Question

(See “Edit 2” below for the solution.)

I need to create SDL surfaces from scratch, instead of loading them from a file. Unfortunately, SDL_BlitSurface() seems to render all colors as black when used with the surface generated through SDL_CreateRGBSurface(). This is my code:

int main(int argc, char** argv)
{
    SDL_Surface* screen = SDL_SetVideoMode(640, 480, 32, SDL_HWSURFACE);
    SDL_Surface* layer = SDL_CreateRGBSurface(SDL_HWSURFACE, 100, 100,
        screen->format->BitsPerPixel,
        screen->format->Rmask,
        screen->format->Gmask,
        screen->format->Bmask,
        screen->format->Amask
    );
    SDL_Rect rect;

    rect.x = 0;
    rect.y = 0;
    rect.w = 100;
    rect.h = 100;

    Uint32 blue = SDL_MapRGB(screen->format, 0, 0, 255);
    SDL_FillRect(layer, &rect, blue);
    SDL_BlitSurface(screen, NULL, layer, NULL);
    SDL_Flip(screen);
    SDL_Delay(3000);
    return 0;
}

What I get is a black screen, instead of a 100×100 blue rectangle. What I could find by Googling doesn’t seem to help me, as those questions either apply to 8bit surfaces (and setting palettes — my bpp is 32 here) or are left unanswered.

So, I would like to know how should I properly blit a generated surface onto a SDL screen.

Edit: I see it was an error in the parameter ordering. The line in question should read

SDL_BlitSurface(layer, NULL, screen, NULL);

Still, I am having trouble to achieve the same effect in my more complex C++ program. I will post the relevant parts of the code here:

main.cpp:

int main(int argc, char** argv)
{
    SDLScreen screen(1024, 700, "Hello, SDL!");
SDL_Event event;
    SDLMenu menu;
    bool shouldQuit = false;

    menu.setBounds(200, 100, 200, 600);
    menu.setFontName("NK211.otf");
    menu.setFontSize(36);
    menu.setEffect(sdlteShadowText);
    menu.addItem("New game");
    menu.addItem("Load game");
    menu.addItem("Save game");
    menu.addItem("Exit");
    menu.render();

    while (!shouldQuit)
    {
        menu.draw(screen.getSurface());
        SDL_Flip(screen.getSurface());
        SDL_Delay(10);
        while (SDL_PollEvent(&event))
        {
            if (event.type == SDL_QUIT)
            {
                shouldQuit = true;
            }
            else if (event.type == SDL_KEYUP)
            {
                if (event.key.keysym.sym == SDLK_q)
                {
                    shouldQuit = true;
                }
            }
        }
    }
}

SDLMenu.cpp:

void
SDLMenu::setSelectionColorRGB(int r, int g, int b)
{
    SDL_VideoInfo* info = (SDL_VideoInfo*)SDL_GetVideoInfo();
    selectionColor = SDL_MapRGB(info->vfmt, r, g, b);
}

void
SDLMenu::render()
{
    SDLText* current = NULL;
    SDL_VideoInfo* info = (SDL_VideoInfo*)SDL_GetVideoInfo();

    if (!items->empty())
    {
        current = getItemAt(currentItem);
        selectionRect = getItemRect(current);
        setSelectionColorRGB(0,0,255);
        selectionCanvas = SDL_CreateRGBSurface(SDL_HWSURFACE,
            selectionRect->w, selectionRect->h,
            info->vfmt->BitsPerPixel,
            info->vfmt->Rmask,
            info->vfmt->Gmask,
            info->vfmt->Bmask,
            info->vfmt->Amask);
        SDL_FillRect(selectionCanvas, selectionRect, selectionColor);
        SDL_SaveBMP(selectionCanvas, "selection.bmp"); // debug
    }

    for (list<SDLText*>::iterator i = items->begin();
        i != items->end(); i++)
    {
        (*i)->render();
    }
}

void
SDLMenu::draw(SDL_Surface* canvas)
{
    int currentY = bounds.y;

    if (selectionCanvas != NULL)
    {
        SDL_BlitSurface(selectionCanvas, NULL, canvas, selectionRect);
    }

    for (list<SDLText*>::iterator i = items->begin();
        i != items->end(); i++)
    {
        (*i)->draw(bounds.x, currentY, canvas);
        currentY += fontSize + itemGap;
    }
}

SDLScreen.cpp:

SDLScreen::SDLScreen(int w, int h, string t, int d)
    : width(w), height(h), depth(d), title(t)
{
    SDL_Init(SDL_INIT_EVERYTHING);
    SDL_WM_SetCaption(title.c_str(), NULL);
    refresh();
}

void
SDLScreen::refresh()
{
    screen = SDL_SetVideoMode(width, height, 32, SDL_HWSURFACE);
}

The selection rectangle for the active menu item should be blue, but it shows up in black. The file selection.bmp is also all black.

Edit 2: I found out what created the problem. The selectionRect was set relative to the screen, while the selectionCanvas had the width and height of a particular menu item. So, the filling was done out of bounds of the selectionCanvas. Adding separate SDL_Rect for filling solved the problem.

SDL_Rect fillRect;
fillRect.x = 0;
fillRect.y = 0;
fillRect.w = selectionRect->w;
fillRect.h = selectionRect->h;
SDL_FillRect(selectionCanvas, &fillRect, selectionColor);

// and later...

SDL_BlitSurface(selectionCanvas, NULL, canvas, selectionRect);

Answer 1

You inverted source and destination. To blit on screen, it should be

SDL_BlitSurface(layer, NULL, screen, NULL);

doc for SDL_BlitSurface