greenbeast greenbeast - 2 months ago 32
C++ Question

Creating GDI+ bitmaps in memory and then saving as png

I am very new to C++ and been having trouble with writing a function that uses the GDI+ library to create a new bitmap in memory ( so not opening/reading an existing bitmap); then drawing on the bitmap; before saving the bitmap as a png. In particular, I am having problems with the bitmap creation and saving code. I am constrained to using codeblocks and so can't use visual studios, even if I wanted to. The code is as follows:

#include "drawImage.h"
#include <windows.h>
#include <objidl.h>
#include <gdiplus.h>
#include <stdio.h>
#include <iostream>
using namespace std;
using namespace Gdiplus;

drawImage::drawImage(){}

void drawImage::DrawBitmap(int width, int height){
GdiplusStartupInput gdiplusStartupInput;
ULONG_PTR gdiplusToken;
GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);

{
//Create a bitmap
Bitmap myBitmap(width, height, PixelFormatCanonical);
Graphics g(&myBitmap);
Pen blackpen(Color(255,0,0,0), 3);

//draw on bitmap
int x1 = 1;
int x2 = 200;
int y1 = 1;
int y2 = 200;
g.DrawLine(&blackpen, x1,y1,x2,y2);

// Save bitmap (as a png)
CLSID pngClsid;
GetEncoderClsid(L"image/png", &pngClsid);
myBitmap.Save(L"C:\\test\\test.png", &pngClsid, NULL);
}

GdiplusShutdown(gdiplusToken);
}


The issues I have been having with this code are as follows:


  1. The 'saving' code does not compile with an error message of "'GetEncoderClsid' was not declared in this scope". However, I got this direct from the Microsoft website here and so don't know what I am doing wrong, although I don't think this is the proper way of converting bitmap to png but I dont know any alternative way?

  2. When the code is compiled and run (by commenting out the saving code), it then crashes on the line "Bitmap *myBitmap = new Bitmap(width, height, PixelFormatCanonical);" with an error message saying my executable has stopped working.



I have added the 'gdi32' linker library and also '-lgdiplus' as a linker option. Also, I have used this website to help with the gdi stuff although the section on bitmaps only deals with loading existing bitmaps (not creating new ones in memory)

I am totally lost on what to do, so any help or advice on this matter is much appreciated.

Answer

The core issue is passing the wrong pixel format to the Bitmap constructor. PixelFormatCanonical is not one of the supported pixel formats. It's a bit mask used to determine whether a pixel format is canonical (see IsCanonicalPixelFormat). You'll have to use a real pixel format, like the default PixelFormat32bppARGB.

The following code produces the desired output:

First up, a small helper class for GDI+ initialization. This ensures, that the d'tor (i.e. the call to GdiplusShutdown) is executed after all other objects have been destroyed. With respect to order of destruction, it serves the same purpose as the additional scope in the OP. In addition, it also allows for exceptions to be thrown.

#include <windows.h>
#include <gdiplus.h>
using namespace Gdiplus;
#include <stdexcept>
using std::runtime_error;

struct GdiplusInit {
    GdiplusInit() {
        GdiplusStartupInput inp;
        GdiplusStartupOutput outp;
        if ( Ok != GdiplusStartup( &token_, &inp, &outp ) )
            throw runtime_error( "GdiplusStartup" );
    }
    ~GdiplusInit() {
        GdiplusShutdown( token_ );
    }
private:
    ULONG_PTR token_;
};

This code was taken from the MSDN sample Retrieving the Class Identifier for an Encoder.

int GetEncoderClsid( const WCHAR* format, CLSID* pClsid )
{
    UINT  num = 0;          // number of image encoders
    UINT  size = 0;         // size of the image encoder array in bytes

    ImageCodecInfo* pImageCodecInfo = NULL;

    GetImageEncodersSize( &num, &size );
    if ( size == 0 )
        return -1;  // Failure

    pImageCodecInfo = (ImageCodecInfo*)( malloc( size ) );
    if ( pImageCodecInfo == NULL )
        return -1;  // Failure

    GetImageEncoders( num, size, pImageCodecInfo );

    for ( UINT j = 0; j < num; ++j )
    {
        if ( wcscmp( pImageCodecInfo[j].MimeType, format ) == 0 )
        {
            *pClsid = pImageCodecInfo[j].Clsid;
            free( pImageCodecInfo );
            return j;  // Success
        }
    }

    free( pImageCodecInfo );
    return -1;  // Failure
}

Finally, the GDI+ rendering code. It uses objects with automatic storage duration throughout, making it more compact and safer.

void drawImage( int width, int height ) {
    GdiplusInit gdiplusinit;

    //Create a bitmap
    Bitmap myBitmap( width, height, PixelFormat32bppARGB );
    Graphics g( &myBitmap );
    Pen blackpen( Color( 255, 0, 0, 0 ), 3 );

    //draw on bitmap
    g.DrawLine( &blackpen, 1, 1, 200, 200 );

    // Save bitmap (as a png)
    CLSID pngClsid;
    int result = GetEncoderClsid( L"image/png", &pngClsid );
    if ( result == -1 )
        throw runtime_error( "GetEncoderClsid" );
    if ( Ok != myBitmap.Save( L"C:\\test\\test.png", &pngClsid, NULL ) )
        throw runtime_error( "Bitmap::Save" );
}

int main()
{
    drawImage( 200, 200 );
    return 0;
}