GSK render nodes can be serialized and deserialized using APIs such as gsk_render_node_serialize() and gsk_render_node_deserialize(). The intended use for this is development - primarily the development of GTK - by allowing things such as creating testsuites and benchmarks, exchanging nodes in bug reports. GTK includes the gtk4-node-editor application for creating such test files.

The format is a text format that follows the CSS syntax rules. In particular, this means that every array of bytes will produce a render node when parsed, as there is a defined error recovery method. For more details on error handling, please refer to the documentation of the parsing APIs.

The grammar of a node text representation using the CSS value definition syntax looks like this:

document: <@-rule>*<node>
@-rule: `cicp` "name" { <property>* }
node: container [ "name" ] { <document> } | <node-type> [ "name" ] { <property>* } | "name"
property: <property-name>: <node> | <value> ;

Each node has its own <node-type> and supports a custom set of properties, each with their own <property-name> and syntax. The following paragraphs document each of the nodes and their properties.

When serializing and the value of a property equals the default value, this value will not be serialized. Serialization aims to produce an output as small as possible.

To embed newlines in strings, use \A. To break a long string into multiple lines, escape the newline with a .

Names

Nodes

Nodes can be given a name by adding a string after the <node-type> in their definition. That same node can then be used further down in the document by specifying just the name identifying the node.

Textures

Just like nodes, textures can be referenced by name. When defining a named texture, the name has to be placed in front of the URL.

Color states

Color states are represented either by an ident (for builtin ones) or a string (for custom ones):

color-state: <ident> | <string>

Custom color states can be defined at the beginning of the document, with an @‌cicp rule.

The format for @‌cicp rules is

@‌cicp "name" {
  ...
}

The following properties can be set for custom color states:

Note that the primaries, transfer and matrix properties always need to be specified, since GTK does not allow creating color state objects with these being set to 2 (== unspecified).

Range can have the following values:

range: narrow | full

Colors

Colors can be specified with a variation of the modern CSS color syntax:

color(<color-state> <number> <number> <number> ["/" <number>])

The traditional syntax for sRGB colors still works as well:

rgba(<number>, <number>, <number>, <number)
rgb(<number, <number>, <number>)

Rectangles

Rectangles can be specified just as four integers for x, y, width and height:

rect: <number> <number> <number> <number>

Rounded rectangles use a CSS-like syntax:

rounded-rect: <rect> [ "/" <number>{1,4} [ "/" <number>{1,4} ] ]

Nodes

container

The container node is a special node that allows specifying a list of child nodes. Its contents follow the same rules as an empty document.

blend

Creates a node like gsk_blend_node_new() with the given properties.

Possible values for the mode property are:

blend-mode: normal | multiply | screen | overlay | darken |
            lighten | color-dodge | color-burn | hard-light |
            soft-light | difference | exclusion | color |
            hue | saturation | luminosity

blur

Creates a node like gsk_blur_node_new() with the given properties.

border

Creates a node like gsk_border_node_new() with the given properties.

For the color and width properties, the values follow the typical CSS order of top, right, bottom, left. If the last/left value isn’t given, the 2nd/right value is used. If the 3rd/bottom value isn’t given, the 1st/top value is used. And if the 2nd/right value also isn’t given, the 1st/top value is used for every 4 values.

cairo

The pixels are a base64-encoded data url of png data. The script is a base64-encoded data url of a cairo script.

clip

Creates a node like gsk_clip_node_new() with the given properties.

As an extension, this node allows specifying a rounded rectangle for the clip property. If that rectangle is indeed rounded, a node like gsk_rounded_clip_node_new() will be created instead.

color

Creates a node like gsk_color_node_new() with the given properties.

The color is chosen as an error pink so it is visible while also reminding people to change it.

color-matrix

Creates a node like gsk_color_matrix_node_new() with the given properties.

The matrix property accepts a for compatibility purposes, but you should be aware that the allowed values are meant to be used on 3D transformations, so their naming might appear awkward. However, it is always possible to use the matrix3d() production to specify all 16 values individually.

conic-gradient

Creates a node like gsk_conic_gradient_node_new() with the given properties.

Possible values for the hue-interpolation property are:

hue-interpolation: shorter | longer | increasing | decreasing

cross-fade

Creates a node like gsk_cross_fade_node_new() with the given properties.

debug

Creates a node like gsk_debug_node_new() with the given properties.

fill

Creates a node like gsk_fill_node_new() with the given properties.

The default child node is the default color node, but created with the bounds of the path.

Possible values for the fill-rule property are:

fill-rule: winding | even-odd

glshader

Creates a GLShader node. The sourcecode must be a GLSL fragment shader. The args must match the uniforms of simple types declared in that shader, in order and comma-separated. The child properties must match the sampler uniforms in the shader.

inset-shadow

Creates a node like gsk_inset_shadow_node_new() with the given properties.

linear-gradient

Creates a node like gsk_linear_gradient_node_new() with the given properties.

mask

Creates a node like gsk_mask_node_new() with the given properties.

Possible values for the mode property are:

mask-mode: alpha | inverted-alpha | luminance | inverted-luminance

opacity

Creates a node like gsk_transform_node_new() with the given properties.

outset-shadow

Creates a node like gsk_outset_shadow_node_new() with the given properties.

radial-gradient

Creates a node like gsk_radial_gradient_node_new() with the given properties.

repeat

Creates a node like gsk_repeat_node_new() with the given properties.

repeating-linear-gradient

Creates a node like gsk_repeating_linear_gradient_node_new() with the given properties.

repeating radial-gradient

Creates a node like gsk_repeating_radial_gradient_node_new() with the given properties.

rounded-clip

Creates a node like gsk_rounded_clip_node_new() with the given properties.

shadow

Creates a node like gsk_shadow_node_new() with the given properties.

stroke

Creates a node like gsk_stroke_node_new() with the given properties.

The default child node is the default color node, but created with the stroke bounds of the path.

Possible values for the line-cap property are:

line-cap: butt | round | square

Possible values for the line-join property are:

line-join: miter | round | bevel

text

Creates a node like gsk_text_node_new() with the given properties.

If a url is specified for the font, it must point to a font file for the font that is specified in the string. It can be either a data url containing a base64-encoded font file, or a regular url that points to a font file.

Glyphs can be specified as an ASCII string, or as a comma-separated list of their glyph IDs. Optionally, the advance width, x and y offsets and flags can be specified as well, like this: 40 10 0 0 color.

If the given font does not exist or the given glyphs are invalid for the given font, an error node will be returned.

Possible values for the hint-style property are:

hint-style: none | slight | full

Possible value for the antialias property are:

antialias:  none | gray

Possible value for hint-metrics are:

hint-metrics:  on | off

texture

Creates a node like gsk_texture_node_new() with the given properties.

If a string is specified for the texture, it will be used as a name for the text. Textures can be reused by specifying the name of a previously used texture. In that case, the url can be omitted.

The default texture is a 10x10 checkerboard with the top left and bottom right 5x5 being in the color FF00CC and the other part being transparent. A possible representation for this texture is url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKCAYAAACNMs+9AAAABmJLR0QA/wD/AP+gvaeTAAAAKUlEQVQYlWP8z3DmPwMaYGQwYUQXY0IXwAUGUCGGoxkYGBiweXAoeAYAz44F3e3U1xUAAAAASUVORK5CYII=").

texture-scale

Creates a node like gsk_texture_scale_node_new() with the given properties.

The default texture is a 10x10 checkerboard, just like for texture.

Possible values for the filter property are:

filter: linear | nearest | trilinear

transform

Creates a node like gsk_transform_node_new() with the given properties.