The GdiPlus API show different behaves in QT

jusinyah

Here is a demo which shows different behaves using GDIPlus API in QT.Pay attention to the Note section

#include <QtWidgets/qapplication.h>
#include <iostream>
#include <comdef.h>
#include <gdiplus.h>
using namespace Gdiplus;
#pragma comment(lib, "gdiplus.lib")
using namespace std;
int main(int args, char** argv)
{
    QApplication app(args, argv);	
    ULONG_PTR gdiplusToken;
    GdiplusStartupInput gdiplusStartupInput;
    GdiplusStartup(&gdiplusToken,&gdiplusStartupInput,NULL);
    Bitmap mBitMap(506, 283);
    Graphics* mGr = Graphics::FromImage(&mBitMap);
    mGr->SetPageUnit(Gdiplus::Unit::UnitPixel);
    mGr->SetPageScale(1.0f);
    Gdiplus::Font font((L"Aharoni"), 14.0f,  (Gdiplus::Unit)1);
    StringFormat* stringFormat = new StringFormat();
    stringFormat->SetAlignment(StringAlignmentCenter);
    stringFormat->SetLineAlignment(StringAlignmentCenter);
    RectF rect(0, 0, 409.05f, 141.0f);
    RectF boundingBox;
    INT codepointsFitted = 0;
    INT linesFilled = 0;
	  /*
      **Note ：When you remove "QApplication app(args, argv);", boundingBox will show different values,please debug and check.**
	  */
    Gdiplus::Status status = mGr->MeasureString((L"Chart Title"), 7 , &font, rect, 
    stringFormat,&boundingBox,&codepointsFitted,&linesFilled);
    delete stringFormat;
    GdiplusShutdown(gdiplusToken);
    return 0;
}

Not only this GDI+ API but others also show different value,Who can tell me why and how to fix it to get coincident value by Gdiplus API ? Thanks!

SGaist

Hi,

Which version of Qt ?
Which version of Windows ?
What are these differences ?
What happens if you create your QApplication object after initializing GdiPlus ?

jusinyah

Hi,
A1: Qt version is qt-opensource-windows-x86-5.12.2
A2: My windows is win10
A3: If I don't remove the code “QApplication app(args, argv);” the value of the variable
boundingBox as follows (Value A):

boundingBox class Gdiplus::RectF
Height 25.96516609
Width 86.74423218
X 161.1528778
Y 57.51741409

if I remove the code “QApplication app(args, argv);” the value of the variable
boundingBox as follows (Value B):

boundingBox class Gdiplus::RectF
Height 20.77213097
Width 69.39538574
X 169.827301
Y 60.11393356

A4: If I create my QApplication object after initializing GdiPlus like this:

...
GdiplusStartup(&gdiplusToken,&gdiplusStartupInput,NULL);
QApplication app(args, argv);
...

the value of boundingBox is the same to Value B
Thanks!

SGaist

So it seems there might be some initialisation done by Qt that interacts with GdiPlus.

What is your use case by the way ?

jusinyah

First of all, thanks for your reply!
Actually we integrated a third part library in QT(5.12) which drawing by Gdiplus but we found the drawing result is different compare to running the third part lib in Visual Studio.
So I give you a simple demo above which has nothing to do with the third part library to explain the different behavior using GDIPlus in QT.
I created the demo in QTCreate 4.8.2, by Application-> QT Console Application by a CMakeLists.txt.

cmake_minimum_required(VERSION 3.1.0)

project(qtdemo VERSION 1.0.0 LANGUAGES CXX)

get_filename_component(PARENT_DIR ./ ABSOLUTE)
message(PARENT_DIR:${PARENT_DIR})

set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

set(CMAKE_AUTOMOC ON)

if(CMAKE_VERSION VERSION_LESS "3.7.0")
    set(CMAKE_INCLUDE_CURRENT_DIR ON)
endif()

find_package(Qt5 COMPONENTS Widgets REQUIRED)

add_executable(qtdemo main.cpp)

target_link_libraries(qtdemo Qt5::Widgets )

SGaist

Maybe this stack overflow thread might help.