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CanvasItem shaders

CanvasItem shaders are used to draw all 2D elements in Redot. These include all nodes that inherit from CanvasItems, and all GUI elements.

CanvasItem shaders contain fewer built-in variables and functionality than Spatial shaders, but they maintain the same basic structure with vertex, fragment, and light processor functions.

Render modes

Render mode	Description
blend_mix	Mix blend mode (alpha is transparency), default.
blend_add	Additive blend mode.
blend_sub	Subtractive blend mode.
blend_mul	Multiplicative blend mode.
blend_premul_alpha	Pre-multiplied alpha blend mode.
blend_disabled	Disable blending, values (including alpha) are written as-is.
unshaded	Result is just albedo. No lighting/shading happens in material.
light_only	Only draw on light pass.
skip_vertex_transform	VERTEX needs to be transformed manually in the vertex() function.
world_vertex_coords	VERTEX is modified in world coordinates instead of local.

Built-ins

Values marked as in are read-only. Values marked as out can optionally be written to and will not necessarily contain sensible values. Values marked as inout provide a sensible default value, and can optionally be written to. Samplers cannot be written to so they are not marked.

Global built-ins

Global built-ins are available everywhere, including custom functions.

Built-in	Description
in float TIME	Global time since the engine has started, in seconds. It repeats after every 3,600 seconds (which can be changed with the rollover setting). It's not affected by time_scale or pausing. If you need a TIME variable that can be scaled or paused, add your own global shader uniform and update it each frame.
in float PI	A PI constant (3.141592). A ratio of a circle's circumference to its diameter and amount of radians in half turn.
in float TAU	A TAU constant (6.283185). An equivalent of PI * 2 and amount of radians in full turn.
in float E	An E constant (2.718281). Euler's number and a base of the natural logarithm.

Vertex built-ins

Vertex data (VERTEX) is presented in local space (pixel coordinates, relative to the Node2D's origin). If not written to, these values will not be modified and be passed through as they came.

The user can disable the built-in model to world transform (world to screen and projection will still happen later) and do it manually with the following code:

shader_type canvas_item;
render_mode skip_vertex_transform;

void vertex() {

    VERTEX = (MODEL_MATRIX * vec4(VERTEX, 0.0, 1.0)).xy;
}

Other built-ins, such as UV and COLOR, are also passed through to the fragment() function if not modified.

For instancing, the INSTANCE_CUSTOM variable contains the instance custom data. When using particles, this information is usually:

• x: Rotation angle in radians.

• y: Phase during lifetime (0.0 to 1.0).

• z: Animation frame.

Built-in	Description
in mat4 MODEL_MATRIX	Local space to world space transform. World space is the coordinates you normally use in the editor.
in mat4 CANVAS_MATRIX	World space to canvas space transform. In canvas space the origin is the upper-left corner of the screen and coordinates ranging from (0.0, 0.0) to viewport size.
in mat4 SCREEN_MATRIX	Canvas space to clip space. In clip space coordinates ranging from (-1.0, -1.0) to (1.0, 1.0).
in int INSTANCE_ID	Instance ID for instancing.
in vec4 INSTANCE_CUSTOM	Instance custom data.
in bool AT_LIGHT_PASS	Always false.
in vec2 TEXTURE_PIXEL_SIZE	Normalized pixel size of default 2D texture. For a Sprite2D with a texture of size 64x32px, TEXTURE_PIXEL_SIZE = vec2(1/64, 1/32)
inout vec2 VERTEX	Vertex position, in local space.
in int VERTEX_ID	The index of the current vertex in the vertex buffer.
inout vec2 UV	Normalized texture coordinates. Range from 0.0 to 1.0.
inout vec4 COLOR	Color from vertex primitive multiplied by CanvasItem's modulate multiplied by CanvasItem's self_modulate.
inout float POINT_SIZE	Point size for point drawing.
in vec4 CUSTOM0	Custom value from vertex primitive.
in vec4 CUSTOM1	Custom value from vertex primitive.

Fragment built-ins

COLOR and TEXTURE

The built-in variable COLOR is used for a few things:

• In the vertex() function, COLOR contains the color from the vertex primitive multiplied by the CanvasItem's modulate multiplied by the CanvasItem's self_modulate.

• In the fragment() function, the input value COLOR is that same value multiplied by the color from the default TEXTURE (if present).

• In the fragment() function, COLOR is also the final output.

Certain nodes (for example, Sprite2D) display a texture by default, for example texture. When using a custom fragment() function, you have a few options on how to sample this texture.

To read only the contents of the default texture, ignoring the vertex COLOR:

void fragment() {
  COLOR = texture(TEXTURE, UV);
}

To read the contents of the default texture multiplied by vertex COLOR:

void fragment() {
  // Equivalent to an empty fragment() function, since COLOR is also the output variable.
  COLOR = COLOR;
}

To read only the vertex COLOR in fragment(), ignoring the main texture, you must pass COLOR as a varying, then read it in fragment():

varying vec4 vertex_color;
void vertex() {
  vertex_color = COLOR;
}
void fragment() {
  COLOR = vertex_color;
}

NORMAL

Similarly, if a normal map is used in the CanvasTexture, Redot uses it by default and assigns its value to the built-in NORMAL variable. If you are using a normal map meant for use in 3D, it will appear inverted. In order to use it in your shader, you must assign it to the NORMAL_MAP property. Redot will handle converting it for use in 2D and overwriting NORMAL.

NORMAL_MAP = texture(NORMAL_TEXTURE, UV).rgb;

Built-in	Description
in vec4 FRAGCOORD	Coordinate of pixel center. In screen space. xy specifies position in viewport. Upper-left of the viewport is the origin, (0.0, 0.0).
in vec2 SCREEN_PIXEL_SIZE	Size of individual pixels. Equal to inverse of resolution.
in vec2 POINT_COORD	Coordinate for drawing points.
sampler2D TEXTURE	Default 2D texture.
in vec2 TEXTURE_PIXEL_SIZE	Normalized pixel size of default 2D texture. For a Sprite2D with a texture of size 64x32px, TEXTURE_PIXEL_SIZE = vec2(1/64, 1/32)
in bool AT_LIGHT_PASS	Always false.
sampler2D SPECULAR_SHININESS_TEXTURE	Specular shininess texture of this object.
in vec4 SPECULAR_SHININESS	Specular shininess color, as sampled from the texture.
in vec2 UV	UV from the vertex() function.
in vec2 SCREEN_UV	Screen UV coordinate for current pixel.
sampler2D SCREEN_TEXTURE	Removed in Redot 4. Use a sampler2D with hint_screen_texture instead.
inout vec3 NORMAL	Normal read from NORMAL_TEXTURE. Writable.
sampler2D NORMAL_TEXTURE	Default 2D normal texture.
out vec3 NORMAL_MAP	Configures normal maps meant for 3D for use in 2D. If used, overrides NORMAL.
out float NORMAL_MAP_DEPTH	Normal map depth for scaling.
inout vec2 VERTEX	Pixel position in screen space.
inout vec2 SHADOW_VERTEX	Same as VERTEX but can be written to alter shadows.
inout vec3 LIGHT_VERTEX	Same as VERTEX but can be written to alter lighting. Z component represents height.
inout vec4 COLOR	COLOR from the vertex() function multiplied by the TEXTURE color. Also output color value.

Light built-ins

Light processor functions work differently in Redot 4.x than they did in Redot 3.x. In Redot 4.x all lighting is done during the regular draw pass. In other words, Redot no longer draws the object again for each light.

Use the unshaded render mode if you do not want the light() function to run. Use the light_only render mode if you only want to see the impact of lighting on an object; this can be useful when you only want the object visible where it is covered by light.

If you define a light() function it will replace the built in light function, even if your light function is empty.

Below is an example of a light shader that takes a CanvasItem's normal map into account:

void light() {
  float cNdotL = max(0.0, dot(NORMAL, LIGHT_DIRECTION));
  LIGHT = vec4(LIGHT_COLOR.rgb * COLOR.rgb * LIGHT_ENERGY * cNdotL, LIGHT_COLOR.a);
}

Built-in	Description
in vec4 FRAGCOORD	Coordinate of pixel center. In screen space. xy specifies position in viewport. Upper-left of the viewport is the origin, (0.0, 0.0).
in vec3 NORMAL	Input normal.
in vec4 COLOR	Input color. This is the output of the fragment() function.
in vec2 UV	UV from the vertex() function, equivalent to the UV in the fragment() function.
sampler2D TEXTURE	Current texture in use for CanvasItem.
in vec2 TEXTURE_PIXEL_SIZE	Normalized pixel size of TEXTURE. For a Sprite2D with a TEXTURE of size 64x32 pixels, TEXTURE_PIXEL_SIZE = vec2(1/64, 1/32)
in vec2 SCREEN_UV	Screen UV coordinate for current pixel.
in vec2 POINT_COORD	UV for Point Sprite.
in vec4 LIGHT_COLOR	Color of the class_Light2D. If the light is a class_PointLight2D, multiplied by the light's texture.
in float LIGHT_ENERGY	Energy multiplier of the class_Light2D.
in vec3 LIGHT_POSITION	Position of the class_Light2D in screen space. If using a class_DirectionalLight2D this is always (0.0, 0.0, 0.0).
in vec3 LIGHT_DIRECTION	Direction of the class_Light2D in screen space.
in bool LIGHT_IS_DIRECTIONAL	true if this pass is a class_DirectionalLight2D.
in vec3 LIGHT_VERTEX	Pixel position, in screen space as modified in the fragment() function.
inout vec4 LIGHT	Output color for this class_Light2D.
in vec4 SPECULAR_SHININESS	Specular shininess, as set in the object's texture.
out vec4 SHADOW_MODULATE	Multiply shadows cast at this point by this color.

SDF functions

There are a few additional functions implemented to sample an automatically generated Signed Distance Field texture. These functions available for the fragment() and light() functions of CanvasItem shaders.

The signed distance field is generated from class_LightOccluder2D nodes present in the scene with the SDF Collision property enabled (which is the default). See the 2D lights and shadows documentation for more information.

Function	Description
float texture_sdf (vec2 sdf_pos)	Performs an SDF texture lookup.
vec2 texture_sdf_normal (vec2 sdf_pos)	Calculates a normal from the SDF texture.
vec2 sdf_to_screen_uv (vec2 sdf_pos)	Converts an SDF to screen UV.
vec2 screen_uv_to_sdf (vec2 uv)	Converts screen UV to an SDF.