1) Do I understand correctly that to draw using vertex arrays or VBOs I need for all my attributes to either call glBindAttribLocation before the shader program linkage or call glGetAttribLocation after the shader program was successfully linked and then use the bound/obtained index in the glVertexAttribPointer and glEnableVertexAttribArray calls?
To be more specific: these three functions - glGetAttribLocation, glVertexAttribPointer and glEnableVertexAttribArray - they all have an input parameter named "index". Is it the same "index" for all the three? And is it the same thing as the one returned by glGetAttribLocation?
If yes: 2) I've been facing a problem on OS X, I described it here: https://stackoverflow.com/questions/28093919/using-default-attribute-location-doesnt-work-on-osx-osx-opengl-bug , but unfortunately didn't get any replies.
The problem is that depending on what attribute locations I bind to my attributes I do or do not see anything on the screen. I only see this behavior on my MacBook Pro with OS X 10.9.5; I've tried running the same code on Linux and Windows and it seems to work on those platforms independently from which locations are my attributes bound to.
Here is a code example (which is supposed to draw a red triangle on the screen) that exhibits the problem:
#include <iostream>
#include <GLFW/glfw3.h>
GLuint global_program_object;
GLint global_position_location;
GLint global_aspect_ratio_location;
GLuint global_buffer_names[1];
int LoadShader(GLenum type, const char *shader_source)
{
GLuint shader;
GLint compiled;
shader = glCreateShader(type);
if (shader == 0)
return 0;
glShaderSource(shader, 1, &shader_source, NULL);
glCompileShader(shader);
glGetShaderiv(shader, GL_COMPILE_STATUS, &compiled);
if (!compiled)
{
GLint info_len = 0;
glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &info_len);
if (info_len > 1)
{
char* info_log = new char[info_len];
glGetShaderInfoLog(shader, info_len, NULL, info_log);
std::cout << "Error compiling shader" << info_log << std::endl;
delete info_log;
}
glDeleteShader(shader);
return 0;
}
return shader;
}
int InitGL()
{
char vertex_shader_source[] =
"attribute vec4 att_position; \n"
"attribute float dummy;\n"
"uniform float uni_aspect_ratio; \n"
"void main() \n"
" { \n"
" vec4 test = att_position * dummy;\n"
" mat4 mat_projection = \n"
" mat4(1.0 / uni_aspect_ratio, 0.0, 0.0, 0.0, \n"
" 0.0, 1.0, 0.0, 0.0, \n"
" 0.0, 0.0, -1.0, 0.0, \n"
" 0.0, 0.0, 0.0, 1.0); \n"
" gl_Position = att_position; \n"
" gl_Position *= mat_projection; \n"
" } \n";
char fragment_shader_source[] =
"void main() \n"
" { \n"
" gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0); \n"
" } \n";
GLuint vertex_shader;
GLuint fragment_shader;
GLuint program_object;
GLint linked;
vertex_shader = LoadShader(GL_VERTEX_SHADER , vertex_shader_source );
fragment_shader = LoadShader(GL_FRAGMENT_SHADER, fragment_shader_source);
program_object = glCreateProgram();
if(program_object == 0)
return 1;
glAttachShader(program_object, vertex_shader );
glAttachShader(program_object, fragment_shader);
// Here any index except 0 results in observing the black screen
glBindAttribLocation(program_object, 1, "att_position");
glLinkProgram(program_object);
glGetProgramiv(program_object, GL_LINK_STATUS, &linked);
if(!linked)
{
GLint info_len = 0;
glGetProgramiv(program_object, GL_INFO_LOG_LENGTH, &info_len);
if(info_len > 1)
{
char* info_log = new char[info_len];
glGetProgramInfoLog(program_object, info_len, NULL, info_log);
std::cout << "Error linking program" << info_log << std::endl;
delete info_log;
}
glDeleteProgram(program_object);
return 1;
}
global_program_object = program_object;
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glUseProgram(global_program_object);
global_position_location = glGetAttribLocation (global_program_object, "att_position");
global_aspect_ratio_location = glGetUniformLocation(global_program_object, "uni_aspect_ratio");
GLfloat vertices[] = {-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 0.0f,
0.0f, 0.5f, 0.0f};
glGenBuffers(1, global_buffer_names);
glBindBuffer(GL_ARRAY_BUFFER, global_buffer_names[0]);
glBufferData(GL_ARRAY_BUFFER, sizeof(GLfloat) * 9, vertices, GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);
return 0;
}
void Render()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
glUseProgram(global_program_object);
glBindBuffer(GL_ARRAY_BUFFER, global_buffer_names[0]);
glVertexAttribPointer(global_position_location, 3, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(global_position_location);
glDrawArrays(GL_TRIANGLES, 0, 3);
glDisableVertexAttribArray(global_position_location);
glUseProgram(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
}
void FreeGL()
{
glDeleteBuffers(1, global_buffer_names);
glDeleteProgram(global_program_object);
}
void SetViewport(int width, int height)
{
glViewport(0, 0, width, height);
glUseProgram(global_program_object);
glUniform1f(global_aspect_ratio_location, static_cast<GLfloat>(width) / static_cast<GLfloat>(height));
}
int main(void)
{
GLFWwindow* window;
if (!glfwInit())
return -1;
window = glfwCreateWindow(640, 480, "Hello World", NULL, NULL);
if (!window)
{
glfwTerminate();
return -1;
}
glfwMakeContextCurrent(window);
InitGL();
// Double the resolution to correctly draw with Retina display
SetViewport(1280, 960);
while (!glfwWindowShouldClose(window))
{
Render();
glfwSwapBuffers(window);
glfwPollEvents();
}
FreeGL();
glfwTerminate();
return 0;
}
Does this look like a bug to you? Can anyone reproduce it? If it's a bug where should I report it?
P.S. I've also tried SDL instead of GLFW, the behavior is the same...
