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SDL_BlitSurface doing nothing


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#1 Leo_Joy   Members   -  Reputation: 100

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Posted 10 April 2012 - 01:37 PM

I can't seem to draw to the screen at all. When I call SDL_BlitSurface, nothing happens. However, it does not return an error code. I've written a class to handle image sprites, and when I step through with a debugger, I see it is returning non-empty values for the surface to blit to the main video surface. When I call my draw function, however, it does nothing. I have confirmed that the pixel values are not changed at all for the surface. What am I doing wrong?

Here's my main.cpp:

[/font][/color]
[color=#000000][font=Arial,]//Standard libs
#include <SDL.h>

//Our gaming base
#include "game_base.h"

//SDL redefines main as winmain, but this is a console application so we don't want that
#undef main

const int MAX_FPS = 80; ///< The maximum frames per second for the game

int main ( int argc, char* argv[] ) {
	//Initialization
	System game( false, 640, 480, "Platformer", MAX_FPS ); //Create a game that is not fullscreen with a resolution of 640x480
	SDL_Surface *buffer = NULL;
	Input *input = NULL;
	Image_Sprite *player_sprite = NULL;

	buffer = game.get_buffer();
	player_sprite = new Image_Sprite( "data/player.bmp", 1, 4 );

	//Setup the hitbox for the player
	SDL_Rect hitbox;
	hitbox.w = 32;
	hitbox.h = 32;
	hitbox.x = 0;
	hitbox.y = 0;

	Player player( 100, 100, hitbox, 1.0, RIGHT, player_sprite ); //Initialize the player at (100, 100)

	//Main game loop
	while( !game.check_is_done() ) {
		game.refresh_top();
		player.draw( buffer );
		game.refresh_bottom();
	}

	//Cleanup
	delete player_sprite;
}[/font][/color]
[color=#000000][font=Arial,]


My draw method looks like this:


[/font][/color][/color]
[color=#000000][color=#000000][font=Arial,]void Character::draw( SDL_Surface *destination ) {
	SDL_Rect coordinates;
	SDL_Surface *sprite;
	sprite = my_sprite->get_frame( my_frame, my_direction );
	coordinates.x = (int)get_x();
	coordinates.y = (int)get_y();

	SDL_BlitSurface( sprite, NULL, destination, NULL );

}[/font][/color][/color]
[color=#000000][color=#000000][font=Arial,]


And my get_frame method is this:


[/font][/color][/color]
[color=#000000][color=#000000][font=Arial,]SDL_Surface* Image_Sprite::get_frame( int x, int y ) {
	SDL_PixelFormat *format;
	format = my_sprite->format;
	SDL_Surface *frame = SDL_CreateRGBSurface( my_sprite->flags, my_frame_width, my_frame_height, format->BitsPerPixel,
													format->Rmask, format->Gmask, format->Bmask, format->Amask );
	SDL_Rect *frame_crop = new SDL_Rect;
	frame_crop->x = my_frame_width * x;
	frame_crop->y = my_frame_height * y;
	frame_crop->w = my_frame_width;
	frame_crop->h = my_frame_height;

	SDL_Rect *coordinates = new SDL_Rect;
	coordinates->x = 0;
	coordinates->y = 0;

	SDL_BlitSurface( my_sprite, frame_crop, frame, coordinates );

	delete frame_crop;
	return frame;

}[/font][/color][/color]
[color=#000000][color=#000000][font=Arial,]


The funny thing is, this code is almost directly copied from another project where it worked fine. And yes, I am calling SDL_Init(), SDL_Quit, and SDL_Flip(). They're all part of my System class. Any help would be appreciated. I tried solving this myself, but I'm stumped.


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#2 Leo_Joy   Members   -  Reputation: 100

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Posted 11 April 2012 - 08:00 AM

Alright, I figured it out. I wasn't initializing my_direction, so I was getting nonsense values for where to crop the image.

#3 Ectara   Crossbones+   -  Reputation: 3066

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Posted 11 April 2012 - 11:11 AM

Don't forget that you are also not initializing the w and h fields in the SDL_Rect named coordinates, so attempting to blit a square of unknown dimensions would invoke undefined behavior.

#4 rip-off   Moderators   -  Reputation: 8764

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Posted 11 April 2012 - 11:32 AM

SDL is documented as not reading the width and height of the rightmost rectangle pointer parameter. In fact, this function actually initialises these values with the clipped range to be blitted! The dimensions to blit are solely based on the source rectangle.

That said, the function would be better written as:

SDL_Surface* Image_Sprite::get_frame( int x, int y ) {
    SDL_PixelFormat *format = my_sprite->format;
    SDL_Surface *frame = SDL_CreateRGBSurface(
        my_sprite->flags, my_frame_width, my_frame_height, 
        format->BitsPerPixel, format->Rmask, format->Gmask, format->Bmask, format->Amask );

    SDL_Rect frame_crop = {
        my_frame_width * x,
        my_frame_height * y,
        my_frame_width,
        my_frame_height
    };

    SDL_BlitSurface( my_sprite, &frame_crop, frame, NULL );

    return frame;
}
This avoids the possibility of undefined behaviour, and also avoids a memory leak. Passing NULL as the destination defaults to the top left.

If the destination were not at the top left corner, writing the following avoids undefined behaviour:

SDL_Rect source = { /* ... */ };
SDL_Rect dest = { x, y };

SDL_BlitSurface( surface, &source, frame, &dest );
Here, the width and height members of the destination rectangle are zeroed (due to the default zero-initialisation of incomplete aggregate initialisers - I cannot recall the technical name for this).

#5 Ectara   Crossbones+   -  Reputation: 3066

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Posted 11 April 2012 - 03:45 PM

Gah, foiled. I must have mistook it for my own image handling; if the width and height don't match, the source rectangle is scaled to match the size of the destination rectangle. However, explicitly setting your members, be it by initialization lists or assignment operators, will ensure that if they do change it to something like setting it to zero will use the original dimensions while a non-zero value will scale to fit, the behavior does not change unexpectedly.




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