[Interest] painter.fillRect() with gradient on PDF printer produces all white?

[Henry Skoglund]

On 2022-04-07 23:08, Israel Brewster wrote:
> On Apr 7, 2022, at 12:59 PM, Henry Skoglund <henry at tungware.se> wrote:
>>
>> On 2022-04-07 22:04, Israel Brewster wrote:
>>> On Apr 7, 2022, at 11:58 AM, Henry Skoglund <henry at tungware.se> wrote:
>>>> On 2022-04-07 20:28, Israel Brewster wrote:
>>>>> Using Qt 5.15 and PySide2, I am working on a project that requires 
>>>>> me to produce a PDF output. My process so far is to create a 
>>>>> QWidget containing the content I want, render it to a QPicture (so 
>>>>> it won’t be rasterized upon “printing”), then create a PDF 
>>>>> QPrinter and use the painter.drawPicture() function to “print” the 
>>>>> widget into a PDF.
>>>>>
>>>>> This works well, giving me a high-quality vectorized PDF output, 
>>>>> with one exception: any item that is filled with a gradient. When 
>>>>> printing to PDF, the gradient always comes out as solid white. 
>>>>> With some digging, I determined that this is even the case when 
>>>>> doing nothing but a simple painter.fillRect() with a gradient.
>>>>>
>>>>> Is there any “fix” for this? Thanks.
>>>>>
>>>>> If it helps, here is some simple code that reproduces the issue:
>>>>>
>>>>> from PySide2.QtWidgets import QApplication
>>>>> from PySide2.QtGui import QPainter, QPageSize, QLinearGradient
>>>>> from PySide2.QtPrintSupport import QPrinter
>>>>> from PySide2.QtCore import QSize, QRect, Qt
>>>>>
>>>>> app = QApplication()
>>>>>
>>>>> gradient_rect = QRect(0, 0, 500, 25)
>>>>> gradient = QLinearGradient(0, 0, 1, 0)
>>>>> gradient.setColorAt(0, Qt.blue)
>>>>> gradient.setColorAt(1, Qt.red)
>>>>>
>>>>> page_size = QPageSize(QSize(500, 25), matchPolicy = 
>>>>> QPageSize.ExactMatch)
>>>>> printer = QPrinter()
>>>>> printer.setOutputFormat(QPrinter.PdfFormat)
>>>>> printer.setPageSize(page_size)
>>>>> printer.setOutputFileName('/tmp/testPDFGradient.pdf')
>>>>>
>>>>> painter = QPainter(printer)
>>>>> painter.fillRect(gradient_rect, gradient)
>>>>> painter.end()
>>>>>
>>>> Hi, try
>>>> ...
>>>> gradient_rect = QRect(0, 0, 500, 25)
>>>> gradient = QLinearGradient(0, 0, 500, 25)
>>>> gradient.setColorAt(0, Qt.blue)
>>>> gradient.setColorAt(1, Qt.red)
>>>> ...
>>>> (i.e. QLinearGradient's ctor expects coords)
>>> No change. The 1 value was picked because that’s what displays 
>>> correctly in testing, but even with the suggested change I still get 
>>> a solid white box.
>>>
>> Hmm, I just tested on Ubuntu 20.04, created an empty vanilla widget 
>> program in Qt 5.15.2. added "printsupport" to the .pro file and 
>> changed mainwindow.cpp to look like this:
>>
>> #include "mainwindow.h"
>> #include "ui_mainwindow.h"
>> #include "QtPrintSupport"
>>
>> MainWindow::MainWindow(QWidget *parent)
>>     : QMainWindow(parent)
>>     , ui(new Ui::MainWindow)
>> {
>>     ui->setupUi(this);
>>
>>     auto gradient_rect = QRect(0,0,500,25);
>>     auto gradient = QLinearGradient(0,0,500,25);
>>     gradient.setColorAt(0,Qt::blue);
>>     gradient.setColorAt(1,Qt::red);
>>
>>     QPrinter printer;
>>     QPageSize ps(QSize(500,25),"GradientTest",QPageSize::ExactMatch);
>> printer.setOutputFormat(QPrinter::PdfFormat);
>>     printer.setPageSize(ps);
>> printer.setOutputFileName("/home/henry/Downloads/testPDFGradient.pdf");
>>
>>     QPainter painter;
>>     painter.begin(&printer);
>> painter.fillRect(gradient_rect,gradient);
>>     painter.end();
>> }
>>
>> MainWindow::~MainWindow()
>> {
>>     delete ui;
>> }
>>
>> screenshot here:https://tungware.se/GradientOnUbuntu.png
>> (also tested on Windows 10 MSVC2019 Qt 5.15.2 and works there as well)
>>
>> Maybe pySide2 messes up something, you could try with some C++ :-)
>
> Interesting…must be a MacOS thing then. I just tried your C++ code 
> with the exact same results - plain white rectangle. So the only 
> difference I can see now is that you tested on Windows and Linux, 
> while I am testing on MacOS.
>
> Wonderfull. Now I apparently need different code depending on which OS 
> I am running on (assuming I can get this to work on MacOS at all)...
> ---
>
I fired up my Mac, tested my code above on Qt 6.2.4 and verified I got 
the same "A whiter shade of pale" as you.

Then I tested by copying a working .pdf from Ubuntu into Monterey, it 
also displayed as totally white (when double-clicked or when viewed in 
Safari).
But when I copied the .PDF generated in Monterey into Ubuntu, guess 
what: it displayed correctly.

But it gets better! Guess what happens if you choose to view the PDF on 
Monterey with either Chrome or Firefox (instead of Safari), then the 
blue-red gradient lights up, yes even on the Mac :-)

Interesting, right?
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