在C++中,我想用Gdiplus创建一个简单的透明图像并将其保存为png。我有以下代码:
// These variables are class members and got initialized and are used elsewhere
BITMAPINFO bmi;
HDC hdc;
void* pvBits;
ZeroMemory(&bmi, sizeof(BITMAPINFO));
bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmi.bmiHeader.biWidth = some_width;
bmi.bmiHeader.biHeight = some_height;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 32;
bmi.bmiHeader.biCompression = BI_RGB;
bmi.bmiHeader.biSizeImage = ((((bmi.bmiHeader.biWidth * bmi.bmiHeader.biBitCount) + 31) & ~31) >> 3) * bmi.bmiHeader.biHeight;
HBITMAP hBM = CreateDIBSection(hDC, &bmi, DIB_RGB_COLORS, &pvBits, NULL, 0x0);
FillMemory(pvBits, bmi.bmiHeader.biSizeImage, 255);
HGDIOBJ oldObj = SelectObject(hDC, hBM);
ReleaseDC(NULL, hDC);
GdiFlush();
GdiPlusBitmap* bitmap = new Gdiplus::Bitmap(&bmi, pvBits);
当我将图像保存为 png 时,我可以看到有一个 alpha channel ,但它设置为 0,因此没有任何透明内容(RGB 均已设置)。我也尝试将 255 更改为 0,但这只会给我一个黑色图像,没有蒸腾效果。为什么 Fillmemory
调用没有填充 Alpha channel ,还是我遗漏了其他内容?
最佳答案
FillMemory(pvBits, bmi.bmiHeader.biSizeImage, 255)
将以纯白色填充内存。如果使用 GDI 函数覆盖图像,Alpha 值将保持不变。即使文件支持,您也不会看到任何透明度。
要创建 32 位图像,您只需要 Gdiplus::Bitmap(w, h, PixelFormat32bppARGB)
。不需要 BITMAPINFO
和 CreateDIBSection
如果将 GDI+ 与 GDI 函数混合使用,则可能需要在使用 GDI 函数写入后重置 alpha。例如:
void test()
{
int w = 100;
int h = 100;
int bitcount = 32;
int size = ((((w * bitcount) + 31) & ~31) >> 3) * h;
BITMAPINFO bmi = { 0 };
bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmi.bmiHeader.biWidth = w;
bmi.bmiHeader.biHeight = -h;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = bitcount;
bmi.bmiHeader.biCompression = BI_RGB;
bmi.bmiHeader.biSizeImage = size;
HDC hdc = GetDC(0);
BYTE* pvBits = NULL;
HBITMAP hbitmap = CreateDIBSection(hdc, &bmi, DIB_RGB_COLORS,
(void**)&pvBits, NULL, 0x0);
FillMemory(pvBits, size, 0);
auto memdc = CreateCompatibleDC(hdc);
auto oldbmp = SelectObject(memdc, hbitmap);
SetBkColor(memdc, RGB(255, 0, 0));
TextOut(memdc, 0, 0, L"123", 3);
//GDI cleanup, don't delete hbitmap yet
SelectObject(memdc, oldbmp);
DeleteDC(memdc);
ReleaseDC(0, hdc);
//make the non-zero colors transparent:
for(int i = 0; i < size; i += 4)
{
int n = *(int*)(pvBits + i);
if (n != 0)
pvBits[i + 3] = 255;
}
CLSID clsid_png;
CLSIDFromString(L"{557cf406-1a04-11d3-9a73-0000f81ef32e}", &clsid_png);
Gdiplus::Bitmap* bitmap = new Gdiplus::Bitmap(w, h, PixelFormat32bppARGB);
Gdiplus::BitmapData data;
bitmap->LockBits(&Gdiplus::Rect(0, 0, w, h),
Gdiplus::ImageLockModeWrite, PixelFormat32bppARGB, &data);
memcpy(data.Scan0, pvBits, size);
bitmap->UnlockBits(&data);
//safe to delete hbitmap
DeleteObject(hbitmap);
bitmap->Save(L"test.png", &clsid_png);
delete bitmap;
}
关于c++ - 在 C++ 中使用 Gdiplus 创建透明位图,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/51555487/