Black package background when transparent used for package and default background (ubuntu)

I'm using Inkscape 1.0.x on Windows and Linux so it does appear to be related to that tool. 

Like you say, it previews well in my VSCode PlantUML plugin both on Windows and Linux (I'm not sure how they show previews under the hood), and Chrome/Firefox with this link.  

If I convert the SVG to PNG with the linux convert tool, the backgrounds are OK (but convert doesn't do a nice job with fonts, etc.). 

I found out a fix to the SVG (at least for inkscape), on the style of the package polygon, if I add "fill:none" to the end. That is,

$ diff sample.svg sample_fill_none.svg
2c2
< cluster a--><polygon fill="#00000000" points="16,6,31,6,38,28.2969,62,28.2969,62,97,16,97,16,6" style="stroke: #000000; stroke-width: 1.5;"/><line style="stroke: #000000; stroke-width: 1.5;" x1="16" x2="38" y1="28.2969" y2="28.2969"/><text fill="#000000" font-family="sans-serif" font-size="14" font-weight="bold" lengthAdjust="spacingAndGlyphs" textLength="9" x="20" y="20.9951">a</text><!--MD5=[69c451c751c414e37549b782190fef46]
---
> cluster a--><polygon fill="#00000000" points="16,6,31,6,38,28.2969,62,28.2969,62,97,16,97,16,6" style="stroke: #000000; stroke-width: 1.5;fill:none"/><line style="stroke: #000000; stroke-width: 1.5;" x1="16" x2="38" y1="28.2969" y2="28.2969"/><text fill="#000000" font-family="sans-serif" font-size="14" font-weight="bold" lengthAdjust="spacingAndGlyphs" textLength="9" x="20" y="20.9951">a</text><!--MD5=[69c451c751c414e37549b782190fef46]
20c20

Otherwise, I will need to pursue a bug report with Inkscape. I'm not sure when this started, as the file where I saw the problem I have not touched in more than a year.

You can try last beta http://beta.plantuml.net/plantuml.jar

We now add "fill:none;" for polygon when fill is "#00000000."

It sounds like a hack, but if it helps Inkscape and does not bother other, we are ok with that.

Tell us if it's working for you and if you find regression !

Thanks

I went a step further to diff the two SVG files produced (one with skinparam backgroundcolor transparent and the other without it), and here is what it looks like:

$ wdiff sample.svg sample-no-background.svg | colordiff
<?xml version="1.0" encoding="UTF-8" standalone="no"?><svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" contentScriptType="application/ecmascript" contentStyleType="text/css" height="104px" preserveAspectRatio="none" [-style="width:79px;height:104px;background:#00000000;"-] {+style="width:79px;height:104px;"+} version="1.1" viewBox="0 0 79 104" width="79px" zoomAndPan="magnify"><defs/><g><!--MD5=[03c9795141fffc1586b10a901fd13311]
cluster a--><polygon [-fill="#00000000"-] {+fill="#FFFFFF"+} points="16,6,31,6,38,28.2969,62,28.2969,62,97,16,97,16,6" style="stroke: #000000; stroke-width: 1.5;"/><line style="stroke: #000000; stroke-width: 1.5;" x1="16" x2="38" y1="28.2969" y2="28.2969"/><text fill="#000000" font-family="sans-serif" font-size="14" font-weight="bold" lengthAdjust="spacingAndGlyphs" textLength="9" x="20" y="20.9951">a</text><!--MD5=[69c451c751c414e37549b782190fef46]
class B--><rect codeLine="5" fill="#FEFECE" height="39.9688" id="B" style="stroke: #A80036; stroke-width: 1.5;" width="14" x="32" y="41"/><text fill="#000000" font-family="sans-serif" font-size="12" lengthAdjust="spacingAndGlyphs" textLength="8" x="35" y="57.1387">B</text><line style="stroke: #A80036; stroke-width: 1.5;" x1="33" x2="45" y1="64.9688" y2="64.9688"/><line style="stroke: #A80036; stroke-width: 1.5;" x1="33" x2="45" y1="72.9688" [-y2="72.9688"/><!--MD5=[1a6d1374294d2653a55137f104111a40]-] {+y2="72.9688"/><!--MD5=[fe362aebce9f6af6177bd7bc421da198]+}
@startuml
[-skinparam-]
{+'skinparam+} backgroundcolor transparent
{+skinparam packagebackgroundcolor transparent
skinparam style strictuml
package a {
class B
}
@enduml

@startuml+}
skinparam packagebackgroundcolor transparent
skinparam style strictuml
package a {
class B
}
@enduml

PlantUML version 1.2020.20(Sat Nov 21 05:02:45 EST 2020)
(GPL source distribution)
Java Runtime: OpenJDK Runtime Environment
JVM: OpenJDK 64-Bit Server VM
Default Encoding: UTF-8
Language: en
Country: null
--></g></svg>

The diff gets the PlantUML source code wrong (at least it presents the diff badly, one line is commented out).

I'm surprised to see the fill="#FFFFFF" appearing in the package background on the version where skinparam backgroundcolor transparent is not specified. Shouldn't it still be "#000000" or is it deducing the background is white since nothing is specified?

I saw you posted another response - so I have not read it yet...

I found one regression - if you use:

skinparam packageStyle rect

Hello again - there are actually many more regressions. I haven't made an exhaustive list, but if you get a hold of Inkscape 1.0.x you can try the examples below.  

To find these bugs it could be useful to rebuild the PlantUML documentation using SVG/Inkscape in the toolchain rather than PNG? It's actually how I build my course notes. It's useful to keep the vector format, because PDF viewers can find the text in the PlantUML diagrams (not possible if they're PNG). 

As a potential work-around, I also tried using -tpdf with all its required libraries. But it's, too, crashing with exceptions like:

CSSEngine: exception property.syntax.error:org.w3c.css.sac.CSSParseException: Invalid RGB color: 00000000.

@startuml
skinparam BackgroundColor transparent
skinparam ClassBackgroundColor transparent
skinparam PackageBackgroundColor transparent
skinparam ComponentBackgroundColor transparent
skinparam NodeBackgroundColor transparent
skinparam PackageStyle rect

package "Some Group" {
HTTP - [First Component]
[Another Component]
}

node "Other Groups..." {
FTP - [Second Component]
[First Component] --> FTP
}
cloud {
[Example 1]
}
database "MySql" {
folder "This is my folder" {
[Folder 3]
}
frame "Foo" {
[Frame 4]
}
}
[Another Component] --> [Example 1]
[Example 1] --> [Folder 3]
[Folder 3] --> [Frame 4]
@enduml

@startuml
skinparam BackgroundColor transparent
skinparam ClassBackgroundColor transparent
skinparam PackageBackgroundColor transparent
skinparam PackageStyle rect

interface B

package a {
    class A
}

cloud c {
    class C
}

folder f {
    class F
}

folder f {
    class F
}

database {
    class D
}

frame "Frame" {
    class frame
}
@enduml

An apparent work-around is to use #transparent on each element, rather than the skinparam. I didn't try it in all the cases, but it works in a lot (except for when you have a namespace in a class, e.g., a.b.C, and PlantUML draws a package for it).

This is weird: there shouldn't be any difference between using #transparent or skinparam.

I'm confused here :-)

Anyway , we figure out that it was useless in the generated SVG to put #00000000 to fill shapes. So here is a new beta http://beta.plantuml.net/plantuml.jar that may work better.

Tell us if it changes something!

Thanks again for the quick reply - that new beta is working on all the examples I found. I hope this has a long-term benefit, too.