如何将模板缓冲区与 alpha 掩码一起使用

我正在尝试使用带有 alpha 蒙版的模板缓冲区来防止绘制部分纹理。

初始化代码

if(SDL_Init(SDL_INIT_EVERYTHING) < 0) {
    fprintf(stderr,"%s:%dn    SDL_Init call failed.n",__FILE__,__LINE__);
    return false;
}
// Request use of the stencil buffer
SDL_GL_SetAttribute( SDL_GL_STENCIL_SIZE, 1 );
if((Surf_Display = SDL_SetVideoMode(WWIDTH, WHEIGHT, 32,  SDL_GL_DOUBLEBUFFER | SDL_OPENGL | SDL_RESIZABLE)) == NULL) {
    fprintf(stderr,"%s:%dn    SDL_SetVideoMode call failed.n",__FILE__,__LINE__);
    return false;
}
// Init GLDebug system
GLDebug::Init();
// Init GL system
glClearColor(0, 0, 0, 1);
glClearDepth(1.0f);
glViewport(0, 0, WWIDTH, WHEIGHT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, WWIDTH,  WHEIGHT, 0, 1, -1);
glMatrixMode(GL_MODELVIEW);
glHint(GL_POINT_SMOOTH_HINT, GL_NICEST);
glHint(GL_LINE_SMOOTH_HINT, GL_NICEST);
glHint(GL_POLYGON_SMOOTH_HINT, GL_NICEST);
glEnable(GL_POINT_SMOOTH);
glEnable(GL_LINE_SMOOTH);
glEnable(GL_POLYGON_SMOOTH);
glLoadIdentity();

源

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
// Enable use of textures
glEnable(GL_TEXTURE_2D);
// 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);
glEnable(GL_ALPHA_TEST);
glEnable(GL_STENCIL_TEST);
glBindTexture(GL_TEXTURE_2D,(&StencilTex)[0]);
// Draw our stencil buffer texture
glBegin(GL_POLYGON);                                   
    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_ALPHA_TEST);
// Re enable drawing of colors
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glStencilFunc(GL_GREATER, 0, -1);
// Bind desired texture for drawing
glBindTexture(GL_TEXTURE_2D,(&texture)[0]);
// Draw the box with colors (this should be masked off)
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();

我将摆脱蒂姆发布的这个问题的参考。我的设置适用于像素的原始绘制，但我无法 A） 制作此过程应该如何工作的正面或反面或 B） 让它从 alpha 纹理工作。我得到的只是一个黑屏，或者完整的纹理。

有人愿意把它分解成合理的步骤吗？我对glStencilOp和glStencilFunc选项的工作原理有一个模糊的理解，但它们与使用alpha测试和模板测试的关系让我感到困惑。我们如何决定仅保留纹理中的 alpha 层？

要查看：

• 我有一个想要绘制到场景中的纹理，但我需要屏蔽非常特定的区域。
• 无法在运行时修改对象的 alpha 纹理来实现此效果。
我
• 正在使用固定管道，我计划稍后学习如何使用 VBO。
• 给定的代码会导致在调用第二个纹理时不绘制任何内容。

[编辑 0]

接受的解决方案有效！这是符合我期望的最终代码。

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
// // Enable use of textures
glEnable(GL_TEXTURE_2D);
// Disable writing to any of the color fields
glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE);
glStencilOp(GL_KEEP, GL_KEEP, GL_INCR); 
glAlphaFunc(GL_GREATER, 0.05);
glStencilFunc(GL_ALWAYS, 0,0);
glEnable(GL_STENCIL_TEST);
glEnable(GL_ALPHA_TEST);
glBindTexture(GL_TEXTURE_2D,(&StencilTex)[0]);
// Draw our blocking poly
glBegin(GL_POLYGON);                                   
    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_ALPHA_TEST);

// Re enable drawing of colors
glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
glStencilFunc(GL_LESS, 0, -1);
// 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();