将片段着色器输出广播到所有FBO颜色附件吗？

Question

Is there some way to get OpenGL ES 3.0 to broadcast the value of a single-output fragment shader to all active (as per glDrawBuffers() ) FBO color attachments?

If possible I'd like to keep my shaders more-or-less as-is and avoid the re-write required by multiple layout 'd outputs:

layout( location = 0 ) out vec4 out_color0;
layout( location = 1 ) out vec4 out_color1;
layout( location = 2 ) out vec4 out_color2;
layout( location = 3 ) out vec4 out_color3;
void main()
{
    out_color0 = vec4( 1.0, 0.2, 0.0, 1.0 );
    out_color1 = vec4( 1.0, 0.2, 0.0, 1.0 );
    out_color2 = vec4( 1.0, 0.2, 0.0, 1.0 );
    out_color3 = vec4( 1.0, 0.2, 0.0, 1.0 );
}

...or an output array:

out vec4 out_color[4];
void main()
{
    out_color[0] = vec4( 1.0, 0.2, 0.0, 1.0 );
    out_color[1] = vec4( 1.0, 0.2, 0.0, 1.0 );
    out_color[2] = vec4( 1.0, 0.2, 0.0, 1.0 );
    out_color[3] = vec4( 1.0, 0.2, 0.0, 1.0 );
}

Here's the program I'm using for testing, it (tries) to draw a red triangle to all four FBO attachments & then blits the 3rd attachment to the default framebuffer:

#include <glad/glad.h>
#define GLFW_INCLUDE_NONE
#include <GLFW/glfw3.h>

#include <cstdlib>
#include <cstdarg>
#include <iostream>
#include <vector>

struct Program
{
    static GLuint Load( const char* shader, ... )
    {
        const GLuint prog = glCreateProgram();
        va_list args;
        va_start( args, shader );
        while( shader )
        {
            AttachShader( prog, va_arg( args, GLenum ), shader );
            shader = va_arg( args, const char* );
        }
        va_end( args );
        glLinkProgram( prog );
        CheckStatus( prog );
        return prog;
    }

private:
    static void CheckStatus( GLuint obj )
    {
        GLint status = GL_FALSE;
        if( glIsShader(obj) ) glGetShaderiv( obj, GL_COMPILE_STATUS, &status );
        if( glIsProgram(obj) ) glGetProgramiv( obj, GL_LINK_STATUS, &status );
        if( status == GL_TRUE ) return;
        GLchar log[ 1 << 15 ] = { 0 };
        if( glIsShader(obj) ) glGetShaderInfoLog( obj, sizeof(log), NULL, log );
        if( glIsProgram(obj) ) glGetProgramInfoLog( obj, sizeof(log), NULL, log );
        std::cerr << log << std::endl;
        std::exit( EXIT_FAILURE );
    }

    static void AttachShader( GLuint program, GLenum type, const char* src )
    {
        const GLuint shader = glCreateShader( type );
        glShaderSource( shader, 1, &src, NULL );
        glCompileShader( shader );
        CheckStatus( shader );
        glAttachShader( program, shader );
        glDeleteShader( shader );
    }
};

const char* vert = 1 + R"GLSL(
#version 300 es
void main()
{
    const vec2 verts[3] = vec2[3]
    (
        vec2( -0.5, -0.5 ),
        vec2(  0.5, -0.5 ),
        vec2(  0.0,  0.5 )
    );
    gl_Position = vec4( verts[ gl_VertexID ], 0.0, 1.0 );
}
)GLSL";

const char* frag = 1 + R"GLSL(
#version 300 es
precision mediump float;
out vec4 out_color;
void main()
{
    out_color = vec4( 1.0, 0.2, 0.0, 1.0 );
}
)GLSL";

int main( int argc, char** argv )
{
    glfwSetErrorCallback( []( int err, const char* desc )
    {
        std::cerr << "GLFW error: " << desc << std::endl;
    } );

    if( !glfwInit() )
        return EXIT_FAILURE;

    glfwWindowHint( GLFW_CLIENT_API, GLFW_OPENGL_ES_API );
    glfwWindowHint( GLFW_CONTEXT_VERSION_MAJOR, 3 );
    glfwWindowHint( GLFW_CONTEXT_VERSION_MINOR, 0 );
    glfwWindowHint( GLFW_CONTEXT_CREATION_API, GLFW_EGL_CONTEXT_API );
    GLFWwindow* window = glfwCreateWindow( 640, 480, "GLFW", NULL, NULL );
    if( nullptr == window )
        return EXIT_FAILURE;

    glfwMakeContextCurrent( window );
    glfwSwapInterval( 1 );
    gladLoadGLES2Loader( (GLADloadproc)glfwGetProcAddress );

    const GLuint prog = Program::Load( vert, GL_VERTEX_SHADER, frag, GL_FRAGMENT_SHADER, NULL );
    glUseProgram( prog );

    // init framebuffer attachments
    std::vector< GLuint > textures( 4, 0 );
    glGenTextures( 4, textures.data() );
    for( size_t i = 0; i < textures.size(); ++ i )
    {
        glBindTexture( GL_TEXTURE_2D, textures[i] );
        glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
        glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
        glTexImage2D( GL_TEXTURE_2D, 0, GL_RGBA, 32, 32, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr );
    }
    GLuint rbDepth = 0;
    glGenRenderbuffers(1, &rbDepth );
    glBindRenderbuffer( GL_RENDERBUFFER, rbDepth );
    glRenderbufferStorage( GL_RENDERBUFFER, GL_DEPTH24_STENCIL8, 32, 32 );

    // init FBO
    GLuint fbo = 0;
    glGenFramebuffers( 1, &fbo );
    glBindFramebuffer( GL_FRAMEBUFFER, fbo );
    for( size_t i = 0; i < textures.size(); ++ i )
    {
        glFramebufferTexture2D( GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + i, GL_TEXTURE_2D, textures[i], 0 );
    }
    glFramebufferRenderbuffer( GL_DRAW_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_RENDERBUFFER, rbDepth );
    if( GL_FRAMEBUFFER_COMPLETE != glCheckFramebufferStatus( GL_FRAMEBUFFER ) )
    {
        std::cerr << "Incomplete framebuffer" << std::endl;
        std::exit( EXIT_FAILURE );
    }

    while( !glfwWindowShouldClose( window ) )
    {
        glfwPollEvents();

        // render to FBO
        glBindFramebuffer( GL_FRAMEBUFFER, fbo );
        GLenum bufs[] =
        {
            GL_COLOR_ATTACHMENT0 + 0,
            GL_COLOR_ATTACHMENT0 + 1,
            GL_COLOR_ATTACHMENT0 + 2,
            GL_COLOR_ATTACHMENT0 + 3,
        };
        glDrawBuffers( 4, bufs );
        glViewport( 0, 0, 32, 32 );
        glClearColor( 0.0f, 0.6f, 1.0f, 1.f );
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
        glDrawArrays( GL_TRIANGLES, 0, 3 );

        // switch back to default framebuffer & clear it with non-black color
        glBindFramebuffer( GL_FRAMEBUFFER, 0 );
        GLenum defaultBuf = GL_BACK;
        glDrawBuffers( 1, &defaultBuf );
        glClearColor( 1.0f, 0.0f, 1.0f, 1.f );
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

        // blit a color attachment to the default framebuffer
        glBindFramebuffer( GL_READ_FRAMEBUFFER, fbo );
        glReadBuffer( GL_COLOR_ATTACHMENT0 + 2 );
        glBindFramebuffer( GL_DRAW_FRAMEBUFFER, 0 );
        glBlitFramebuffer( 0, 0, 32, 32, 0, 0, 640, 480, GL_COLOR_BUFFER_BIT, GL_LINEAR );

        glfwSwapBuffers( window );
    }

    glfwMakeContextCurrent( NULL );
    glfwDestroyWindow( window );

    glfwTerminate();
    return EXIT_SUCCESS;
}

On this Windows 10 machine using a recent-ish ANGLE build I get the blue clear color but no triangle ("undefined") for the 3rd attachment:

The first attachment is fine:

Answer 1

No such functionality exists in the API :

3) Should we support broadcast from gl_FragColor to all gl_FragData[x] or should it be synonymous with gl_FragData[0]?

DISCUSSION: With NV_draw_buffers, writing to gl_FragColor writes to all the enabled draw buffers (ie broadcast). In OpenGL ES 3.0 when using ESSL 1.0, gl_FragColor is equivalent to writing a single output to gl_FragData[0] and multiple outputs are not possible. When using ESSL 3.0, only user-defined out variables may be used.

If broadcast is supported, some implementations may have to replace writes to gl_FragColor with replicated writes to all possible gl_FragData locations when this extension is enabled.

RESOLVED: Writes to gl_FragColor are broadcast to all enabled color buffers. ES 3.0 using ESSL 1.0 doesn't support broadcast because ESSL 1.0 was not extended to have multiple color outputs (but that is what this extension adds). ESSL 3.0 doesn't support the broadcast because it doesn't have the gl_FragColor variable at all, and only has user- defined out variables. This extension extends ESSL 1.0 to have multiple color outputs. Broadcasting from gl_FragColor to all enabled color buffers is the most consistent with existing draw buffer extensions to date (both NV_draw_buffers and desktop GL).