Image on screen dragging on `

[bijan311]

This is a 2 part question
1.So when I move my block around it doesn't move the image just extends it for example:
this is the image when the program starts up

~

this is the image when you press the right key

~~

while it should be

 ~

why does that happen.(I'll add attachments).

2. you have to keep on tapping a key for it to move in that direction, you can't just hold it down.

here is my code

#include <SDL/SDL.h>
#include <SDL/SDL_image.h>
#include <string>

using namespace std;

//block coordinates
int x = 0;
int y = 0;

SDL_Surface *screen = NULL;
SDL_Surface *block = NULL;
SDL_Surface *wall = NULL;

bool quit = false;

SDL_Event event;

void handle_events(){
    if(SDL_PollEvent(&event)){
            if(event.type == SDL_QUIT){
                quit = true;
            }
            if(event.type = SDL_KEYDOWN){
                switch(event.key.keysym.sym){
                    case SDLK_UP: y--; break;
                    case SDLK_DOWN: y++;break;
                    case SDLK_RIGHT: x++; break;
                    case SDLK_LEFT: x--; break;
                    case SDLK_ESCAPE: quit = true; break;
                    default: break;
                }
        }

    }
}


void apply_surface(int X, int Y, SDL_Surface *source, SDL_Surface *destination){
    SDL_Rect pos;
    pos.x = X;
    pos.y = Y;

    SDL_BlitSurface(source, NULL, destination, &pos);
}

SDL_Surface *load_image(string filename){
    SDL_Surface *image;

    image = IMG_Load(filename.c_str());

    return image;
}

bool init(){
    if(SDL_Init(SDL_INIT_EVERYTHING) == -1){
        return false;
    }

    screen = SDL_SetVideoMode(640, 480, 32, SDL_SWSURFACE);

    if(screen == NULL){
        return false;
    }

    SDL_WM_SetCaption("Test", NULL);

    return true;
}

int main( int argc, char* args[] )
{
    if(init() == false){
        return 1;
    }

    block = load_image("block.png");
    wall = load_image("wall.png");

    if(block == NULL){
        return 1;
    }
    if(wall == NULL){
        return 1;
    }
    //make screen color
    SDL_FillRect(screen, &screen->clip_rect, SDL_MapRGB(screen->format, 0x12, 0x24, 0x48));



    while(quit == false){
        handle_events();

        apply_surface(x, y, block, screen);
        apply_surface(320, 0, wall, screen);

        SDL_Flip(screen);
    }

    //Quit SDL
    SDL_Quit();

    return 0;
}

Thank you

[jdragyn]

Not sure about part 1, but the second part of your question is easy to answer. When you press a key repeatedly, how many times do you press the KEY DOWN? How about when you press the KEY DOWN and hold it?

[bijan311]

Oh I get it, when you hold it down it only presses it once, so how do I make it so you can hold it down

[jeffcobb]

I will be curious to see the answer to this one...the first part has so little information about what is going on, platform, language, code intent, everything that I don't see how to even begin answering the question...not even sure what the question is...

[VirtualAce]

Looks like lack of double buffering to me or perhaps failure to clear the back buffer per frame.

[bijan311]

OK I figured out #1, i just but fill rect in the loop

[bijan311]

OK, I still need an answer to #2

[VirtualAce]

Sounds like you have a bad key handler.

[rodrigorules]

i know exactly what is wrong ...

when an event registers as KEYDOWN it means you just pressed the key down, and it alters x by 1 , just because you are holding it doesnt incur another event ... when you release the key it sends another event KEYUP.

Since only 1 event is passed when you press the KEYDOWN, x is only altered one time, by 1.

make a variable called velocity_x, velocity_y

when keydown is registered change velocity to 1, now in your while loop alter x+=velocity.

when keyup is registered change velocity back to 0.

heres what your while loop would look like

    while(quit == false){
        handle_events();
         x+=velocity_x;
         y+=velocity_y;
        apply_surface(x, y, block, screen);
        apply_surface(320, 0, wall, screen);

        SDL_Flip(screen);
    }

NOTE: your block's speed of movement will be relative to how fast your computer can render, you need frame control for this to work 'as intended' (google SDL_Delay)

To sum everything up: Pressing a key , holding it down for whatever time, and releasing it is only 2 events .. and that is all SDL sees, therefore you have to handle 2 events not just KEYDOWN

[bijan311]

now all the block goes is going diagonal
here are my additions:

            if(event.type == SDL_KEYDOWN){
                velocity_x = 1;
                velocity_y = 1;
                switch(event.key.keysym.sym){

if(event.type == SDL_KEYUP){
    velocity_x = 0;
    velocity_y = 0;
}

handle_events();
x += velocity_x;
y += velocity_y;

[rodrigorules]

You are changing both x and y to 1 , obviously it is gonna change BOTH x and y directions, pick one or be more specific;

KEYDOWN means any key was pressed down, you have to be more specific than that

example:

switch(event.type)
			{
				case SDL_KEYUP:
					if(event.key.keysym.sym == SDLK_UP)
					{
						veloy1=0;
					}
					else	if(event.key.keysym.sym == SDLK_DOWN)
					{
						veloy1=0;
					}

					break;
				case SDL_KEYDOWN:
					if(event.key.keysym.sym == SDLK_UP)
					{
						veloy1=-1;
					}
					else if(event.key.keysym.sym == SDLK_DOWN)
					{
						veloy1=1;
					}

					break;
			}

You are never specifing which key it is that your pressing.


You have it set so that if you pressed ANY KEY DOWN then you set velo_x =1 AND velo_y =1 , can't you tell that it doesn't make sense to change both velocities if you press the DOWN key only?