DPI metadata for png (#4812)

* remove special cairomakie png method

* convert px_per_unit to dpi and store in png

* bump pngfiles compat to version introducing dpi keyword

* add changelog

* fix test by always going to rgba

* retrieve px_per_unit in more robust way

CHANGELOG.md

@@ -24,6 +24,7 @@
 - Changed `inspectable` to be inherited from the parent scenes theme. [#4739](https://github.com/MakieOrg/Makie.jl/pull/4739)
 - Reverted change to `poly` which disallowed 3D geometries from being plotted [#4738](https://github.com/MakieOrg/Makie.jl/pull/4738)
 - Enabled autocompletion on Block types, e.g. `?Axis.xti...` [#4786](https://github.com/MakieOrg/Makie.jl/pull/4786)
+- Added `dpi` metadata to all rendered png files, where `px_per_unit = 1` means 96dpi, `px_per_unit = 2` means 192dpi, and so on. This gives frontends a chance to show plain Makie png images with the correct scaling [#4812](https://github.com/MakieOrg/Makie.jl/pull/4812).
 
 ## [0.22.1] - 2025-01-17
 

CairoMakie/src/display.jl

@@ -101,13 +101,6 @@ function Makie.backend_show(screen::Screen{EPS}, io::IO, ::MIME"application/post
     return screen
 end
 
-function Makie.backend_show(screen::Screen{IMAGE}, io::IO, ::MIME"image/png", scene::Scene)
-    Makie.push_screen!(scene, screen)
-    cairo_draw(screen, scene)
-    Cairo.write_to_png(screen.surface, io)
-    return screen
-end
-
 # Disabling mimes and showable
 
 const DISABLED_MIMES = Set{String}()

CairoMakie/src/screen.jl

@@ -305,6 +305,9 @@ function Makie.apply_screen_config!(screen::Screen, config::ScreenConfig, scene:
     Makie.apply_screen_config!(screen, config, scene, nothing, MIME"image/png"())
 end
 
+function Makie.px_per_unit(s::Screen)::Float64
+    return s.config.px_per_unit
+end
 
 function Screen(scene::Scene; screen_config...)
     config = Makie.merge_screen_config(ScreenConfig, Dict{Symbol, Any}(screen_config))

GLMakie/src/screen.jl

@@ -463,6 +463,12 @@ function Screen(;
     return screen
 end
 
+function Makie.px_per_unit(s::Screen)::Float64
+    config = s.config
+    config === nothing && return 1.0
+    return something(config.px_per_unit, 1.0)
+end
+
 function set_screen_visibility!(screen::Screen, visible::Bool)
     if !screen.owns_glscreen
         error(unimplemented_error)

Project.toml

@@ -102,7 +102,7 @@ Markdown = "1.0, 1.6"
 MathTeXEngine = "0.5, 0.6"
 Observables = "0.5.5"
 OffsetArrays = "1"
-PNGFiles = "0.4.3"
+PNGFiles = "0.4.4"
 Packing = "0.5"
 PlotUtils = "1.4.2"
 PolygonOps = "0.1.1"

ReferenceTests/src/runtests.jl

@@ -2,8 +2,7 @@ function get_frames(a, b)
     return (get_frames(a), get_frames(b))
 end
 
-rgbf_convert(x::AbstractMatrix{<:RGB}) = convert(Matrix{RGBf}, x)
-rgbf_convert(x::AbstractMatrix{<:RGBA}) = convert(Matrix{RGBAf}, x)
+rgbaf_convert(x::AbstractMatrix{<:Union{RGB,RGBA}}) = convert(Matrix{RGBAf}, x)
 
 function get_frames(video::AbstractString)
     mktempdir() do folder
@@ -25,8 +24,8 @@ end
 
 function compare_images(a::AbstractMatrix{<:Union{RGB,RGBA}}, b::AbstractMatrix{<:Union{RGB,RGBA}})
 
-    a = rgbf_convert(a)
-    b = rgbf_convert(b)
+    a = rgbaf_convert(a)
+    b = rgbaf_convert(b)
 
     if size(a) != size(b)
         @warn "images don't have the same size, difference will be Inf"

WGLMakie/src/display.jl

@@ -66,6 +66,10 @@ mutable struct Screen <: Makie.MakieScreen
     end
 end
 
+function Makie.px_per_unit(s::Screen)::Float64
+    return something(s.config.px_per_unit, 1.0)
+end
+
 function Screen(; config...)
     config = Makie.merge_screen_config(ScreenConfig, Dict{Symbol,Any}(config))
     return Screen(nothing, config)

src/display.jl

@@ -480,10 +480,14 @@ function colorbuffer(fig::FigureLike, format::ImageStorageFormat = JuliaNative;
     end
 end
 
+px_per_unit(screen::MakieScreen)::Float64 = 1.0 # fallback for backends who don't have upscaling
+
 # Fallback for any backend that will just use colorbuffer to write out an image
 function backend_show(screen::MakieScreen, io::IO, ::MIME"image/png", scene::Scene)
     img = colorbuffer(screen)
-    FileIO.save(FileIO.Stream{FileIO.format"PNG"}(Makie.raw_io(io)), img)
+    px_per_unit = Makie.px_per_unit(screen)::Float64
+    dpi = px_per_unit * 96 # attach dpi metadata corresponding to 1 unit == 1 CSS pixel
+    FileIO.save(FileIO.Stream{FileIO.format"PNG"}(Makie.raw_io(io)), img; dpi)
     return
 end