blending alpha textures in gluOrtho2D

Question

i'm doing a simple game/application with some minimal 2d graphics: png textures and ploygons with standard glVertex calls

my setting:

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, w, 0, h);
glMatrixMode(GL_MODELVIEW);

glEnable(GL_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc( GL_ONE_MINUS_SRC_ALPHA, GL_SRC_ALPHA );

code for textures:

texture = glGenTextures(1)
glBindTexture(GL_TEXTURE_2D, texture)
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR)
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR)
glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA,     GL_UNSIGNED_BYTE, textureData )
newList = glGenLists(1)
glNewList(newList,GL_COMPILE);
glBindTexture(GL_TEXTURE_2D, texture)
glBegin(GL_QUADS)
glTexCoord2f(0, 0); glVertex2f(0, 0)
glTexCoord2f(0, 1); glVertex2f(0, height)
glTexCoord2f(1, 1); glVertex2f( width,  height)
glTexCoord2f(1, 0); glVertex2f(width, 0)
glEnd()
glEndList()

...

glCallList(newList)

i've found this code here: http://disruption.ca/gutil/introduction.html

if i render any texture with glCallList i no more visualize plygons! what could it be? i'm stucking with some combinations of glBlendFunc but i'm getting nothing goos..

Answer 1

I have tested the example2a and it worked without changes. These are the files that I have used:

gutil.py:

import pygame

from OpenGL.GL import *
from OpenGL.GLU import *

def initializeDisplay( w, h ):
    pygame.display.set_mode((w,h),pygame.OPENGL|pygame.DOUBLEBUF)

    glClearColor( 0.0, 0.0, 0.0, 1.0 )
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT )

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    gluOrtho2D( 0, w, 0, h );
    glMatrixMode( GL_MODELVIEW );

    #set up texturing
    glEnable( GL_TEXTURE_2D )
    glEnable( GL_BLEND )
    glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA )

def loadImage( image ):
    textureSurface = pygame.image.load( image )

    textureData = pygame.image.tostring( textureSurface, "RGBA", 1 )

    width = textureSurface.get_width()
    height = textureSurface.get_height()

    texture = glGenTextures(1)
    glBindTexture( GL_TEXTURE_2D, texture )
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR )
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR )
    glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, textureData )

    return texture, width, height

def delTexture( texture ):
    glDeleteTextures( texture )

def createTexDL(texture, width, height):
    newList = glGenLists(1)
    glNewList(newList,GL_COMPILE);
    glBindTexture(GL_TEXTURE_2D, texture)
    glBegin(GL_QUADS)
    glTexCoord2f(0, 0); glVertex2f(0, 0)    # Bottom Left Of The Texture and Quad
    glTexCoord2f(0, 1); glVertex2f(0, height)    # Top Left Of The Texture and Quad
    glTexCoord2f(1, 1); glVertex2f( width,  height)    # Top Right Of The Texture and Quad
    glTexCoord2f(1, 0); glVertex2f(width, 0)    # Bottom Right Of The Texture and Quad
    glEnd()
    glEndList()

    return newList

def delDL(list):
    glDeleteLists(list, 1)

example2a.py:

import gutil
import pygame
from pygame.locals import *
from OpenGL.GL import *

def main():
    pygame.init()
    gutil.initializeDisplay(800, 600)

    glColor4f(1.0,1.0,1.0,1.0)

    done = False

    cowTex, w, h = gutil.loadImage('put here a correct file path to a png file')
    cow = gutil.createTexDL(cowTex, w, h)
    alienTex, w, h = gutil.loadImage('put here a correct file path to a png file')
    alien = gutil.createTexDL(alienTex, w, h)

    while not done:

        glColor4f(1.0,1.0,1.0,1.0)

        glEnable( GL_TEXTURE_2D ) # enable texture
        glLoadIdentity()
        glTranslatef(0, 0, 0)
        glCallList(cow)
        glTranslatef(400, 400, 0)
        glCallList(alien)
        glDisable( GL_TEXTURE_2D ) # disable texture


        glLoadIdentity();  # reset the MODEL_VIEW matrix

        glColor3f( 1.0, 0.0, 0.0 ) # set color to red
        glBegin( GL_TRIANGLES )
        glVertex2i( 0, 0 )
        glVertex2i( 100, 0 )
        glVertex2i( 100, 100 )
        glEnd()


        pygame.display.flip()

        eventlist = pygame.event.get()
        for event in eventlist:
            if event.type == QUIT \
                or event.type == KEYDOWN and event.key == K_ESCAPE:
                    done = True

if __name__ == '__main__':
    main()

Review that your code is exactly the same. Put extreme caution in the identation. If copy and past the code below, convert the spaces into tab. If you try this and have a problem you can write a comment.

Answer 2

OpenGL is a state machine, whenever you set it to a state you need to revert that state after you've used it. You have set current texture to be "texture", once you've called the list you need to restore the 0 texture (no texture state).

Textures aren't stored with display lists, so there's no need to bind them when making lists.

Your code should look like so:

//Create lists and fill them up
glNewList(newList,GL_COMPILE)
glBegin(GL_QUADS)
glTexCoord2f(0, 0); glVertex2f(0, 0)
.....
glTexCoord2f(1, 0); glVertex2f(width, 0)
glEnd()
glEndList()

...

//Bind texture and call the list
glBindTexture(GL_TEXTURE_2D, texture)
glCallList(newList)

//Bind another texture and call another list
glBindTexture(GL_TEXTURE_2D, texture2)
glCallList(newList2)

//UnBind texture
glBindTexture(GL_TEXTURE_2D, 0)

//Render any other untextured polygons/triangles in any fashion (Call lists, begin/end)

The reason why you had your polygons invisible most probably is that your texture is black/transparent at 0;0 coordinates. And when you draw your polygons they had been using 0;0 texture coords (default).