[dvipdfmx] Color stacks and transformations

[Joseph Wright]

On 16/03/2021 18:05, Joseph Wright wrote:
> On 16/03/2021 17:26, Shunsaku Hirata wrote:
>> Hi,
>>
>> Thank you for the report.
>>
>> The problem occurs because dvipdfmx tries to re-install the color
>> recorded in the pdf:color color stack (default color in the example)
>> just after graphics state restore operation.
>> This behavior is to ensure the color change induced by pdf:color
>> within a graphics state save-restore block to be propagated outside
>> of the block.
>>
>> I will consider a solution to this issue. Probably it takes some time.
>>
>> Thanks,
>> Shunsaku Hirata
> 
> Hello Shunsaku,
> 
> Thanks for the feedback.
> 
> ----
> 
> On the 'restore the color' business, I am slightly confused by the 
> intent. I can see it does what you describe, in for example
> 
>      \leavevmode
>      \special{x:gsave}%
>      \special{pdf:bc [1 0 0]}%
>      Hello
>      \special{x:grestore}%
>      world
>      \special{pdf:ec}%
>      \bye
> 
> but that behaviour contrasts with pdfTeX
> 
>      \leavevmode
>      \pdfsave
>      \pdfcolorstack 0 push{1 0 0 rg}%
>      Hello
>      \pdfrestore
>      world
>      \pdfcolorstack 0 pop
>      \bye
> 
> which admittedly is a lot more aggressive all-round here. However, both 
> of these demos look unbalanced to me: I'd expect in a well-behaved real 
> example that the color stack would be popped *inside* the graphics state 
> save/restore pair.
> 
> ----
> 
> I can think of at least a couple of approaches that would work for me if 
> available from the driver.
> 
> - You could disable re-insertion if pdfcolorstackinit is encountered. 
> The moment someone is setting \special{pdfcolorstackinit 1 ...} they are
> likely to be using that approach exclusively.
> 
> - You could have a keyword-based setting to do the same, for example
> 
>      \special{pdfcolorstackpriority on}
> 
>   (would allow \special{pdfcolorstackpriority off} if people really need
>    the old behavior back)
> 
> ---
> 
> At the macro level I might try setting the 'classical' stack to hold the 
> same color as first 'new' one, as they should be equivalent anyway. The 
> only slight wrinkle is that as the two take different color spec 
> formats, there will be places that this is more challenging. That is 
> likely the best way to make things work right now.

Actually, there is a much easier macro-level solution. With the 
knowledge that color is applied immediately after x:grestore, one can 
simply add a line that then reapplies the stack color

     \leavevmode
     \special{pdfcolorstackinit 1 page direct (0 g 0 g)}
     \special{x:gsave}%
     \special{pdfcolorstack 1 push (1 0 0 rg)}%
     Hello
     \special{x:grestore}%
     \special{pdfcolorstack 1 current}%
     world
     \special{pdfcolorstack 1 pop}%
     end
     \bye

In a richer format, that can be wrapped up such that this is transparent 
to the end user.

Probably I will use this solution for the present and we can revisit if 
there is a way to do the same in the driver.

Joseph