C++ Language Error? Pixel Drawing - SDL

Unidentified · 2004-01-04T17:29:48

First I copied the code from a tutorial and converted it to C++, it didn't work. So i copied and pasted it and it worked. I re wrote it in C++ again today and it doesn't work. I checked the tutorial and it seems fine. The error is a runtime error. I'm thinking from the main function since all I had to do before was copy the tutorials main code. I don't know what the deal is. Does it have to do with C++ or not? Do I have to do something different when I'm programming in C++ with SDL than C? Here's the code if it doesn't have to do with main.cpp #include <iostream> #include <SDL/SDL.h> #include "Draw.h" using namespace std; int main(int argc, char* argv[]) { if(SDL_Init(SDL_INIT_AUDIO | SDL_INIT_VIDEO) < 0) { cerr << "Error initailizing SDL: " << SDL_GetError(); return -1; } SDL_Surface* screen; screen = SDL_SetVideoMode(640, 480, 32, SDL_HWSURFACE | SDL_DOUBLEBUF); if(screen = NULL) { cerr << "Could not set video mode, 640 x 480, 32 bbps: " << SDL_GetError(); SDL_Quit(); return -1; } SDL_Event event; bool done = false; while(done == false) { while(SDL_PollEvent(&event)) { if(event.type == SDL_QUIT) done = true; else if(event.type == SDL_KEYDOWN) if(event.key.keysym.sym == SDLK_ESCAPE) done = true; } LockSurface(screen); DrawScene(screen); UnlockSurface(screen); } SDL_Quit(); return 0; } Draw.h ///////////////////////////////////////////////////// //Draw.h - Drawing Related Functions ///////////////////////////////////////////////////// #ifndef _GAURD_DRAW_H_ #define _GAURD_DRAW_H_ #include <iostream> #include <SDL/SDL.h> //------------------------------------------- //Surface Locking/Unlocking Functions //------------------------------------------- void LockSurface(SDL_Surface* screen); void UnlockSurface(SDL_Surface* screen); //----------------------------------------------- //Pixel Drawing Functions //----------------------------------------------- void DrawPixel(SDL_Surface* screen, int x, int y, Uint8 R, Uint8 G, Uint8 B); void DrawScene(SDL_Surface* screen); #endif Draw.cpp ///////////////////////////////////////////////////// //Draw.cpp - Drawing Related Functions ///////////////////////////////////////////////////// #include "Draw.h" //------------------------------------------- //Surface Locking/Unlocking Functions //------------------------------------------- void LockSurface(SDL_Surface* screen) { if(SDL_MUSTLOCK(screen)) if(SDL_LockSurface(screen) < 0) { std::cerr << "Error Locking Screen: " << SDL_GetError(); return; } } void UnlockSurface(SDL_Surface* screen) { if(SDL_MUSTLOCK(screen)) SDL_UnlockSurface(screen); } //----------------------------------------------- //Pixel Drawing Functions //----------------------------------------------- void DrawPixel(SDL_Surface* screen, int x, int y, Uint8 R, Uint8 G, Uint8 B) { Uint32 color = SDL_MapRGB(screen->format, R, G, B); Uint8 bpp = screen->format->BytesPerPixel; Uint32* pxp = (Uint32*)screen->pixels + y * screen->pitch + x * bpp; *pxp = color; } void DrawScene(SDL_Surface* screen) { for(int x = 0; x < 640; ++x) for(int y = 0; y < 480; ++y) DrawPixel(screen, x, y, x, y, x + y % 256); } I'm using VC++ .Net 2002 Standard Edition to complie it. PS: I also have a question about the adress of the pixel. Does the (SDL_Surface*)->pitch * y give the pixel down and x * BitsPerPixel give the pixel right. And does that mean (SDL_Surface*)->pixels) give the adress of the top left pixel. I just read info about what the pitch is(scanlines I think) so I'm not use if it's correct. I got that equation from a the SDL site and it's different than from the tutorial. So not sure if it's the best way to do it but I understand it better. [edited by - Unidentified on January 4, 2004 5:31:13 PM]

For Beginners

Started by Unidentified January 04, 2004 04:25 PM

13 comments, last by Unidentified 20 years, 3 months ago

Unidentified

136

Author

January 04, 2004 04:54 PM

There another error, when in the line with *pxp = color. I guess there is something wrong with either the equation or the value I convert it to or possibly the color.

Stephen R

721

January 04, 2004 04:59 PM

Uint32* pxp = (Uint32*)screen->pixels + y * screen->pitch + x * bpp;

should be

Uint32* pxp = (Uint32*)screen->pixels + y * screen->pitch + x;

[size=1]Website

Unidentified

136

Author

January 04, 2004 05:13 PM

I had to put

bufp = (Uint32 *)screen->pixels + y * screen->pitch/4 + x;

Which is the one from the Cone3D tutoiral. I don't get why you have to dived screen->pitch by the number of bytes per pixel The one from the SDL Doc Project didn't work which was the one I was using, it multiplied the bytes per pixel by x.

[edited by - Unidentified on January 4, 2004 6:16:57 PM]

Stephen R

721

January 04, 2004 05:17 PM

Screen->pixels points to 0,0.
To get to the right Y coordinate you mutliply y * the number of pixels per line. screen->pitch was the number of BYTES per line. Since you were using 32 bit graphics that meant divide by 4 to get the number of pixels. Then you simply add on the x and you''re there.

Does that answer your question?

[size=1]Website

Unidentified

136

Author

January 04, 2004 05:29 PM

quote:Original post by stro
Screen->pixels points to 0,0.
To get to the right Y coordinate you mutliply y * the number of pixels per line. screen->pitch was the number of BYTES per line. Since you were using 32 bit graphics that meant divide by 4 to get the number of pixels. Then you simply add on the x and you''re there.

Does that answer your question?

Ok. I understand it now.

C++ Language Error? Pixel Drawing - SDL

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

C++ Language Error? Pixel Drawing - SDL

This topic is closed to new replies.

Popular Topics

Recommended Tutorials

Reticulating splines