How to convert from Cairo to Imager? - image

Question

For historical reasons we are forced to use Cairo and Imager together.
Converting from Cairo to Imager and converting it back to Imager for some reason makes the color strange.
The reason for converting to Imager again is to combine with another Imager object after this.
# Create a yellow fill image as Cairo object.
# And output as a png file.
my $testSurface = Cairo::ImageSurface->create(
'argb32',
$width,
$height
);
my $testContext = Cairo::Context->create($testSurface);
$testContext->rectangle(0, 0, $width, $height);
$testContext->set_source_rgba(1.0, 1.0, .0, 1);
$testContext->fill();
# This is a yellow png file.
$testSurface->write_to_png("output/fill_yellow.png");
# Convert the Cairo object to an Imager object.
my $testData = $testSurface->get_data;
my $testImager = Imager->new(
xsize => $width,
ysize => $height,
channels => 4,
);
my $testRes = $testImager->read(
data => $testData,
type => "raw",
xsize => $width,
ysize => $height,
raw_datachannels => 4,
raw_storechannels => 4,
raw_interleave => 0,
);
# Output Imager object as a PNG file.
# ! This PNG file becomes blue unexpectedly
$testRes->write(
file => "output/fill_yellow_imager.png",
type => "png"
);

Answer 1

Cairo is using ARGB for its raw format, while Imager is using RGBA. The difference between them is the order that the samples are stored within each pixel. Cairo also uses premultiplied alpha, while Imager uses non-premultiplied alpha. Neither library seems to have any option to change either of these things.
The sample ordering thing could be fixed fairly easily by re-ordering the bytes within the raw image data, but the premultiplication thing starts to get into the territory of not worth bothering with. Therefore I recommend that you simply save a PNG file in Cairo and load it in Imager. It may be slightly slower, but it's easy to understand and recognize that it's correct.
Below: code from a previous version of this answer that recommended swapping the byte order, before I realized about the premultiplied alpha issue:
for (my $i = 0 ; $i < length($testData) ; $i += 4) {
substr($testData, $i, 4,
substr($testData, $i+1, 3) . substr($testData, $i, 1)
);
}
I don't recommend using it.