Multitexturing problem in Age of Empires

OpenGL OpenGL: Basic Coding
1

I am trying to make a terrain engine like age of empires. Its 2D isometric.I plan to produce the terrain like this.
1.Render a base texture like grass
2.Render another texture on top like sand but control its alpha using an alpha texture(or map i think its called).

i tried this

// render grass
glActiveTextureARB(GL_TEXTURE0_ARB);
glBindTexture(GL_TEXTURE_2D, textures[0]);

RenderTerrain();

// render dirt

// TEXTURE-UNIT #0:Alpha Map
glActiveTextureARB(GL_TEXTURE0_ARB);
glBindTexture(GL_TEXTURE_2D, textures[2]);

// TEXTURE-UNIT #1:Farm
glActiveTextureARB(GL_TEXTURE1_ARB);
glBindTexture(GL_TEXTURE_2D, textures[1]);

glTexEnvi(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_COMBINE_EXT);
glTexEnvi(GL_TEXTURE_ENV,GL_COMBINE_RGB_EXT, GL_MODULATE );

glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_ALPHA_EXT,GL_PREVIOUS_EXT);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_ALPHA_EXT,GL_SRC_ALPHA);

glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE1_RGB_EXT,GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND1_RGB_EXT,GL_SRC_COLOR);

glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
glBegin(GL_QUADS);

glMultiTexCoord2fARB(GL_TEXTURE0_ARB,0.0,0.0);
glMultiTexCoord2fARB(GL_TEXTURE1_ARB,0.0,0.0);
glVertex2i(0,0);

glMultiTexCoord2fARB(GL_TEXTURE0_ARB,0.378,0.0);
glMultiTexCoord2fARB(GL_TEXTURE1_ARB,0.378,0.0);
glVertex2i(TILE_WIDTH,0.0);

glMultiTexCoord2fARB(GL_TEXTURE0_ARB,0.378,0.765);
glMultiTexCoord2fARB(GL_TEXTURE1_ARB,0.378,0.765);
glVertex2i(TILE_WIDTH,TILE_HEIGHT);

glMultiTexCoord2fARB(GL_TEXTURE0_ARB,0.0,0.765);
glMultiTexCoord2fARB(GL_TEXTURE1_ARB,0.0,0.765);
glVertex2i(0.0,TILE_HEIGHT);

glEnd();

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

RenderTerrain();

glDisable(GL_BLEND);

// disable 2nd texture unit
glActiveTextureARB(GL_TEXTURE1_ARB);
glDisable(GL_TEXTURE_2D);

This should render the base grass texture.Then multiply the alpha channel from textures[2] with rgb channel of textures[1]but i only get the 1st texture(grass) on the screen…yet i have seen other programs use multitexturing perfectly on my card.But no matter wat i use i seem to get only the 1st texture…am i missing something?

2

Somebody told me to try this

// render grass
glActiveTextureARB(GL_TEXTURE0_ARB);
glBindTexture(GL_TEXTURE_2D, grassID);

RenderTerrain();

// render dirt
glActiveTextureARB(GL_TEXTURE0_ARB);
glBindTexture(GL_TEXTURE_2D, alphaMapID);

glActiveTextureARB(GL_TEXTURE1_ARB);
glBindTexture(GL_TEXTURE_2D, dirtID);
glEnable(GL_TEXTURE_2D);
glTexEnvi(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_COMBINE_EXT);
glTexEnvi(GL_TEXTURE_ENV,GL_COMBINE_RGB_EXT, GL_MODULATE );

glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_ALPHA_EXT,GL_PREVIOUS);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_ALPHA_EXT,GL_SRC_ALPHA);
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE1_ALPHA_EXT,GL_PREVIOUS);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND1_ALPHA_EXT,GL_SRC_ALPHA);

glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_RGB_EXT,GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_RGB_EXT,GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE1_RGB_EXT,GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND1_RGB_EXT,GL_SRC_COLOR);

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

RenderTerrain();

glDisable(GL_BLEND);

// disable 2nd texture unit
glActiveTextureARB(GL_TEXTURE1_ARB);
glDisable(GL_TEXTURE_2D);

But i cannot understand for the dirt+alpha map part…why he has multiplied the alpha channel of tex unit 1 with itself in :-
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_ALPHA_EXT,GL_PREVIOUS);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_ALPHA_EXT,GL_SRC_ALPHA);
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE1_ALPHA_EXT,GL_PREVIOUS);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND1_ALPHA_EXT,GL_SRC_ALPHA);

Then the rgb of tex unit 2 with itself in:-
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE0_RGB_EXT,GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND0_RGB_EXT,GL_SRC_COLOR);
glTexEnvi(GL_TEXTURE_ENV,GL_SOURCE1_RGB_EXT,GL_TEXTURE);
glTexEnvi(GL_TEXTURE_ENV,GL_OPERAND1_RGB_EXT,GL_SRC_COLOR);

…i dont get why this has to be done

Also 1 more question.The following segment is to blend in the alpha modulated dirt texture with the already existing grass texture in frame buffer right?

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

RenderTerrain();

glDisable(GL_BLEND);

And of course i would like to store the alpha map as 8 bit textures…can i mix that with my 24 bit dirt texture?(i dont need alpha channel in dirt texture …so 24 bit)

3

Also currently my alpha map is done like this

//Texture 3-------------Alpha Map
SDL_LockSurface(TextureImage[2]);
for( i=0; i<TextureImage[2]->h; i++)
for( j=0; j<TextureImage[2]->w; j++){

x = Uint32( 128<<24 | 0<<16 | 0<<8 | 0<<0);
printf("
Alpha set:%p",x);
((unsigned int*)TextureImage[2]->pixels)[i*TextureImage[2]->w+j] = x;


}
SDL_UnlockSurface(TextureImage[2]);

i simply take an rgb image loaded using SDL_image & make the alpha component 128 & rgb=0 in all pixels…but surely there is a better way to produce alpha maps…because here i am wasting space.Ideally there shud be only 1 8bit component(the alpha values)…but then how do i multiply it with the rgb of the other texture unit?