Help obtaining accurate data values from ImageSurface.data with alpha values

[cormullion]

Hoping you can help me work out how to use Cairo.jl to get better code for this. I'm trying to extract the data from a copied image surface. The values match if alpha is 1, but not if alpha is less than 1.0.

Here's the MWE:

using Cairo

alpha = 0.5
rgba(k) = reinterpret(UInt8,[k]) ./ 0xFF

############### source image

surface = CairoARGBSurface(256, 256)
cr = CairoContext(surface)

save(cr)
set_source_rgba(cr, 0.8, 0.8, 0.8, alpha)    
rectangle(cr, 0.0, 0.0, 256.0, 256.0) 
fill(cr)
restore(cr)

save(cr)
set_source_rgba(cr, 1.0, 0.5, 0.0, alpha)    
rectangle(cr, 64.0, 64.0, 64.0, 64.0) 
fill(cr)
restore(cr)

save(cr)
set_source_rgba(cr, 0.0, 1.0, 1.0, alpha)    
rectangle(cr, 100.0, 100.0, 64.0, 64.0) 
fill(cr)
restore(cr)

Cairo.write_to_png(surface,"/tmp/image1.png")

############## make a copy

z = zeros(UInt32, 256, 256)
imagesurface = CairoImageSurface(z, Cairo.FORMAT_ARGB32)
crdest = CairoContext(imagesurface)

set_source_surface(crdest, surface, 0, 0)
set_operator(crdest, Cairo.OPERATOR_SOURCE)
paint(crdest)

data = imagesurface.data              # this works

println(rgba(data[1]))                # but values are different

Cairo.write_to_png(imagesurface,"/tmp/image2.png")

If alpha is 1.0, the value printed is [0.8, 0.8, 0.8, 1.0], matching the first pixel's value, but if alpha is 0.5, the printed value is [0.4, 0.4, 0.4, 0.5019607843137255].

What I want to do is copy an RGB image to a matrix with accurate values. I think I'm missing some information about copying alpha color values...

[lobingera]

Recomended read https://cairographics.org/operators/