How to properly use SDL_BlitSurface() with SDL_CreateRGBSurface()? - c++

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 100x100 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