Mask part of a texture on draw with OpenGL in a fixed pipeline

Question

I am trying to figure out the best way to mask of sections of a texture when they ar drawn. My issue comes in the fact that I seem to have run our of alpha masks!

We are using openGL to draw a custom built 2D game engine. The game is built up off of sprites and simple block textures.

My desired outcome is like this:

• A character sprite is drawn in place (using it's alpha color to not just be a box)
• An item is drawn into the players hand (also using it's alpha color to draw into the scene without being a box)
• The item should appear behind the characters arm/hand, but above the rest of the body.

For the moment the only way I can figure out how to accomplish this, is by drawing them in order (Body, Item, Arm) but I would like to avoid this to make art assets a bit easier to deal with. My idea solution would be to draw the character, then draw the item with an alpha mask that blocks out areas of the texture that should be "under" the arm.

Other solutions that I have seen are like this, where the glBlendFuncSeparate() function is used. I am trying to avoid bringing in extensions, as my current version of OpenGL doesn't support it. Not to say that I am opposed to the idea, but it seems a bit of a handle to brig it in just to draw an alpha mask?

I fully admit that this is a learning process for me, and I am using it as an excuse to really see how OpenGL handles. Any suggestions as to where I should head to get this to draw correctly? Is there a way for OpenGL in the fixed pipeline to take a texture, apply an alpha mask on top of it, and THEN draw it into the buffer? Should I give in and separate my character into several parts of its model?

[UPDATE: 8/12/12]

Tried to add the code suggested by Tim, but I seem to be having an issue. When I enable the stencil buffer, everything just gets blocked out, NOT just what I wanted. Here is my test example code.

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

// Disable writing to any of the color fields
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);

glStencilOp(GL_KEEP, GL_KEEP, GL_INCR); 

glStencilFunc(GL_ALWAYS, 0,0);

// Draw our blocking poly
glBegin(GL_POLYGON);                                   
    glVertex2f( 50,     50 );
    glVertex2f( 50,     50+128 );
    glVertex2f( 50+128, 50+128 );
glEnd();

glStencilFunc(GL_GREATER, 0, -1);

glEnable(GL_STENCIL_TEST);

// Re enable drawing of colors
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);

// Enable use of textures
glEnable(GL_TEXTURE_2D);

// Bind desired texture for drawing
glBindTexture(GL_TEXTURE_2D,(&texture)[0]);

// Draw the box with colors
glBegin(GL_QUADS);                                   
    glTexCoord2d( 0, 0 );    glVertex2f( 50,     50 );
    glTexCoord2d( 0, 1 );    glVertex2f( 50,     50+128 );
    glTexCoord2d( 1, 1 );    glVertex2f( 50+128, 50+128 );
    glTexCoord2d( 1, 0 );    glVertex2f( 50+128, 50 );
glEnd();

// Swap buffers and display!
SDL_GL_SwapBuffers();

Just to be clear, here is my init code as well to set this system up.

When the code is run with stencil disabled, I get this:

When I use glEnable(GL_STENCIL_TEST), I get this:

I've tried playing around with various options, but I cannot see a clear reason why my stencil buffer is blocking everything.

[Update#2 8/12/12]

We got some working code, Thanks tim! Here is what I ended up running to work correctly.

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

// Disable writing to any of the color fields
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);

glStencilOp(GL_INCR, GL_INCR, GL_INCR);

glEnable(GL_STENCIL_TEST);
// Draw our blocking poly
glBegin(GL_POLYGON);                                   
    glVertex2f( 50,     50 );
    glVertex2f( 50,     50+128 );
    glVertex2f( 50+128, 50+128 );
glEnd();

glStencilFunc(GL_EQUAL, 1, 1);

glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP); 

// Re enable drawing of colors
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);

// Enable use of textures
glEnable(GL_TEXTURE_2D);

// Bind desired texture for drawing
glBindTexture(GL_TEXTURE_2D,(&texture)[0]);

// Draw the box with colors
glBegin(GL_QUADS);                                   
    glTexCoord2d( 0, 0 );    glVertex2f( 50,     50 );
    glTexCoord2d( 0, 1 );    glVertex2f( 50,     50+128 );
    glTexCoord2d( 1, 1 );    glVertex2f( 50+128, 50+128 );
    glTexCoord2d( 1, 0 );    glVertex2f( 50+128, 50 );
glEnd();
glDisable(GL_STENCIL_TEST);

// Swap buffers and display!
SDL_GL_SwapBuffers();

Answer 1

Tim was pretty clear in his comment, but I want to present you the solution I find the most intuitive. It's 3D, so hold on... ;)

Basically, you can just use the Z coordinate of your images to create virtual "layers". It then doesnt' matter, in which order you draw them. Just alphatest every image individually, and draw it on correct Z value. If it still isn't enough, you could use separate texture containing "depth" of every pixel, and then use the 2nd texture to perform some sort of depth-testing.

Be sure to call glEnable(GL_DEPTH_TEST); if you want to use this approach.

Answer 2

Here's my idea for the situation where you have one texture and one alpha mask:

1. Draw the character onto the scene like normal.
2. Lock the RGB color channels so that it cannot be changed with glColorMask
3. Setup the stencil buffer with glStencilOp(GL_KEEP, GL_KEEP, GL_INCR); glStencilFunc(GL_ALWAYS, 0,0);
4. Draw the alpha mask with alpha testing enabled. This will increment the stencil buffer anywhere the alpha test passes (you may have to flip this based on your mask polarity)
5. At this point, you have a character texture in the framebuffer, and a mask outline in the stencil buffer.
6. Reenable the color channels with glColorMask
7. Setup the stencil buffer for the weapon with glStencilFunc(GL_GREATER, 0, -1); This will only draw the weapon texels where the stencil buffer is greater than zero, and reject pixels where the stencil is not updated.
8. Draw the weapon texture as normal.

Answer 3

As I see it, the problem is that you have one texture, but part of it represents the arm and part of it the rest of the character. The issue is that you want to draw the weapon over the character, but draw the arm over both.

This means, while drawing two objects, you want to put them into three different "layers". This fundamentally doesn't make sense, so you're kind of stuck.

Here's an idea though: use a fragment program (i.e., a shader).

I suggest you overload the character's texture's alpha channel to encode both transparency and layer. For example, let's use 0=transparent body, 64=opaque body, 128=transparent arm, 255=opaque arm.

From here, you draw your objects, but conditionally set the depth of your objects into three layers. Basically, you write a fragment program that draws your character into two different layers, the character gets pushed backward while the arm gets pulled forward. When the weapon is drawn, it is drawn without a shader, but it's tested against the characters' pixels' depths. It works something like this (untested, obviously).

Define a shader my_shader, which contains a fragment program:

uniform sampler2D character_texture;
void main(void) {
    vec4 sample = texture2D(character_texture,gl_TexCoord[0].st);

    int type; //Figure out what type of character texel we're looking at
    if      (fabs(sample.a-0.00)<0.01) type = 0; //transparent body
    else if (fabs(sample.a-0.25)<0.01) type = 1; //opaque body
    else if (fabs(sample.a-0.50)<0.01) type = 2; //transparent arm
    else if (fabs(sample.a-1.00)<0.01) type = 3; //opaque arm

    //Don't draw transparent pixels.
    if (type==0 || type==2) discard;

    gl_FragColor = vec4(sample.rgb,1.0);

    //Normally, you (can) write "gl_FragDepth = gl_FragCoord.z".  This
    //is how OpenGL will draw your weapon.  However, for the character,
    //we alter that so that the arm is closer and the body is farther.

    //Move body farther
    if      (type==1) gl_FragDepth = gl_FragCoord.z * 1.1;
    //Move arm closer
    else if (type==3) gl_FragDepth = gl_FragCoord.z * 0.9;
}

Here's some pseudocode for your draw function:

//...

//Algorithm to draw your character
glUseProgram(my_shader);
glBindTexture(GL_TEXTURE_2D,character.texture.texture_gl_id);
glUniform1i(glGetUniformLocation(my_shader,"character_texture"),1);
character.draw();
glUseProgram(0);

//Draw your weapon
glEnable(GL_DEPTH_TEST);
character.weapon.draw();
glDisable(GL_DEPTH_TEST);

//...