P_POSITION vec2 VertexKernel( P_POSITION vec2 position )
{
    P_POSITION float amplitude = 10;
    position.y += sin( 3.0 * CoronaTotalTime + CoronaTexCoord.x ) * amplitude * CoronaTexCoord.y;

    return position;
}
]]

Time

The same vertex kernel time uniforms can be accessed by the fragment kernel.

Size

The same vertex kernel size uniforms can be accessed by the fragment kernel.

Samplers

• P_COLOR sampler2D CoronaSampler0 — The texture sampler for the first texture.
• P_COLOR sampler2D CoronaSampler1 — The texture sampler for the second texture (requires a composite paint).

Alpha/Tint

All display objects have an alpha property. In addition, shape objects have a tint which is set either via object:setFillColor() or the color channel properties (r, g, b, a) of the object's fill property.

Generally, your shader should incorporate the effect of these properties into the color that your fragment kernel returns. You can do this by calling the following function to calculate the correct color:

P_COLOR vec4 CoronaColorScale( P_COLOR vec4 color );

This function takes an input color vector (red, green, blue, and alpha channels) and returns a color vector modulated by the display object's tint and alpha, as shown in the fragment kernel examples. Generally, you should call this function at the end of the fragment kernel so that you can properly calculate the color vector your fragment kernel should return.

Example

The following example brightens an image by a fixed amount per color component:

local kernel = {}

-- "filter.custom.myBrighten"
kernel.category = "filter"
kernel.name = "myBrighten"

kernel.fragment =
[[
P_COLOR vec4 FragmentKernel( P_UV vec2 texCoord )
{
    P_COLOR float brightness = 0.5;
    P_COLOR vec4 texColor = texture2D( CoronaSampler0, texCoord );

    // Pre-multiply the alpha to brightness
    brightness = brightness * texColor.a;

    // Add the brightness
    texColor.rgb += brightness;

    // Modulate by the display object's combined alpha/tint.
    return CoronaColorScale( texColor );
}
]]

Example

The following example combines the wobble vertex and brighten fragment kernels. Unlike the "myBrighten" fragment example above, this version does not use a fixed value for brightness. Instead, the vertex shader calculates an oscillating brightness value for each vertex and the fragment shader linearly interpolates the brightness value according to the pixel it's shading.

local kernel = {}
kernel.category = "filter"
kernel.name = "wobbleAndBrighten"

-- Shader code uses time environment variable CoronaTotalTime
kernel.isTimeDependent = true

kernel.vertex =
[[
varying P_COLOR float delta; // Custom varying variable

P_POSITION vec2 VertexKernel( P_POSITION vec2 position )
{
    P_POSITION float amplitude = 10;

    position.y += sin( 3.0 * CoronaTotalTime + CoronaTexCoord.x ) * amplitude * CoronaTexCoord.y;

    // Calculate value for varying
    delta = 0.4*(CoronaTexCoord.y + sin( 3.0 * CoronaTotalTime + 2.0 * CoronaTexCoord.x ));

    return position;
}
]]

kernel.fragment =
[[
varying P_COLOR float delta; // Matches declaration in vertex shader

P_COLOR vec4 FragmentKernel( P_UV vec2 texCoord )
{
    // Brightness changes based on interpolated value of custom varying variable
    P_COLOR float brightness = delta;

    P_COLOR vec4 texColor = texture2D( CoronaSampler0, texCoord );

    // Pre-multiply the alpha to brightness
    brightness *= texColor.a;

    // Add the brightness
    texColor.rgb += brightness;

    // Modulate by the display object's combined alpha/tint.
    return CoronaColorScale( texColor );
}

Vertex Userdata

When using vertex userdata to pass effect parameters, the effect parameters are copied for each vertex. To minimize the data size impact, the effect parameters are limited to a vec4 (vector of 4 floats). This is available as the following read-only vector variable in both the vertex and fragment kernels:

P_DEFAULT vec4 CoronaVertexUserData

For example, suppose you want to modify the above "filter.custom.myBrighten" effect example so that, in Lua, there is a "brightness" parameter for the effect:

object.fill.effect = "filter.custom.myBrighten"
object.fill.effect.brightness = 0.3

To accomplish this, you must instruct Corona to map the parameter name in Lua with the corresponding component in the vector returned by CoronaVertexUserData. The following code tells Corona that the "brightness" parameter is the first component (index = 0) of the CoronaVertexUserData vector.

kernel.vertexData =
{
    {
        name = "brightness",
        default = 0, 
        min = 0,
        max = 1,
        index = 0,  -- This corresponds to "CoronaVertexUserData.x"
    },
}

In the above array (kernel.vertexData), each element is a table and each table specifies:

• name — The string name for the parameter exposed in Lua.
• default — The default value.
• min — The minimum value.
• max — The maximum value.

• index — The index for the corresponding vector component in CoronaVertexUserData:

  index = 0 → CoronaVertexUserData.x
index = 1 → CoronaVertexUserData.y
index = 2 → CoronaVertexUserData.z
index = 3 → CoronaVertexUserData.w

Finally, modify the FragmentKernel to read the parameter value, accessing the parameter value via CoronaVertexUserData:

kernel.fragment =
[[
P_COLOR vec4 FragmentKernel( P_UV vec2 texCoord )
{
    P_COLOR float brightness = CoronaVertexUserData.x;

    ...
}
]]

Uniform Userdata

(forthcoming feature)

Corona Simulator vs Device

Precision Qualifier Macros

Unlike other flavors of GLSL, GLSL ES (OpenGL ES 2.0) generally requires precision qualifiers to be specified in variable declarations. Thus, it's a good practice to be explicit about precision.

Instead of using raw precision qualifiers like lowp, you should use one of the following precision qualifier macros. The defaults are optimized for the type of data:

• P_DEFAULT — For generic values; default is highp.
• P_RANDOM — For random values; default is highp.
• P_POSITION — For positions; default is mediump.
• P_NORMAL — For normals; default is mediump.
• P_UV — For texture coordinates; default is mediump.
• P_COLOR — For pixel colors; default is lowp.

We strongly recommend you use Corona's defaults for shader precision, all of which have been optimized to balance performance and fidelity. However, your project can override these settings in config.lua (guide).

High-Precision Devices

Not all devices support high precision. Therefore, if your kernel requires high precision, you should use the GL_FRAGMENT_PRECISION_HIGH macro. This is 1 if high precision is supported on the device, or undefined otherwise.

If the device does not support highp, your kernel can gracefully degrade by writing two implementations:

P_COLOR vec4 FragmentKernel( P_UV vec2 texCoord )
{
#ifdef GL_FRAGMENT_PRECISION_HIGH
    // Code path for high precision calculations
#else
    // Code path for fallback
#endif
}

Pre-Multiplied Alpha

Corona provides textures with pre-multiplied alpha. Therefore, you may need to divide by the alpha to recover the original RGB values. However, for performance reasons, you should try to perform calculations to avoid the divide. Compare the following two kernels that brighten an image:

1. In the following, the original RGB values are recovered by undoing the pre-multiplied alpha, and later, the alpha is re-applied. This is not ideal because it generates a lot of additional operations on the GPU for every pixel.

// Non-optimal Version
P_COLOR vec4 FragmentKernel( P_UV vec2 texCoord )
{
    P_COLOR float brightness = 0.5;
    P_COLOR vec4 texColor = texture2D( CoronaSampler0, texCoord );

    // BAD: Recover original RGBs via divide
    texColor.rgb /= texColor.a;

    // Add the brightness
    texColor.rgb += brightness;

    // BAD: Re-apply the pre-multiplied alpha
    texColor.rgb *= texColor.a;

    return CoronaColorScale( texColor );
}

2. This version pre-multiplies the alpha of the texture to the brightness variable so that it can be added directly to the texture's RGB values. This circumvents the deficiencies of the above implementation.

// Optimal Version
P_COLOR vec4 FragmentKernel( P_UV vec2 texCoord )
{
    P_COLOR float brightness = 0.5;
    P_COLOR vec4 texColor = texture2D( CoronaSampler0, texCoord );

    // GOOD: Pre-multiply the alpha to brightness
    brightness = brightness * texColor.a;

    // Add the brightness
    texColor.rgb += brightness;

    return CoronaColorScale( texColor );
}

Vector Calculations

Some devices do not have GPUs with vector processors. In those cases, vector calculations may be performed on a scalar processor. Generally, you should carefully consider the order of operations in your shader to ensure that unnecessary calculations can be avoided on a scalar processor.

P_DEFAULT float f0, f1;
P_DEFAULT vec4 v0, v1;
v0 = (v1 * f0) * f1; // BAD: Multiply each scalar to a vector

highp float f0, f1;
highp vec4 v0, v1;
v0 = v1 * (f0 * f1); // GOOD: Multiply scalars first

highp vec4 v0;
highp vec4 v1;
highp vec4 v2;
v2.xz = v0 * v1; // GOOD: Write mask limits calculations

Avoid Dynamic Texture Lookups

When a fragment shader samples textures at a location different from the texture coordinate passed to the shader, it causes a dynamic texture lookup, also known as "dependent texture reads." In OpenGL-ES 2.0, dependent texture reads can delay texel data loading and reduce performance. This is why certain effects that sample a region of texels, for instance blur effects, are slower.

In contrast, effects that have no dependent texture reads enable the GPU to pre-fetch texel data before the shader executes, reducing I/O latency.

Avoid Branching and Loops

Branching instructions (if conditions) are expensive. When possible, for loops should be unrolled or replaced by vector operations.