SDL/OpenGL Texture Transparency - opengl

Question

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.

Answer 1

I don't see you enabling blending anywhere. You'll need some variation of:
glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
See here: http://www.opengl.org/resources/faq/technical/transparency.htm

Answer 2

To hav transparency, you need an alpha channel, on your texture, but more important, in your framebuffer. This is done with SDL_GL_SetAttribute, for example:
SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 8 );
SDL_GL_SetAttribute( SDL_GL_ALPHA_SIZE, 8 );
This will request a framebuffer with at least those sizes, and you'll be able to use blending. Remember to enable blending and set a blending function.

Answer 3

Have a look at your call glTexImage2D( GL_TEXTURE_2D, 0, 3, surface->w, surface->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, surface->pixels );. You specify 3 color components for the internal format. Try 4 (with alpha channel), or GL_RGBA (more about this).

Answer 4

i've been check your code in my computer and thats run correctly. Have you set 'opengl32.lib' in your linker?

Answer 5

Being the brilliant mind that I am, my issue was that i had glEnable(GL_DEPTH_TEST) on. Removing that line made everything work and me worry about later issues. Have fun!