Rainmeter plugin for making taskbar translucent. http://arkenthera.deviantart.com/art/TranslucentTaskbar-1-0-656402039

PluginTranslucentTaskbar.cpp

#include <Windows.h>
#include "../../API/RainmeterAPI.h"

struct ACCENTPOLICY {
	int nAccentState;
	int nFlags;
	int nColor;
	int nAnimationId;
};
struct WINCOMPATTRDATA {
	int nAttribute;
	PVOID pData;
	ULONG ulDataSize;
};

enum AccentTypes {
	ACCENT_DISABLE = 0,
	ACCENT_ENABLE_GRADIENT = 1,
	ACCENT_ENABLE_TRANSPARENTGRADIENT = 2,
	ACCENT_ENABLE_BLURBEHIND = 3
};

bool IsWindows10() {
	OSVERSIONINFOA info;
	ZeroMemory(&info, sizeof(OSVERSIONINFOA));
	info.dwOSVersionInfoSize = sizeof(OSVERSIONINFOA);

	GetVersionExA(&info);

	return info.dwMajorVersion == 10;
}

void SetBlurBehind(HWND hwnd, AccentTypes desiredType) {
	if (IsWindows10()) {
		const HINSTANCE hModule = LoadLibrary(TEXT("user32.dll"));
		if (hModule) {
			typedef BOOL(WINAPI*pSetWindowCompositionAttribute)(HWND,
				WINCOMPATTRDATA*);
			const pSetWindowCompositionAttribute
				SetWindowCompositionAttribute =
				(pSetWindowCompositionAttribute)GetProcAddress(
					hModule,
					"SetWindowCompositionAttribute");

			// Only works on Win10
			if (SetWindowCompositionAttribute) {
				ACCENTPOLICY policy =
				{ desiredType
				,0,desiredType == (ACCENT_ENABLE_TRANSPARENTGRADIENT) ? 255 : 0,0 };
				WINCOMPATTRDATA data = { 19,&policy,sizeof(ACCENTPOLICY) };
				SetWindowCompositionAttribute(hwnd, &data);
			}
			FreeLibrary(hModule);
		}
	}
	else {
		//Nothing
	}
}

HWND TaskbarHandle = FindWindow(L"Shell_TrayWnd", NULL);
HWND SecondaryTaskbar = FindWindow(L"Shell_SecondaryTrayWnd", NULL);


struct Measure
{
	AccentTypes State;

	Measure() : State(ACCENT_ENABLE_BLURBEHIND) {}
};

PLUGIN_EXPORT void Initialize(void** data, void* rm)
{
	Measure* measure = new Measure;
	*data = measure;
}

PLUGIN_EXPORT void Reload(void* data, void* rm, double* maxValue)
{
	Measure* measure = (Measure*)data;

	LPCWSTR value = RmReadString(rm, L"AccentState", L"");
	if (_wcsicmp(value, L"0") == 0)
	{
		measure->State = ACCENT_DISABLE;
	}
	else if (_wcsicmp(value, L"1") == 0)
	{
		measure->State = ACCENT_ENABLE_GRADIENT;
	}
	else if (_wcsicmp(value, L"2") == 0)
	{
		measure->State = ACCENT_ENABLE_TRANSPARENTGRADIENT;
	}
	else if (_wcsicmp(value, L"3") == 0)
	{
		measure->State = ACCENT_ENABLE_BLURBEHIND;
	}
	else
	{
		measure->State = ACCENT_ENABLE_BLURBEHIND;
	}
}

PLUGIN_EXPORT double Update(void* data)
{
	Measure* measure = (Measure*)data;

	SetBlurBehind(TaskbarHandle, measure->State);
	SetBlurBehind(SecondaryTaskbar, measure->State);

	SecondaryTaskbar = FindWindow(L"Shell_SecondaryTrayWnd", NULL);

	// Just in case 
	int GuardCounter = 0;
	while (SecondaryTaskbar = FindWindowEx(0, SecondaryTaskbar, L"Shell_SecondaryTrayWnd", L""))
	{
		if (GuardCounter > 8)
			break;
		SetBlurBehind(SecondaryTaskbar, measure->State);
		GuardCounter++;
	}
	return 0.0;
}

PLUGIN_EXPORT void Finalize(void* data)
{
	Measure* measure = (Measure*)data;

	SetBlurBehind(TaskbarHandle, ACCENT_DISABLE);
	SetBlurBehind(SecondaryTaskbar, ACCENT_DISABLE);

	SecondaryTaskbar = FindWindow(L"Shell_SecondaryTrayWnd", NULL);

	int GuardCounter = 0;
	while (SecondaryTaskbar = FindWindowEx(0, SecondaryTaskbar, L"Shell_SecondaryTrayWnd", L""))
	{
		if (GuardCounter > 8)
			break;
		SetBlurBehind(SecondaryTaskbar, ACCENT_DISABLE);
		GuardCounter++;
	}
	delete measure;
}