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OpenGL SDL/OpenGL Texture Transparency

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So I have a seemingly simple problem, and I've searched around, but none of the solutions I've found have worked (though I probably applied them incorrectly). I'm just wondering how I can convert an SDL_Surface (loaded from a png via IMG_Load) and stick it on a quad. Here is what I have (mostly just copy pasted from a tutorial I found).



#include "SDL.h"
#include "SDL_opengl.h"
#include "SDL_image.h"

#include <stdio.h>

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

// Slightly different SDL initialization
if ( SDL_Init(SDL_INIT_VIDEO) != 0 ) {
printf("Unable to initialize SDL: %s\n", SDL_GetError());
return 1;
}

SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 ); // *new*

screen = SDL_SetVideoMode( 640, 480, 16, SDL_OPENGL ); // *changed*
if ( !screen ) {
printf("Unable to set video mode: %s\n", SDL_GetError());
return 1;
}

// Set the OpenGL state after creating the context with SDL_SetVideoMode

glClearColor( 0, 0, 0, 0 );

glEnable( GL_TEXTURE_2D ); // Need this to display a texture

glViewport( 0, 0, 640, 480 );

glMatrixMode( GL_PROJECTION );
glLoadIdentity();

glOrtho( 0, 640, 480, 0, -1, 1 );

glMatrixMode( GL_MODELVIEW );
glLoadIdentity();

// Load the OpenGL texture

GLuint texture; // Texture object handle
SDL_Surface *surface; // Gives us the information to make the texture

if ( (surface = IMG_Load("img/star.png")) ) {

// Check that the image's width is a power of 2
if ( (surface->w & (surface->w - 1)) != 0 ) {
printf("warning: image.bmp's width is not a power of 2\n");
}

// Also check if the height is a power of 2
if ( (surface->h & (surface->h - 1)) != 0 ) {
printf("warning: image.bmp's height is not a power of 2\n");
}

// Have OpenGL generate a texture object handle for us
glGenTextures( 1, &texture );

// Bind the texture object
glBindTexture( GL_TEXTURE_2D, texture );

// Set the texture's stretching properties
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );

// Edit the texture object's image data using the information SDL_Surface gives us
glTexImage2D( GL_TEXTURE_2D, 0, 3, surface->w, surface->h, 0,
GL_RGBA, GL_UNSIGNED_BYTE, surface->pixels );
}
else {
printf("SDL could not load image.bmp: %s\n", SDL_GetError());
SDL_Quit();
return 1;
}

// Free the SDL_Surface only if it was successfully created
if ( surface ) {
SDL_FreeSurface( surface );
}

// Clear the screen before drawing
glClear( GL_COLOR_BUFFER_BIT );

// Bind the texture to which subsequent calls refer to
glBindTexture( GL_TEXTURE_2D, texture );

glColor3f(1,0,0);
glDisable( GL_TEXTURE_2D );
glBegin( GL_QUADS );
// Top-left vertex (corner)
glTexCoord2i( 0, 0 );
glVertex3f( 0, 0, 0 );

// Bottom-left vertex (corner)
glTexCoord2i( 1, 0 );
glVertex3f( 328, 0, 0 );

// Bottom-right vertex (corner)
glTexCoord2i( 1, 1 );
glVertex3f( 328, 328, 0 );

// Top-right vertex (corner)
glTexCoord2i( 0, 1 );
glVertex3f( 0, 328, 0 );
glEnd();
glColor3f(1,1,1);
glEnable( GL_TEXTURE_2D );

glBegin( GL_QUADS );
// Top-left vertex (corner)
glTexCoord2i( 0, 0 );
glVertex3f( 100, 100, 0 );

// Bottom-left vertex (corner)
glTexCoord2i( 1, 0 );
glVertex3f( 228, 100, 0 );

// Bottom-right vertex (corner)
glTexCoord2i( 1, 1 );
glVertex3f( 228, 228, 0 );

// Top-right vertex (corner)
glTexCoord2i( 0, 1 );
glVertex3f( 100, 228, 0 );
glEnd();

SDL_GL_SwapBuffers();

// Wait for 3 seconds to give us a chance to see the image
SDL_Delay(3000);

// Now we can delete the OpenGL texture and close down SDL
glDeleteTextures( 1, &texture );

SDL_Quit();

return 0;
}




The texture renders, but where there should be transparency, there is just black. Thanks in advance for any tips.



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Well, just in case you're curious, it seems you forgot to call SDL_DisplayFormatAlpha on your loaded image. Plus, did you call glEnable(GL_BLEND)?

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