# Use assets
In the previous tutorial you learned how to describe UI with `Emerge.UI`.
The next step is to bring real media into that UI: images, SVGs, background
images, and custom fonts.
Assets in Emerge are still declarative. You describe a source in your UI tree,
and the renderer resolves it for you.
## Static assets live under `priv/`
Logical asset paths resolve from your app's `priv/` directory.
For example:
- `priv/images/logo.png`
- `priv/images/hero.jpg`
- `priv/icons/check.svg`
- `priv/fonts/Inter-Regular.ttf`
You can refer to those files by logical path string, or you can use the `~m`
sigil for compile-time verification.
## Start with logical path strings
The simplest source form is a logical path string:
```elixir
image([width(px(120)), height(px(120))], "images/logo.png")
svg([width(px(24)), height(px(24))], "icons/check.svg")
el(
[
width(px(320)),
height(px(180)),
Background.image("images/hero.jpg", fit: :cover)
],
none()
)
```
Logical paths are resolved from the `otp_app` you pass to `EmergeSkia.start/1`
or that the viewport infers for you.
## Prefer `~m` in app code
Import `Emerge.Assets.Path` in the module where you describe UI:
```elixir
defmodule MyApp.UI do
use Emerge.Assets.Path, otp_app: :my_app
use Emerge.UI
end
```
Then you can write compile-time verified paths:
```elixir
~m"images/logo.png"
~m"images/hero.jpg"
~m"icons/check.svg"
```
`~m` verifies that the file exists under `priv/` at compile time and tracks it
as an external resource.
## Show an image and a background image
This example uses a normal image element and a background image on a framed
element:
```elixir
column(
[
width(fill()),
height(fill()),
padding(16),
spacing(16),
Background.color(color(:slate, 900)),
Border.rounded(14)
],
[
column([spacing(8)], [
el([Font.color(color(:slate, 50)), Font.size(14)], text("image/2")),
el(
[
padding(10),
Background.color(color(:slate, 800)),
Border.rounded(12)
],
image([width(px(120)), height(px(120)), Border.rounded(10)], "sample_assets/static.jpg")
)
]),
column([spacing(8)], [
el([Font.color(color(:slate, 50)), Font.size(14)], text("Background.image/2")),
el(
[
width(px(288)),
height(px(160)),
padding(12),
Background.image("sample_assets/fallback.jpg", fit: :cover),
Border.rounded(12)
],
column([height(fill()), spacing(8)], [
el(
[
padding_xy(10, 6),
Background.color(color_rgba(15, 23, 42, 0.7)),
Border.rounded(999),
Font.color(color(:slate, 50))
],
text("Featured trail")
),
el(
[
align_bottom(),
padding(10),
Background.color(color_rgba(15, 23, 42, 0.58)),
Border.rounded(10),
Font.color(color(:slate, 50))
],
column([spacing(4)], [
el([Font.size(18)], text("Background image host")),
el([Font.size(12), Font.color(color(:slate, 200))], text("Foreground content sits on top."))
])
)
])
)
])
]
)
```
`image/2` creates an image element.
`Background.image/2` paints an image inside another element's frame.
## Use SVG files
Use `svg/2` when the source is an SVG:
```elixir
row(
[
width(fill()),
height(fill()),
padding(16),
spacing(12),
Background.color(color(:slate, 900)),
Border.rounded(14)
],
[
el(
[
width(fill()),
padding(12),
Background.color(color(:slate, 800)),
Border.rounded(12)
],
column([center_x(), spacing(8)], [
svg([width(px(48)), height(px(48))], "sample_assets/template_cloud.svg"),
el([Font.color(color(:slate, 50)), Font.size(13)], text("Original SVG"))
])
),
el(
[
width(fill()),
padding(12),
Background.color(color(:slate, 800)),
Border.rounded(12)
],
column([center_x(), spacing(8)], [
svg(
[width(px(48)), height(px(48)), Svg.color(color(:sky, 500))],
"sample_assets/template_cloud.svg"
),
el([Font.color(color(:slate, 50)), Font.size(13)], text("Svg.color/1"))
])
)
]
)
```
By default, SVGs keep their original colors.
Use `Svg.color/1` when you want template-style tinting.
## Background image fit modes
`Background.image/2` defaults to `fit: :cover`.
Use:
- `fit: :cover` to fill the frame and crop if needed
- `fit: :contain` to keep the whole image visible
- `Background.tiled/1`, `Background.tiled_x/1`, and `Background.tiled_y/1` for repeat modes
Example:
```elixir
el(
[
width(px(220)),
height(px(120)),
Background.image("images/logo.png", fit: :contain),
Border.rounded(12)
],
none()
)
```
## Configure fonts at renderer startup
Fonts work a little differently from images.
Images and SVGs are referenced directly in the UI tree.
Fonts are registered once when the renderer starts, and then selected in UI code
by `family`, `weight`, and `italic`.
If you want multiple variants of the same family, register each variant:
```elixir
{:ok, renderer} =
EmergeSkia.start(
otp_app: :my_app,
title: "My App",
assets: [
fonts: [
[family: "Inter", source: "fonts/Inter-Regular.ttf", weight: 400],
[family: "Inter", source: "fonts/Inter-Bold.ttf", weight: 700],
[family: "Inter", source: "fonts/Inter-Italic.ttf", weight: 400, italic: true]
]
]
)
```
After that, use the configured family in UI code:
```elixir
column([spacing(8)], [
el([Font.family("Inter"), Font.size(22), Font.bold()], text("Release notes")),
el([Font.family("Inter"), Font.regular()], text("Design system updated")),
el([Font.family("Inter"), Font.italic(), Font.color(color(:slate, 300))], text("Beta"))
])
```
The key idea is:
- `family` selects the registered family
- `Font.bold/0` or `Font.weight(700)` selects the bold variant
- `Font.italic/0` selects the italic variant
If you want a family to support multiple weights or italics, register those
variants in `assets.fonts`.
## Runtime filesystem paths
Emerge also supports runtime filesystem paths:
```elixir
image([width(px(160)), height(px(96))], {:path, "/data/photos/photo.jpg"})
```
Runtime path loading is disabled by default.
Enable it only when needed, and use an explicit allowlist in
`EmergeSkia.start/1`:
```elixir
assets: [
runtime_paths: [
enabled: true,
allowlist: ["/data/photos"],
follow_symlinks: false
]
]
```
## What happens while assets load
Asset loading is asynchronous.
While a source is still loading, Emerge shows a loading placeholder. If loading
fails, Emerge shows a failed placeholder.
You do not need to block rendering while assets are being resolved.