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Transparent png not transparent with sdl_image

Started by SDL_jesse Mar 3, 2012 at 10:33 AM 7 replies 6.8k views

Original Post

Mar 3, 2012 at 10:33 AM

I created a png in photoshop with a transparent background, and when I load up the png it has transparency. But when I load it into my game using SDL_image it appears white where it should be transparent.I am not use SDL_BlitSurface(),I use getPiexl() and putPiexl() instead,here is my code:
[color=#0000cd]#include
#include
#include "SDL/SDL_image.h"
using namespace std;

void putPixel(SDL_Surface *surface, int x, int y, Uint32 pixel)
{
int bpp = surface->format->BytesPerPixel;
/* Here p is the address to the pixel we want to set */
Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;

switch(bpp) {
case 1:
*p = pixel;
break;

case 2:
*(Uint16 *)p = pixel;
break;

case 3:
if(SDL_BYTEORDER == SDL_BIG_ENDIAN) {
p[0] = (pixel >> 16) & 0xff;
p[1] = (pixel >> 8) & 0xff;
p[2] = pixel & 0xff;
} else {
p[0] = pixel & 0xff;
p[1] = (pixel >> 8) & 0xff;
p[2] = (pixel >> 16) & 0xff;
}
break;

case 4:
*(Uint32 *)p = pixel;
break;
}
}

Uint32 getPixel(SDL_Surface *surface, int x, int y)
{
int bpp = surface->format->BytesPerPixel;
/* Here p is the address to the pixel we want to retrieve */
Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;

switch(bpp) {
case 1:
return *p;

case 2:
return *(Uint16 *)p;

case 3:
if(SDL_BYTEORDER == SDL_BIG_ENDIAN)
return p[0] << 16 | p[1] << 8 | p[2];
else
return p[0] | p[1] << 8 | p[2] << 16;

case 4:
return *(Uint32 *)p;

default:
return 0; /* shouldn't happen, but avoids warnings */
}
}

int main(int argc, char *argv[]) {

SDL_Init(SDL_INIT_VIDEO);
SDL_Surface *pScreen = ::SDL_SetVideoMode(640, 480, 0, SDL_SWSURFACE | SDL_DOUBLEBUF);
SDL_Surface *image = IMG_Load("jbutton.png");
SDL_DisplayFormatAlpha(image);
for(int i=0;iw;i++)
{
for(int j=0;jh;j++)
{

Uint32 color=getPixel(image,i,j);
putPixel(pScreen,i,j,color);

}
}
// SDL_BlitSurface(img, 0, video, 0); //this will be ok
SDL_Flip(pScreen);
SDL_Delay(5000);

SDL_Quit();
return 0;
}

SDL_jesse

Mar 3, 2012 at 10:36 AM

Does any one has ideas about it ? thank you for your reply!

munchor

Mar 3, 2012 at 10:45 AM

As I recall, you can set alpha:

//SDL_SetAlpha(surface_name, SDL_SRCALPHA, SDL_ALPHA_TRANSPARENT);
//SDL_DisplayFormatAlpha(surface_name);

However, I never needed to use it to get true transparency.

SDL_jesse

Mar 3, 2012 at 11:33 AM

sorry , I am a freshman in SDL ,it does not help,the main code is here,:
[color=#0000cd]int main(int argc, char *argv[]) {

SDL_Init(SDL_INIT_VIDEO);
SDL_Surface *pScreen = ::SDL_SetVideoMode(640, 480, 0, SDL_SWSURFACE | SDL_DOUBLEBUF);
SDL_Surface *image = IMG_Load("jbutton.png");
SDL_SetAlpha(image, SDL_SRCALPHA, SDL_ALPHA_TRANSPARENT);
SDL_Surface *screen = SDL_DisplayFormatAlpha(image);
for(int i=0;iw;i++)
{
for(int j=0;jh;j++)
{
Uint32 color=getPixel(screen,i,j);
putPixel(pScreen,i,j,color);

}
}
// SDL_BlitSurface(img, 0, video, 0); //this will be ok
SDL_Flip(pScreen);
SDL_Delay(5000);

SDL_Quit();
return 0;
}

munchor

Mar 3, 2012 at 11:44 AM

You shouldn't loop the image and draw it pixel by pixel...


 
 SDL_RWops *gold_rwops = SDL_RWFromFile("img/gold.png", "rb");
 SDL_Surface *gold = IMG_LoadPNG_RW(gold_rwops);
 
 SDL_Rect tile = {250 + x * TILE_SIZE, y * TILE_SIZE, TILE_SIZE, TILE_SIZE};
 SDL_BlitSurface(gold, NULL, screen, &tile);

Here's how I did it.

SDL_jesse

Mar 3, 2012 at 11:55 AM

thank you for that , but I should change the size of the image ,so I can't use SDL_BlitSurface() to do this in my game.I have to draw it pixel by pixel.
the code above can work with PNG-8 ,but it can't work with PNG-24 or PNG-32

munchor

Mar 3, 2012 at 12:02 PM

When you use SDL_BlitSurface you can also change the size of the image...

Maybe you should take a look at http://sdl.beuc.net/sdl.wiki/SDL_BlitSurface

SDL_jesse

Mar 3, 2012 at 12:20 PM

not only size , I have to flip the image also and some other things according to the game logic . Can I use putPiexl() and getPiexl() to do this?

Storyyeller

Mar 18, 2012 at 3:05 AM

SDL doesn't handle blitting to a surface with alpha properly (it ignores alpha of the destination surface), so alpha to alpha blits have to be done pixel by pixel. Other than that, you should be fine with using regular blits.

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