Re: [Qemu-devel] [PATCH 1/9] shaders: initialize vertexes once

[Max Reitz]

On 14.09.2015 18:00, Max Reitz wrote:
> On 09.09.2015 13:20, Gerd Hoffmann wrote:
>> Create a buffer for the vertex data and place vertexes
>> there at initialization time.  Then just use the buffer
>> for each texture blit.
>>
>> Signed-off-by: Gerd Hoffmann <address@hidden>
>> ---
>>  include/ui/shader.h |  4 +++-
>>  ui/console-gl.c     |  7 ++++++-
>>  ui/shader.c         | 32 +++++++++++++++++++++++++++-----
>>  3 files changed, 36 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/ui/shader.h b/include/ui/shader.h
>> index 8509596..f7d8618 100644
>> --- a/include/ui/shader.h
>> +++ b/include/ui/shader.h
>> @@ -3,7 +3,9 @@
>>  
>>  #include <epoxy/gl.h>
>>  
>> -void qemu_gl_run_texture_blit(GLint texture_blit_prog);
>> +GLuint qemu_gl_init_texture_blit(GLint texture_blit_prog);
>> +void qemu_gl_run_texture_blit(GLint texture_blit_prog,
>> +                              GLint texture_blit_vao);
>>  
>>  GLuint qemu_gl_create_compile_shader(GLenum type, const GLchar *src);
>>  GLuint qemu_gl_create_link_program(GLuint vert, GLuint frag);
>> diff --git a/ui/console-gl.c b/ui/console-gl.c
>> index cb45cf8..baf397b 100644
>> --- a/ui/console-gl.c
>> +++ b/ui/console-gl.c
>> @@ -33,6 +33,7 @@
>>  
>>  struct ConsoleGLState {
>>      GLint texture_blit_prog;
>> +    GLint texture_blit_vao;
>>  };
>>  
>>  /* ---------------------------------------------------------------------- */
>> @@ -47,6 +48,9 @@ ConsoleGLState *console_gl_init_context(void)
>>          exit(1);
>>      }
>>  
>> +    gls->texture_blit_vao =
>> +        qemu_gl_init_texture_blit(gls->texture_blit_prog);
>> +
>>      return gls;
>>  }
>>  
>> @@ -131,7 +135,8 @@ void surface_gl_render_texture(ConsoleGLState *gls,
>>      glClearColor(0.1f, 0.1f, 0.1f, 0.0f);
>>      glClear(GL_COLOR_BUFFER_BIT);
>>  
>> -    qemu_gl_run_texture_blit(gls->texture_blit_prog);
>> +    qemu_gl_run_texture_blit(gls->texture_blit_prog,
>> +                             gls->texture_blit_vao);
>>  }
>>  
>>  void surface_gl_destroy_texture(ConsoleGLState *gls,
>> diff --git a/ui/shader.c b/ui/shader.c
>> index 52a4632..ada1c4c 100644
>> --- a/ui/shader.c
>> +++ b/ui/shader.c
>> @@ -29,21 +29,43 @@
>>  
>>  /* ---------------------------------------------------------------------- */
>>  
>> -void qemu_gl_run_texture_blit(GLint texture_blit_prog)
>> +GLuint qemu_gl_init_texture_blit(GLint texture_blit_prog)
>>  {
>> -    GLfloat in_position[] = {
>> +    static const GLfloat in_position[] = {
>>          -1, -1,
>>          1,  -1,
>>          -1,  1,
>>          1,   1,
>>      };
>>      GLint l_position;
>> +    GLuint vao, buffer;
>> +
>> +    glGenVertexArrays(1, &vao);
>> +    glBindVertexArray(vao);
>> +
>> +    /* this is the VBO that holds the vertex data */
>> +    glGenBuffers(1, &buffer);
>> +    glBindBuffer(GL_ARRAY_BUFFER, buffer);
>> +    glBufferData(GL_ARRAY_BUFFER, sizeof(in_position), in_position,
>> +                 GL_STATIC_DRAW);
>>  
>> -    glUseProgram(texture_blit_prog);
>>      l_position = glGetAttribLocation(texture_blit_prog, "in_position");
>> -    glVertexAttribPointer(l_position, 2, GL_FLOAT, GL_FALSE, 0, 
>> in_position);
>> +    glVertexAttribPointer(l_position, 2, GL_FLOAT, GL_FALSE, 0, 0);
>>      glEnableVertexAttribArray(l_position);
>> -    glDrawArrays(GL_TRIANGLE_STRIP, l_position, 4);
>> +
>> +    glBindBuffer (GL_ARRAY_BUFFER, 0);
>> +    glBindVertexArray (0);
>> +    glDeleteBuffers (1, &buffer);
> 
> Although I may be wrong, I don't think glVertexAttribPointer() loads the
> buffer data into the given vertex attribute.
> 
> As far as I can see from the specification regarding vertex array
> objects, they only represent which data is to be used for vertex
> attributes ("All state related to the definition of data used by the
> vertex processor is encapsulated in a vertex array object.").
> 
> glVertexAttribPointer():
> (1) determines the attribute's format,
> (2) binds a vertex attribute index to a shader attribute index,
> (3) binds the data to the vertex attribute.
> 
> Step (3) (glBindVertexBuffer()) does not appear to load the data, but
> only define its origin ("The source of data for a generic vertex
> attribute may be determined by attaching a buffer object to a vertex
> array object with [glBindVertexBuffer()]").
> 
> Therefore, I'm not sure whether deleting the buffer is right here. Maybe
> OpenGL uses reference counting here, too, so it remembers that the
> buffer is still in use by the VAO, and so the glDeleteBuffers()
> operation will only decrease its refcount, but not actually end up
> deleting it. But I don't think so (compare the documentation of
> glDeleteBuffers() with glDeleteShader(); the former says it will delete
> the buffer, whereas the latter explicitly mentions that the shader will
> not be deleted as long as it is attached to a program).
> 
> All in all I don't think we should delete the buffer as long as it is in
> use by the VAO.

Oh, and I just noticed: With glDeleteBuffers(), I get a segmentation
fault when qemu exits (somewhere deep in fglrx_dri.so). Without, the
segfault disappears.

Max

>> +
>> +    return vao;
>> +}
>> +
>> +void qemu_gl_run_texture_blit(GLint texture_blit_prog,
>> +                              GLint texture_blit_vao)
>> +{
>> +    glUseProgram(texture_blit_prog);
>> +    glBindVertexArray(texture_blit_vao);
>> +    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
>>  }
>>  
>>  /* ---------------------------------------------------------------------- */
>>
> 
>

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