A shader is a computer program that is used to do image processing such as special effects, color effects, lighting, and, well, shading. The position, brightness, contrast, hue, and other effects on all pixels, vertices, or textures used to produce the final image on the screen can be altered during runtime, using algorithms constructed in the shader program(s). These days, most shader programs are built to run directly on the Graphical Processing Unit (GPU). In this article, we are going to get acquainted with shaders and implement our own shader infrastructure for the example engine.
Shader programs are executed in parallel. This means, for example, that a shader might be executed once per pixel, with each of the executions running simultaneously on different threads on the GPU. The amount of simultaneous threads depends on the graphics card specific GPU, with modern cards sporting processors in the thousands. This all means that shader programs can be very performant and provide developers with lots of creative flexibility.
The following article is a part of the book Mastering C++ game Development written by Mickey Macdonald. With this book, you can create advanced games with C++.
Shader languages
With advances in graphics card technology, more flexibility has been added to the rendering pipeline. Where at one time developers had little control over concepts such as fixed-function pipeline rendering, new advancements have allowed programmers to take deeper control of graphics hardware for rendering their creations. Originally, this deeper control was achieved by writing shaders in assembly language, which was a complex and cumbersome task. It wasn’t long before developers yearned for a better solution. Enter the shader programming languages. Let’s take a brief look at a few of the more common languages in use.
C for graphics (Cg) is a shading language originally developed by the Nvidia graphics company. Cg is based on the C programming language and, although they share the same syntax, some features of C were modified and new data types were added to make Cg more suitable for programming GPUs. Cg compilers can output shader programs supported by both DirectX and OpenGL. While Cg was mostly deprecated, it has seen a resurgence in a new form with its use in the Unity game engine.
High-Level Shading Language (HLSL) is a shading language developed by the Microsoft Corporation for use with the DirectX graphics API. HLSL is again modeled after the C programming language and shares many similarities to the Cg shading language. HLSL is still in development and continues to be the shading language of choice for DirectX. Since the release, DirectX 12 the HLSL language supports even lower level hardware control and has seen dramatic performance improvements.
OpenGL Shading Language (GLSL) is a shading language that is also based on the C programming language. It was created by the OpenGL Architecture Review Board (OpenGL ARB) to give developers more direct control of the graphics pipeline without having to use ARB assembly language or other hardware-specific languages. The language is still in open development and will be the language we will focus on in our examples.
Building a shader program infrastructure
Most modern shader programs are composed of up to five different types of shader files: fragment or pixel shaders, vertex shaders, geometry shaders, compute shaders, and tessellation shaders. When building a shader program, each of these shader files must be compiled and linked together for use, much like how a C++ program is compiled and linked. Next, we are going to walk you through how this process works and see how we can build an infrastructure to allow for easier interaction with our shader programs.
To get started, let’s look at how we compile a GLSL shader. The GLSL compiler is part of the OpenGL library itself, and our shaders can be compiled within an OpenGL program. We are going to build an architecture to support this internal compilation. The whole process of compiling a shader can be broken down into some simple steps. First, we have to create a shader object, then provide the source code to the shader object. We can then ask the shader object to be compiled. These steps can be represented in the following three basic calls to the OpenGL API.
First, we create the shader object:
GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
We create the shader object using the glCreateShader() function. The argument we pass in is the type of shader we are trying to create. The types of shaders can be GL_VERTEX_SHADER, GL_FRAGMENT_SHADER, GL_GEOMETRY_SHADER, GL_TESS_EVALUATION_SHADER, GL_TESS_CONTROL_SHADER, or GL_COMPUTE_SHADER. In our example case, we are trying to compile a vertex shader, so we use the GL_VERTEX_SHADER type.
Next, we copy the shader source code into the shader object:
GLchar* shaderCode = LoadShader("shaders/simple.vert");
glShaderSource(vertexShader, 1, shaderCode, NULL);
Here we are using the glShaderSource() function to load our shader source to memory. This function accepts an array of strings, so before we call glShaderSource(), we create a pointer to the start of the shaderCode array object using a still-to-be-created method. The first argument to glShaderSource() is the handle to the shader object. The second is the number of source code strings that are contained in the array. The third argument is a pointer to an array of source code strings. The final argument is an array of GLint values that contains the length of each source code string in the previous argument.
Finally, we compile the shader:
glCompileShader(vertexShader);
The last step is to compile the shader. We do this by calling the OpenGL API method, glCompileShader(), and passing the handle to the shader that we want compiled.
Of course, because we are using memory to store the shaders, we should know how to clean up when we are done. To delete a shader object, we can call the glDeleteShader() function.
Deleting a Shader ObjectShader objects can be deleted when no longer needed by calling glDeleteShader(). This frees the memory used by the shader object. It should be noted that if a shader object is already attached to a program object, as in linked to a shader program, it will not be immediately deleted, but rather flagged for deletion. If the object is flagged for deletion, it will be deleted when it is detached from the linked shader program object.
Once we have compiled our shaders, the next step we need to take before we can use them in our program is to link them together into a complete shader program. One of the core aspects of the linking step involves making the connections between input variables from one shader to the output variables of another and making the connections between the input/output variables of a shader to appropriate locations in the OpenGL program itself.
Linking is much like compiling the shader. We create a new shader program and attach each shader object to it. We then tell the shader program object to link everything together. The steps to accomplish this in the OpenGL environment can be broken down into a few calls to the API, as follows:
First, we create the shader program object:
GLuint shaderProgram = glCreateProgram();
To start, we call the glCreateProgram() method to create an empty program object. This function returns a handle to the shader program object which, in this example, we are storing in a variable named shaderProgram.
Next, we attach the shaders to the program object:
glAttachShader(shaderProgram, vertexShader); glAttachShader(shaderProgram, fragmentShader);
To load each of the shaders into the shader program, we use the glAttachShader() method. This method takes two arguments. The first argument is the handle to the shader program object, and the second is the handle to the shader object to be attached to the shader program.
Finally, we link the program:
glLinkProgram(programHandle);
When we are ready to link the shaders together we call the glLinkProgram() method. This method has only one argument: the handle to the shader program we want to link.
It’s important that we remember to clean up any shader programs that we are not using anymore. To remove a shader program from the OpenGL memory, we call glDeleteProgram() method. The glDeleteProgram() method takes one argument: the handle to the shader program that is to be deleted. This method call invalidates the handle and frees the memory used by the shader program. It is important to note that if the shader program object is currently in use, it will not be immediately deleted, but rather flagged for deletion.
This is similar to the deletion of shader objects. It is also important to note that the deletion of a shader program will detach any shader objects that were attached to the shader program at linking time. This, however, does mean the shader object will be deleted immediately unless those shader objects have already been flagged for deletion by a previous call to the glDeleteShader() method.
So those are the simplified OpenGL API calls required to create, compile, and link shader programs. Now we are going to move onto implementing some structure to make the whole process much easier to work with. To do this, we are going to create a new class called ShaderManager. This class will act as the interface for compiling, linking, and managing the cleanup of shader programs.
To start with, let’s look at the implementation of the CompileShaders() method in the ShaderManager.cpp file. I should note that I will be focusing on the important aspects of the code that pertain to the implementation of the architecture. The full source code for this chapter can be found in the Chapter07 folder in the GitHub repository.
void ShaderManager::CompileShaders(const std::string&
vertexShaderFilePath, const std::string&
fragmentShaderFilepath)
{
m_programID = glCreateProgram();
m_vertexShaderID = glCreateShader(GL_VERTEX_SHADER);
if (m_vertexShaderID == 0){
Exception("Vertex shader failed to be created!");
}
m_fragmentShaderID = glCreateShader(GL_FRAGMENT_SHADER);
if (m_fragmentShaderID == 0){
Exception("Fragment shader failed to be created!");
}
CompileShader(vertexShaderFilePath, m_vertexShaderID);
CompileShader(fragmentShaderFilepath, m_fragmentShaderID);
}
To begin, for this example we are focusing on two of the shader types, so our ShaderManager::CompileShaders() method accepts two arguments. The first argument is the file path location of the vertex shader file, and the second is the file path location to the fragment shader file. Both are strings. Inside the method body, we first create the shader program handle using the glCreateProgram() method and store it in the m_programID variable.
Next, we create the handles for the vertex and fragment shaders using the glCreateShader() command. We check for any errors when creating the shader handles, and if we find any we throw an exception with the shader name that failed. Once the handles have been created, we then call the CompileShader() method, which we will look at next. The CompileShader() function takes two arguments: the first is the path to the shader file, and the second is the handle in which the compiled shader will be stored.
The following is the full CompileShader() function. It handles the look and loading of the shader file from storage, as well as calling the OpenGL compile command on the shader file. We will break it down chunk by chunk:
void ShaderManager::CompileShader(const std::string& filePath, GLuint id)
{
std::ifstream shaderFile(filePath);
if (shaderFile.fail()){
perror(filePath.c_str());
Exception("Failed to open " + filePath);
}
//File contents stores all the text in the file
std::string fileContents = "";
//line is used to grab each line of the file
std::string line;
//Get all the lines in the file and add it to the contents
while (std::getline(shaderFile, line)){
fileContents += line + "n";
}
shaderFile.close();
//get a pointer to our file contents c string
const char* contentsPtr = fileContents.c_str(); //tell opengl that
we want to use fileContents as the contents of the shader file
glShaderSource(id, 1, &contentsPtr, nullptr);
//compile the shader
glCompileShader(id);
//check for errors
GLint success = 0;
glGetShaderiv(id, GL_COMPILE_STATUS, &success);
if (success == GL_FALSE){
GLint maxLength = 0;
glGetShaderiv(id, GL_INFO_LOG_LENGTH, &maxLength);
//The maxLength includes the NULL character
std::vector<char> errorLog(maxLength);
glGetShaderInfoLog(id, maxLength, &maxLength, &errorLog[0]);
//Provide the infolog in whatever manor you deem best.
//Exit with failure.
glDeleteShader(id); //Don't leak the shader.
//Print error log and quit
std::printf("%sn", &(errorLog[0]));
Exception("Shader " + filePath + " failed to compile");
}
}
To start the function, we first use an ifstream object to open the file with the shader code in it. We also check to see if there were any issues loading the file and if, there were, we throw an exception notifying us that the file failed to open:
std::ifstream shaderFile(filePath);
if (shaderFile.fail()) {
perror(filePath.c_str());
Exception("Failed to open " + filePath);
}
Next, we need to parse the shader. To do this, we create a string variable called fileContents that will hold the text in the shader file. We then create another string variable named line; this will be a temporary holder for each line of the shader file we are trying to parse. Next, we use a while loop to step through the shader file, parsing the contents line by line and saving each loop into the fileContents string. Once all the lines have been read into the holder variable, we call the close method on the shaderFile ifstream object to free up the memory used to read the file:
std::string fileContents = "";
std::string line;
while (std::getline(shaderFile, line)) {
fileContents += line + "n";
}
shaderFile.close();
You might remember from earlier in the chapter that I mentioned that when we are using the glShaderSource() function, we have to pass the shader file text as a pointer to the start of a character array. In order to meet this requirement, we are going to use a neat trick where we use the C string conversation method built into the string class to allow us to pass back a pointer to the start of our shader character array. This, in case you are unfamiliar, is essentially what a string is:
const char* contentsPtr = fileContents.c_str();
Now that we have a pointer to the shader text, we can call the glShaderSource() method to tell OpenGL that we want to use the contents of the file to compile our shader. Then, finally, we call the glCompileShader() method with the handle to the shader as the argument:
glShaderSource(id, 1, &contentsPtr, nullptr); glCompileShader(id);
That handles the compilation, but it is a good idea to provide ourselves with some debug support. We implement this compilation debug support by closing out the CompileShader() function by first checking to see if there were any errors during the compilation process. We do this by requesting information from the shader compiler through glGetShaderiv() function, which, among its arguments, takes an enumerated value that specifies what information we would like returned. In this call, we are requesting the compile status:
GLint success = 0; glGetShaderiv(id, GL_COMPILE_STATUS, &success);
Next, we check to see if the returned value is GL_FALSE, and if it is, that means we have had an error and should ask the compiler for more information about the compile issues. We do this by first asking the compiler what the max length of the error log is. We use this max length value to then create a vector of character values called errorLog. Then we can request the shader compile log by using the glGetShaderInfoLog() method, passing in the handle to the shader file the number of characters we are pulling, and where we want to save the log:
if (success == GL_FALSE){
GLint maxLength = 0;
glGetShaderiv(id, GL_INFO_LOG_LENGTH, &maxLength);
std::vector<char> errorLog(maxLength);
glGetShaderInfoLog(id, maxLength, &maxLength, &errorLog[0]);
Once we have the log file saved, we go ahead and delete the shader using the glDeleteShader() method. This ensures we don’t have any memory leaks from our shader:
glDeleteShader(id);
Finally, we first print the error log to the console window. This is great for runtime debugging. We also throw an exception with the shader name/file path, and the message that it failed to compile:
std::printf("%sn", &(errorLog[0]));
Exception("Shader " + filePath + " failed to compile");
}
...
That really simplifies the process of compiling our shaders by providing a simple interface to the underlying API calls. Now, in our example program, to load and compile our shaders we use a simple line of code similar to the following:
shaderManager.CompileShaders("Shaders/SimpleShader.vert",
"Shaders/SimpleShader.frag");
Having now compiled the shaders, we are halfway to a useable shader program. We still need to add one more piece, linking. To abstract away some of the processes of linking the shaders and to provide us with some debugging capabilities, we are going to create the LinkShaders() method for our ShaderManager class. Let’s take a look and then break it down:
void ShaderManager::LinkShaders() {
//Attach our shaders to our program
glAttachShader(m_programID, m_vertexShaderID);
glAttachShader(m_programID, m_fragmentShaderID);
//Link our program
glLinkProgram(m_programID);
//Note the different functions here: glGetProgram* instead of glGetShader*.
GLint isLinked = 0;
glGetProgramiv(m_programID, GL_LINK_STATUS, (int *)&isLinked);
if (isLinked == GL_FALSE){
GLint maxLength = 0;
glGetProgramiv(m_programID, GL_INFO_LOG_LENGTH, &maxLength);
//The maxLength includes the NULL character
std::vector<char> errorLog(maxLength);
glGetProgramInfoLog(m_programID, maxLength, &maxLength,
&errorLog[0]);
//We don't need the program anymore.
glDeleteProgram(m_programID);
//Don't leak shaders either.
glDeleteShader(m_vertexShaderID);
glDeleteShader(m_fragmentShaderID);
//print the error log and quit
std::printf("%sn", &(errorLog[0]));
Exception("Shaders failed to link!");
}
//Always detach shaders after a successful link.
glDetachShader(m_programID, m_vertexShaderID);
glDetachShader(m_programID, m_fragmentShaderID);
glDeleteShader(m_vertexShaderID);
glDeleteShader(m_fragmentShaderID);
}
To start our LinkShaders() function, we call the glAttachShader() method twice, using the handle to the previously created shader program object, and the handle to each shader we wish to link, respectively:
glAttachShader(m_programID, m_vertexShaderID); glAttachShader(m_programID, m_fragmentShaderID);
Next, we perform the actual linking of the shaders into a usable shader program by calling the glLinkProgram() method, using the handle to the program object as its argument:
glLinkProgram(m_programID);
We can then check to see if the linking process has completed without any errors and provide ourselves with any debug information that we might need if there were any errors. I am not going to go through this code chunk line by line since it is nearly identical to what we did with the CompileShader() function. Do note, however, that the function to return the information from the linker is slightly different and uses glGetProgram* instead of the glGetShader* functions from before:
GLint isLinked = 0;
glGetProgramiv(m_programID, GL_LINK_STATUS, (int *)&isLinked);
if (isLinked == GL_FALSE){
GLint maxLength = 0;
glGetProgramiv(m_programID, GL_INFO_LOG_LENGTH, &maxLength);
//The maxLength includes the NULL character
std::vector<char> errorLog(maxLength);
glGetProgramInfoLog(m_programID, maxLength, &maxLength,
&errorLog[0]);
//We don't need the program anymore.
glDeleteProgram(m_programID);
//Don't leak shaders either.
glDeleteShader(m_vertexShaderID);
glDeleteShader(m_fragmentShaderID);
//print the error log and quit
std::printf("%sn", &(errorLog[0]));
Exception("Shaders failed to link!");
}
Lastly, if we are successful in the linking process, we need to clean it up a bit. First, we detach the shaders from the linker using the glDetachShader() method. Next, since we have a completed shader program, we no longer need to keep the shaders in memory, so we delete each shader with a call to the glDeleteShader() method. Again, this will ensure we do not leak any memory in our shader program creation process:
glDetachShader(m_programID, m_vertexShaderID); glDetachShader(m_programID, m_fragmentShaderID); glDeleteShader(m_vertexShaderID); glDeleteShader(m_fragmentShaderID); }
We now have a simplified way of linking our shaders into a working shader program. We can call this interface to the underlying API calls by simply using one line of code, similar to the following one:
shaderManager.LinkShaders();
So that handles the process of compiling and linking our shaders, but there is another key aspect to working with shaders, which is the passing of data to and from the running program/the game and the shader programs running on the GPU. We will look at this process and how we can abstract it into an easy-to-use interface for our engine next.
Working with shader data
One of the most important aspects of working with shaders is the ability to pass data to and from the shader programs running on the GPU. This can be a deep topic, and much like other topics in this book has had its own dedicated books. We are going to stay at a higher level when discussing this topic and again will focus on the two needed shader types for basic rendering: the vertex and fragment shaders.
To begin with, let’s take a look at how we send data to a shader using the vertex attributes and Vertex Buffer Objects (VBO). A vertex shader has the job of processing the data that is connected to the vertex, doing any modifications, and then passing it to the next stage of the rendering pipeline. This occurs once per vertex. In order for the shader to do its thing, we need to be able to pass it data. To do this, we use what are called vertex attributes, and they usually work hand in hand with what is referred to as VBO.
For the vertex shader, all per-vertex input attributes are defined using the keyword in. So, for example, if we wanted to define a vector 3 input attribute named VertexColour, we could write something like the following:
in vec3 VertexColour;
Now, the data for the VertexColour attribute has to be supplied by the program/game. This is where VBO come in. In our main game or program, we make the connection between the input attribute and the vertex buffer object, and we also have to define how to parse or step through the data. That way, when we render, the OpenGL can pull data for the attribute from the buffer for each call of the vertex shader.
Let’s take a look a very simple vertex shader:
#version 410
in vec3 VertexPosition;
in vec3 VertexColour;
out vec3 Colour;
void main(){
Colour = VertexColour;
gl_Position = vec4(VertexPosition, 1.0);
}
In this example, there are just two input variables for this vertex shader, VertexPosition and VertexColor. Our main OpenGL program needs to supply the data for these two attributes for each vertex. We will do so by mapping our polygon/mesh data to these variables. We also have one output variable named Colour, which will be sent to the next stage of the rendering pipeline, the fragment shader. In this example, Colour is just an untouched copy of VertexColour. The VertexPosition attribute is simply expanded and passed along to the OpenGL API output variable gl_Position for more processing.
Next, let’s take a look at a very simple fragment shader:
#version 410
in vec3 Colour;
out vec4 FragColour;
void main(){
FragColour = vec4(Colour, 1.0);
}
In this fragment shader example, there is only one input attribute, Colour. This input corresponds to the output of the previous rendering stage, the vertex shader’s Colour output. For simplicity’s sake, we are just expanding the Colour and outputting it as the variable FragColour for the next rendering stage.
That sums up the shader side of the connection, so how do we compose and send the data from inside our engine? We can accomplish this in basically four steps.
First, we create a Vertex Array Object (VAO) instance to hold our data:
GLunit vao;
Next, we create and populate the VBO for each of the shaders’ input attributes. We do this by first creating a VBO variable, then, using the glGenBuffers() method, we generate the memory for the buffer objects. We then create handles to the different attributes we need buffers for, assigning them to elements in the VBO array. Finally, we populate the buffers for each attribute by first calling the glBindBuffer() method, specifying the type of object being stored. In this case, it is a GL_ARRAY_BUFFER for both attributes.
Then we call the glBufferData() method, passing the type, size, and handle to bind. The last argument for the glBufferData() method is one that gives OpenGL a hint about how the data will be used so that it can determine how best to manage the buffer internally. For full details about this argument, take a look at the OpenGL documentation:
GLuint vbo[2];
glGenBuffers(2, vbo);
GLuint positionBufferHandle = vbo[0];
GLuint colorBufferHandle = vbo[1];
glBindBuffer(GL_ARRAY_BUFFER,positionBufferHandle);
glBufferData(GL_ARRAY_BUFFER,
9 * sizeof(float),
positionData,
GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER,
colorBufferHandle);
glBufferData(GL_ARRAY_BUFFER,
9 * sizeof(float),
colorData,
GL_STATIC_DRAW);
The third step is to create and define the VAO. This is how we will define the relationship between the input attributes of the shader and the buffers we just created. The VAO contains this information about the connections. To create a VAO, we use the glGenVertexArrays() method. This gives us a handle to our new object, which we store in our previously created VAO variable.
Then, we enable the generic vertex attribute indexes 0 and 1 by calling the glEnableVertexAttribArray() method. By making the call to enable the attributes, we are specifying that they will be accessed and used for rendering. The last step makes the connection between the buffer objects we have created and the generic vertex attribute indexes the match too:
glGenVertexArrays( 1, &vao ); glBindVertexArray(vao); glEnableVertexAttribArray(0); glEnableVertexAttribArray(1); glBindBuffer(GL_ARRAY_BUFFER, positionBufferHandle); glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, NULL); glBindBuffer(GL_ARRAY_BUFFER, colorBufferHandle); glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 0, NULL);
Finally, in our Draw() function call, we bind to the VAO and call glDrawArrays() to perform the actual render:
glBindVertexArray(vaoHandle);glDrawArrays(GL_TRIANGLES, 0, 3 );
Before we move on to another way to pass data to the shader, there is one more piece of this attribute connection structure we need to discuss. As mentioned, the input variables in a shader are linked to the generic vertex attribute we just saw, at the time of linking. When we need to specify the relationship structure, we have a few different choices. We can use what are known as layout qualifiers within the shader code itself. The following is an example:
layout (location=0) in vec3 VertexPosition;
Another choice is to just let the linker create the mapping when linking, and then query for them afterward. The third and the one I personally prefer is to specify the relationship prior to the linking process by making a call to the glBindAttribLocation() method. We will see how this is implemented shortly when we discuss how to abstract these processes.
We have described how we can pass data to a shader using attributes, but there is another option: uniform variables. Uniform variables are specifically used for data that changes infrequently. For example, matrices are great candidates for uniform variables. Within a shader, a uniform variable is read-only. That means the value can only be changed from outside the shader.
They can also appear in multiple shaders within the same shader program. They can be declared in one or more shaders within a program, but if a variable with a given name is declared in more than one shader, its type must be the same in all shaders. This gives us insight into the fact that the uniform variables are actually held in a shared namespace for the whole of the shader program.
To use a uniform variable in your shader, you first have to declare it in the shader file using the uniform identifier keyword. The following is what this might look like:
uniform mat4 ViewMatrix;
We then need to provide the data for the uniform variable from inside our game/program. We do this by first finding the location of the variable using the glGetUniformLocation() method. Then we assign a value to the found location using one of the glUniform() methods. The code for this process could look something like the following:
GLuint location = glGetUniformLocation(programHandle," ViewMatrix ");
if( location >= 0 )
{
glUniformMatrix4fv(location, 1, GL_FALSE, &viewMatrix [0][0])
}
We then assign a value to the uniform variable’s location using the glUniformMatrix4fv() method. The first argument is the uniform variable’s location. The second argument is the number of matrices that are being assigned. The third is a GL bool type specifying whether or not the matrix should be transposed. Since we are using the GLM library for our matrices, a transpose is not required. If you were implementing the matrix using data that was in row-major order, instead of column-major order, you might need to use the GL_TRUE type for this argument. The last argument is a pointer to the data for the uniform variable.
Uniform variables can be any GLSL type, and this includes complex types such as structures and arrays. The OpenGL API provides a glUniform() function with the different suffixes that match each type. For example, to assign to a variable of type vec3, we would use glUniform3f() or glUniform3fv() methods. (the v denotes multiple values in the array).
So, those are the concepts and techniques for passing data to and from our shader programs. However, as we did for the compiling and linking of our shaders, we can abstract these processes into functions housed in our ShaderManager class. We are going to focus on working with attributes and uniform variables.
First, we will look at the abstraction of adding attribute bindings using the AddAttribute() function of the ShaderManger class. This function takes one argument, the attribute’s name, to be bound as a string. We then call the glBindAttribLocation() function, passing the program’s handle and the current index or number of attributes, which we increase on call, and finally the C string conversion of the attributeName string, which provides a pointer to the first character in the string array. This function must be called after compilation, but before the linking of the shader program:
void ShaderManager::AddAttribute(const std::string& attributeName)
{
glBindAttribLocation(m_programID,
m_numAttributes++,
attributeName.c_str());
}
For the uniform variables, we create a function that abstracts looking up the location of the uniform in the shader program, the GetUniformLocation() function. This function again takes only one variable which is a uniform name in the form of a string. We then create a temporary holder for the location and assign it the returned value of the glGetUniformLocation() method call. We check to make sure the location is valid, and if not we throw an exception letting us know about the error. Finally, we return the valid location if found:
GLint ShaderManager::GetUniformLocation(const std::string& uniformName)
{
GLint location = glGetUniformLocation(m_programID,
uniformName.c_str());
if (location == GL_INVALID_INDEX)
{
Exception("Uniform " + uniformName + " not found in shader!");
}
return location;
}
This gives us the abstraction for binding our data, but we still need to assign which shader should be used for a certain draw call, and to activate any attributes we need. To accomplish this, we create a function in the ShaderManager called Use(). This function will first set the current shader program as the active one using the glUseProgram() API method call. We then use a for loop to step through the list of attributes for the shader program, activating each one:
void ShaderManager::Use(){
glUseProgram(m_programID);
for (int i = 0; i < m_numAttributes; i++) {
glEnableVertexAttribArray(i);
}
}
Of course, since we have an abstracted way to enable the shader program, it only makes sense that we should have a function to disable the shader program. This function is very similar to the Use() function, but in this case, we are setting the program in use to 0, effectively making it NULL, and we use the glDisableVertexAtrribArray() method to disable the attributes in the for loop:
void ShaderManager::UnUse() {
glUseProgram(0);
for (int i = 0; i < m_numAttributes; i++) {
glDisableVertexAttribArray(i);
}
}
The net effect of this abstraction is we can now set up our entire shader program structure with a few simple calls. Code similar to the following would create and compile the shaders, add the necessary attributes, link the shaders into a program, locate a uniform variable, and create the VAO and VBO for a mesh:
shaderManager.CompileShaders("Shaders/SimpleShader.vert",
"Shaders/SimpleShader.frag");
shaderManager.AddAttribute("vertexPosition_modelspace");
shaderManager.AddAttribute("vertexColor");
shaderManager.LinkShaders();
MatrixID = shaderManager.GetUniformLocation("ModelViewProjection");
m_model.Init("Meshes/Dwarf_2_Low.obj", "Textures/dwarf_2_1K_color.png");
Then, in our Draw loop, if we want to use this shader program to draw, we can simply use the abstracted functions to activate and deactivate our shader, similar to the following code:
shaderManager.Use(); m_model.Draw(); shaderManager.UnUse();
This makes it much easier for us to work with and test out advanced rendering techniques using shaders.
Here in this article, we have discussed how advanced rendering techniques, hands-on practical knowledge of game physics and shaders and lighting can help you to create advanced games with C++.
If you have liked this above article, check out the complete book Mastering C++ game Development.
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