4 - Lecture - OpenGL Intro - Practical

8/10/2019 4 - Lecture - OpenGL Intro - Practical

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First OpenGL program


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Introduction

Lets get started on how to make a simple coloured

triangle using OpenGL

Well talk about the different pieces of the project:

Using the libraries

Brief overview of shaders

Loading the shaders Draw code


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GLFW and GLEW

To use these helper libraries we need to corrinclude the libraries and header paths.


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Helper Libraries

See todays exercises for info on how to intiathese libraries in order to use their functions


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The graphics pipeline


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Shaders

In order to draw to the screenwe need to program parts of t

pipeline. In our 2D games we only need to worry about vertexshaders a

fragmentshaders.

Shaders run on the graphics card in parallel. That is, each verteoperated on by a shader at the same time.

These are separate files that we compile into a separate shade We output data from our OpenGL program and send this to the vertex sh

moves our vertex positions from worldspace into screenspace).

The vertex shader then sends data to the fragment shader (which determcolour value of each pixel depending on many factors).


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World space, screen space? What?

Dont worry about it toomuchdoesnt effectyou much until you start3D games. Will becovered at the end of

this subject. Basically determining

where things are inrelation to the camera.


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Adding shaders to our project

Our C++ code needs to open the shader files, rea

contents, compile the shaders and link them toginto one program before using them.

This is all covered in the exercises


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Vertex Arrays

For now, we need to store the info

for our geometry in arrays.

Well be using one for the vertex

positions, and one for the colours

There are 4 components for

position: x, y, z and w

Also 4 components for colour: red,

green, blue and alpha

constfloatvertexPosi

{

0.0f, 0.03f, 0.0f, 1.0

-0.025f, -0.05f, 0.0

0.025f, -0.05f, 0.0f,

};

constfloatvertexColo

{

1.0f, 0.0f, 0.0f, 1.0f0.0f, 1.0f, 0.0f, 1.0f

0.0f, 0.0f, 1.0f, 1.0f

};


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The main game loop

By now you should have implemented the basic game loop via

tutorial exercise.

Before we draw anything, we need to set a few OpenGL states

transfer of data to the graphics card.

glEnableVertexAttribArray(n)enables us to send data to loca

the vertex shader

glVertexAttribPointerspecify to which variable in the vertex s

are sending data to, the data to send, and info about the data t


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glDrawArrays

Finally, we can draw our data to the screen!

We only need to specify the OpenGL primitiv

want to use, and the number of points well

Er, primitive?


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OpenGL Primitives

Here are the different

options we can use in

the glDrawArrays call.

Triangle Strip re-uses

the last 2 pointsalong with a new

point to create the

next triangle.


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Next

Try to draw some different shapes

Change the colours and locations of these shapes

4 - Lecture - OpenGL Intro - Practical

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