C++ – How to draw transparent BMP with GDI+

bmpcgdi+transparency

I'm currently editing some old GDI code to use GDI+ and ran into a problem when it comes to draw a BMP file with transparent background. The old GDI code did not use any obvious extra code to draw the background transparent so I'm wondering how to achieve this using GDI+.

My current code looks like this

HINSTANCE hinstance = GetModuleHandle(NULL);
bmp = Gdiplus::Bitmap::FromResource(hinstance, MAKEINTRESOURCEW(IDB_BMP));
Gdiplus::Graphics graphics(pDC->m_hDC);
graphics.DrawImage(&bmp, posX, posY);

I also tried to create a new bitmap from the resource by using the clone method and by drawing the bitmap to a newly created one but neither did help. Both times I used PixelFormat32bppPARGB.

Then I tried to use alpha blending but this way the whole image gets transparent and not only the background:

Gdiplus::ColorMatrix clrMatrix = { 
    1.0f, 0.0f, 0.0f, 0.0f, 0.0f,
    0.0f, 1.0f, 0.0f, 0.0f, 0.0f,
    0.0f, 0.0f, 1.0f, 0.0f, 0.0f,
    0.0f, 0.0f, 0.0f, 0.5f, 0.0f,
    0.0f, 0.0f, 0.0f, 0.0f, 1.0f
};

Gdiplus::ImageAttributes imgAttr;
imgAttr.SetColorMatrix(&clrMatrix);

graphics.DrawImage(&bmp, destRect, 0, 0, width(), height(), Gdiplus::UnitPixel, &imgAttr);

The transparency information is already contained in the image but I don't have a clue how to apply it when drawing the image. How does one achieve this?

Best Answer

A late answer but:

ImageAttributes imAtt;    
imAtt.SetColorKey(Color(255,255,255), Color(255,255,255), ColorAdjustTypeBitmap);

Will make white (255,255,255) transparent on any bitmap you use this image attribute with.

Setting a bit

Use the bitwise OR operator (|) to set a bit.

number |= 1UL << n;

That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit.

Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples.

Clearing a bit

Use the bitwise AND operator (&) to clear a bit.

number &= ~(1UL << n);

That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

The XOR operator (^) can be used to toggle a bit.

number ^= 1UL << n;

That will toggle the nth bit of number.

Checking a bit

You didn't ask for this, but I might as well add it.

To check a bit, shift the number n to the right, then bitwise AND it:

bit = (number >> n) & 1U;

That will put the value of the nth bit of number into the variable bit.

Changing the nth bit to x

Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation:

number ^= (-x ^ number) & (1UL << n);

Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1.

To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.

number ^= (-(unsigned long)x ^ number) & (1UL << n);

unsigned long newbit = !!x;    // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);

It's generally a good idea to use unsigned types for portable bit manipulation.

number = (number & ~(1UL << n)) | (x << n);

(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x.

It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.

C++ – How to iterate over the words of a string

I use this to split string by a delimiter. The first puts the results in a pre-constructed vector, the second returns a new vector.

#include <string>
#include <sstream>
#include <vector>
#include <iterator>

template <typename Out>
void split(const std::string &s, char delim, Out result) {
    std::istringstream iss(s);
    std::string item;
    while (std::getline(iss, item, delim)) {
        *result++ = item;
    }
}

std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    split(s, delim, std::back_inserter(elems));
    return elems;
}

Note that this solution does not skip empty tokens, so the following will find 4 items, one of which is empty:

std::vector<std::string> x = split("one:two::three", ':');