The maxLength specifies the size of the buffer passed via the infoLog pointer. The number of characters actually written to this buffer will be stored in the value pointed to by length. Even if compilation succeeds, the compiler may store warnings or other informational messages in the information log. It is common practice during development and in debug builds to either display or log to disc any information logs generated by the compiler.
The parallels between shader programs and C programs continue. After compiling a collection of C source files, the resulting object files must be linked together to form an executable program. Similarly, after compiling a collection of shader source files, the resulting shader objects must be linked together to form a program object. Given a collection of compiled shader objects, creating a usable program object consists of four steps. Placement of attributes will be discussed in a later chapter.
The interface to glAttachShader is very simple. It takes the name of the program object and the name of a shader object to attach to it. In this context attach means that the shader is added to the list of shaders that will be linked to form the final program. Shaders may be attached to multiple programs. After attaching all of the shaders to the program, the program must be linked by calling glLinkProgram. Notice that, like glCompileShader , glLinkProgram does not return value.
Just like compiling a program, linking a program can generate errors and warning. Linker diagnostics are written to a program information log.
Querying this log is nearly identical to querying the shader information log. At a bare minimum, a program must have a complete vertex shader with a main and a complete fragment shader with a main. Some versions of OpenGL support additional, optional shader stages. These shader stages are beyond the scope of this chapter. Additional requirements and restrictions will be covered in later chapters. Once a program object is successfully linked, it can be used for rendering.
A program is activated by calling glUseProgram. A program remains active until another program is used. In doing so, a library routine could save the current program, activate a new program, perform some rendering, and restore the original program. Producing a program object for even the most sophisticated rendering algorithm is only half of the equation.
Every program needs data on which to operate. In this section one fundamental form of program data will be introduced: attributes. Other forms of program data will be covered in later chapters. Each vertex of each triangle rendered in a scene has some data associated with it. At the very least, the vertex has a position. It may also have colors, a normal, texture coordinates, and other data. This data is supplied in the form of attributes. All data to be used as attributes must come from special buffers allocated from the GL.
These buffer objects are regions of memory controlled by the GL. In addition, it allows the implementation to place the buffer in the fastest memory for a particular operation. This interface consists of several elements, but only a few of these elements will be covered here.
Buffer objects also offer the unique ability to provide direct, pointer based access to the object's data. This will be discussed in more detail below.
Buffer object names are created by calling glGenBuffers. This function creates n handles to buffer objects and stores the names in the array specified by buffers. A name can be activated by calling glBindBuffer.
The buffer specifies the name to be bound. The target specifies the binding point. The binding point determines which part of the GL will access the buffer. Once a name is bound, storage for the object's data can be created by calling glBufferData.
Notice that glBufferData does not take a buffer name has a parameter. Instead it uses the name most recently bound to target. In addition to creating storage for the object's data, this function can be used to provide initial values for the data.
This data is read from the data. If a non- NULL pointer is supplied, size bytes will be copied from that pointer into the object's storage. If this pointer is NULL , the buffer will contain uninitialized values. The usage is one of the most vexing parts of the buffer object interface for both application developers and driver writers. It will be discussed in detail in a later chapter. Notice the careful distinction between the name of an object and its storage. This analogy works rather well for many OpenGL objects.
Once we have a storage created for a buffer object, we can perform read and write through the buffer object using glGetBufferSubData and glBufferSubData.
For many uses, this interface is very cumbersome. Image an application that wants to use a 3D model loading library. This library has several interfaces that take pointers to buffers as parameters, and the buffers are filled with data from the 3D model file. To use these interfaces with glBufferSubData , the application would have to allocate a temporary buffer, call into the 3D model loading library, copy the data into the buffer object, then free the temporary buffer.
This usage is inefficient, cumbersome, and error prone. Asked 3 years, 4 months ago. Active 3 years, 4 months ago. Viewed 1k times. Improve this question. The Guest The Guest 59 6 6 bronze badges. Looks as if you are drawing a black triangle on a black background. What happens when you change the background color? Just for the records. This is not modern OpenGL.
The code you have will use the fixed function pipeline. Welcome to SO! I don't consider your code as "modern" OpenGL. It seems it uses the old fixed-pipeline with "steroids", something that OpenGL 2. Prefer a version starting with OpenGL 3. Hi Nothing changes when i change the background color besides the background color. And yes it is not truly modern opengl. Active Oldest Votes. Improve this answer.
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