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Export: PNG with a transparent background (#784)

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The renderer lays every slide background over white (as PowerPoint does),
so a PNG export was always opaque. A new transparentBackground option leaves
the slide background out (both the white underlay and the background fill),
so only what is on the slide is drawn over transparency. Shapes filled with
the background (useBgFill) still show it.

- render: RenderOpts::transparent_background
- engine: file.export {transparentBackground} (PNG only; other formats
  refuse it and write nothing), file.render {transparentBackground}
- UI: Transparent background checkbox in the Export dialog (PNG) and in
  File > Export > Change File Type > PNG; the web download honours it
- CLI: render --transparent
This commit is contained in:
Gert Pellin
2026-10-16 16:30:52 +02:00
parent 1dc69720db
commit 39e7fb1d4f
11 changed files with 237 additions and 17 deletions
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crates/engine/src/cmd/file.rs
+19 -5
View File
@@ -33,7 +33,7 @@ pub fn specs() -> Vec<CommandSpec> {
cmd!(noundo "file.saveAs", "Save As…", ["File"], Some("Cmd+Shift+S"), "{path, format?}", has_doc, save_as),
cmd!(noundo "file.saveTemplate", "Save as Template…", ["File"], None, "{path}", has_doc, save_as),
cmd!(query "file.saveBytes", "Save to Data", [], None, "{format?: deckcraft|pptx} → {data: base64}", has_doc, save_bytes_cmd),
cmd!(noundo "file.export", "Export…", ["File"], None, "{path, format?: png|jpeg|pptx|deckcraft|outline|pdf|gif (else by path), slide?: index, all?: bool, scale?: px per pt, layout?: slides|notes|handouts (pdf), perPage?: 1|2|3|4|6|9, paper?: letter|a4|legal|a3 (pdf), orientation?: portrait|landscape (pdf), dpi?, slides?: [index]|range (\"1-3,5\"), includeHidden?, textLayer?, frame?, width?: px 16-3840|small|medium|large|extraLarge (gif), secondsPerSlide?: 0.1-600 (gif; 5), loop?: bool (gif; true: forever)} → {path, bytes} or {files} (png, jpeg); gif adds frames, width, height, secondsPerSlide", has_doc, export),
cmd!(noundo "file.export", "Export…", ["File"], None, "{path, format?: png|jpeg|pptx|deckcraft|outline|pdf|gif (else by path), slide?: index, all?: bool, scale?: px per pt, layout?: slides|notes|handouts (pdf), perPage?: 1|2|3|4|6|9, paper?: letter|a4|legal|a3 (pdf), orientation?: portrait|landscape (pdf), dpi?, slides?: [index]|range (\"1-3,5\"), includeHidden?, textLayer?, frame?, transparentBackground?: bool (png: leave the slide background out), width?: px 16-3840|small|medium|large|extraLarge (gif), secondsPerSlide?: 0.1-600 (gif; 5), loop?: bool (gif; true: forever)} → {path, bytes} or {files} (png, jpeg); gif adds frames, width, height, secondsPerSlide", has_doc, export),
cmd!(
query "file.info",
"Info",
@@ -43,7 +43,7 @@ pub fn specs() -> Vec<CommandSpec> {
has_doc,
info
),
cmd!(query "file.render", "Render Slide", [], None, "{slide?: index, scale?, edit?: bool} → {png: base64, width, height}", has_doc, render),
cmd!(query "file.render", "Render Slide", [], None, "{slide?: index, scale?, edit?: bool, transparentBackground?: bool} → {png: base64, width, height}", has_doc, render),
cmd!(
query "slide.colorAt",
"Color At",
@@ -431,8 +431,14 @@ pub fn image_scale(doc: &Presentation, scale: f64) -> f64 {
/// Render slide `index` (current when absent) to PNG bytes.
pub fn render_png(doc: &Presentation, index: usize, scale: f64, edit: bool) -> (Vec<u8>, u32, u32) {
render_png_with(doc, index, scale, edit, false)
}
/// [`render_png`]; `transparent` leaves the slide background out (#784).
pub fn render_png_with(doc: &Presentation, index: usize, scale: f64, edit: bool, transparent: bool) -> (Vec<u8>, u32, u32) {
let scale = image_scale(doc, scale);
let img = deckcraft_render::render_slide(doc, index, &deckcraft_render::RenderOpts { scale, edit, ..Default::default() });
let opts = deckcraft_render::RenderOpts { scale, edit, transparent_background: transparent, ..Default::default() };
let img = deckcraft_render::render_slide(doc, index, &opts);
(img.to_png_at(deckcraft_render::dpi_for_scale(scale)), img.width, img.height)
}
@@ -451,7 +457,7 @@ fn render(s: &mut Session, p: &Value) -> Result<Value> {
if i >= st.doc.slides.len() {
return Err(bad("file.render", format!("no slide {i}")));
}
let (png, w, h) = render_png(&st.doc, i, f64_or(p, "scale", 1.0), bool_or(p, "edit", false));
let (png, w, h) = render_png_with(&st.doc, i, f64_or(p, "scale", 1.0), bool_or(p, "edit", false), bool_or(p, "transparentBackground", false));
Ok(json!({"png": base64_encode(&png), "width": w, "height": h}))
}
@@ -483,6 +489,10 @@ fn export(s: &mut Session, p: &Value) -> Result<Value> {
let format = str_param(p, "format").unwrap_or(format_for_path(&path)).to_string();
let st = s.doc()?;
let scale = finite_scale(f64_or(p, "scale", 2.0));
let transparent = bool_or(p, "transparentBackground", false);
if transparent && format != "png" {
return Err(bad("file.export", format!("a transparent background needs PNG; {format} can't keep transparency")));
}
match format.as_str() {
"png" | "jpeg" | "jpg" => {
let slides: Vec<usize> = match usize_param(p, "slide") {
@@ -497,7 +507,11 @@ fn export(s: &mut Session, p: &Value) -> Result<Value> {
let scale = image_scale(&st.doc, scale);
let mut written = vec![];
for (k, i) in slides.iter().enumerate() {
let img = deckcraft_render::render_slide(&st.doc, *i, &deckcraft_render::RenderOpts { scale, ..Default::default() });
let img = deckcraft_render::render_slide(
&st.doc,
*i,
&deckcraft_render::RenderOpts { scale, transparent_background: transparent, ..Default::default() },
);
let bytes = picture_bytes(&img, &format, scale);
let out = if slides.len() == 1 {
path.clone()