GDI GradientFill not working on offscreen bitmap

Question

Hi,

I am trying to use the GDI GradientFill function to draw on a offscreen bitmap, then BitBlt that to the screen.

But I always get a black bitmap... if I GradientFill directly to the screen it works.

Below is a sample app to see what I mean.

#pragma comment(lib, "msimg32.lib")
#include <windows.h>

const CHAR c_szWndClass[] = "GradientTestWnd";
const CHAR c_szWndTitle[] = "GradientTest";
const int c_nWndWidth = 1024;
const int c_nWndHeight = 768;

int WINAPI WinMain(      
                   HINSTANCE hInstance,
                   HINSTANCE hPrevInstance,
                   LPSTR lpCmdLine,
                   int nCmdShow
                   )
{
    WNDCLASSEX wcx;
    ZeroMemory(&wcx, sizeof(wcx));
    wcx.cbSize = sizeof(wcx);
    wcx.style = CS_HREDRAW | CS_VREDRAW | CS_OWNDC;
    wcx.lpfnWndProc = DefWindowProc;
    wcx.hInstance = hInstance;
    wcx.lpszClassName = c_szWndClass;

    RegisterClassEx(&wcx);

    HWND hwndMain = CreateWindowEx(
        0,
        c_szWndClass,
        c_szWndTitle,
        WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT,
        CW_USEDEFAULT,
        c_nWndWidth,
        c_nWndHeight,
        NULL,
        NULL,
        hInstance,
        NULL);

    ShowWindow(hwndMain, SW_SHOW);

    HDC hdc;
    hdc = GetDC(hwndMain);

    HDC hdcOffscreen = CreateCompatibleDC(hdc);
    HBITMAP bitmap = CreateCompatibleBitmap(hdcOffscreen, c_nWndWidth, c_nWndHeight);
    HBITMAP old_bitmap = (HBITMAP) SelectObject(hdcOffscreen, bitmap);

    TRIVERTEX vertices[2];
    ZeroMemory(&vertices, sizeof(vertices));
    vertices[0].Red = 0xFF00;
    vertices[0].Green = 0x0000;
    vertices[0].Blue = 0x0000;
    vertices[0].x = 0;
    vertices[0].y = 0;

    vertices[1].Red = 0x0000;
    vertices[1].Green = 0x0000;
    vertices[1].Blue = 0xFF00;
    vertices[1].x = c_nWndWidth;
    vertices[1].y = c_nWndHeight;

    GRADIENT_RECT rects[1];
    ZeroMemory(&rects, sizeof(rects));
    rects[0].UpperLeft = 0;
    rects[0].LowerRight = 1;

    // This works
    //GradientFill(hdc, vertices, 2, rects, 1, GRADIENT_FILL_RECT_V);

    // This doesn't
    GradientFill(hdcOffscreen, vertices, 2, rects, 1, GRADIENT_FILL_RECT_V);
    BitBlt(hdc, 0, 0, c_nWndWidth, c_nWndHeight, hdcOffscreen, 0, 0, SRCCOPY);

    Sleep(5000);

    SelectObject(hdcOffscreen, old_bitmap);
    DeleteObject(bitmap);
    DeleteDC(hdcOffscreen);

    return 0;
}

Answer 1

Create a DIB instead of a compatible bitmap.

Replace

HBITMAP bitmap = CreateCompatibleBitmap(hdcOffscreen, c_nWndWidth, c_nWndHeight);

with

BITMAPINFO BitmapInfo;
memset(&BitmapInfo, 0, sizeof(BITMAPINFOHEADER));
BitmapInfo.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
BitmapInfo.bmiHeader.biWidth = c_nWndWidth;
BitmapInfo.bmiHeader.biHeight = c_nWndHeight;
BitmapInfo.bmiHeader.biPlanes = 1;
BitmapInfo.bmiHeader.biBitCount = 32;
BitmapInfo.bmiHeader.biCompression = BI_RGB;
HBITMAP bitmap = CreateDIBSection(hdcOffscreen, &BitmapInfo, DIB_RGB_COLORS, NULL, NULL, 0);

Answer 2

the problem here is actually due to the initial state of the device context from which you are creating the compatible bitmap - in this line:

HBITMAP bitmap = CreateCompatibleBitmap(hdcOffscreen, c_nWndWidth, c_nWndHeight);

hdcOffscreen should instead be hdc - this is because the device context created here:

HDC hdcOffscreen = CreateCompatibleDC(hdc);

has by default a 1x1 monochrome bitmap selected into it - when you attempt to create a compatible bitmap from that, you will get a monochrome bitmap also. so if you do this instead:

HBITMAP bitmap = CreateCompatibleBitmap(hdc, c_nWndWidth, c_nWndHeight);

you should see your gradient :) old question that appears to be answered but i thought id just help clarify on exactly why its not working!

details/links:

http://msdn.microsoft.com/en-us/library/dd183489%28VS.85%29.aspx

When the memory DC is created, its display surface is exactly one monochrome pixel wide and one monochrome pixel high

http://msdn.microsoft.com/en-us/library/dd183488%28v=VS.85%29.aspx

The color format of the bitmap created by the CreateCompatibleBitmap function matches the color format of the device identified by the hdc parameter

hth :)