Comp 410/510 Computer Graphics Spring Programming with OpenGL Part 2: First Program

Transcription

1 Comp 410/510 Computer Graphics Spring 2017 Programming with OpenGL Part 2: First Program

2 Objectives Refine the first program Introduce a standard program structure - Initialization

3 Program Structure Most OpenGL programs have a similar structure that consists of the following functions - main(): defines the callback functions opens one or more windows with the required properties enters event loop (last executable statement) - init(): sets the state and loads shaders Viewing Attributes callbacks Display function Input and window functions

4 main.c (with GLUT) #include <GL/glut.h> includes gl.h int main(int argc, char** argv) { glutinit(&argc,argv); glutinitdisplaymode(glut_single GLUT_RGB); glutinitwindowsize(500,500); glutinitwindowposition(0,0); glutcreatewindow("simple"); glutdisplayfunc(mydisplay); } init(); glutmainloop(); define window properties display callback set OpenGL state enter event loop

5 GLUT functions glutinit allows application to get command line arguments and initializes system gluinitdisplaymode requests properties for the window (the rendering context) RGB color Single/Double buffering Properties logically OR ed together glutwindowsize in pixels glutwindowposition from top-left corner of display glutcreatewindow create window with title simple glutdisplayfunc define display callback glutmainloop enter infinite event loop

6 Immediate vs Retained Mode Graphics Geometry specified by vertices - Locations in space (2 or 3 dimensional) - Points, lines, circles, polygons, curves, surfaces Immediate mode - Each time a vertex is specified in application, its location is sent to the GPU - Old style used glvertex - Creates bottleneck between CPU and GPU - Removed from OpenGL 3.1 (deprecated in 3.0) Retained mode - Put all vertex and attribute data in array - Send array over and store on GPU for multiple renderings

7 Display Callback Once we get data to GPU, we can initiate the rendering with a simple callback void mydisplay() { glclear(gl_color_buffer_bit); gldrawarrays(gl_triangles, 0, NumVertices); glflush(); }

8 How to get data to GPU: Vertex Arrays Vertices can have many attributes - Position - Color - Texture Coordinates - Application data A vertex array holds this data using types in vec.h GLfloat vertices[numvertices][2] = { { -0.5, -0.5 }, { -0.5, 0.5 }, { 0.5, 0.5 }, { 0.5, -0.5 }, { -0.5, -0.5 }, { 0.5, 0.5 } };

9 Vertex Array Object Bundles all vertex attribute data (positions, colors,..,) Get name for buffer then bind Gluint abuffer; glgenvertexarrays(1,&abuffer); glbindvertexarray(abuffer); At this point we have a current vertex array but no contents Use of glbindvertexarray lets us switch between VAOs

10 Vertex Buffer Object Buffer objects allow us to transfer large amounts of data to the GPU Hold attribute data for vertex array Need to create, bind and identify data Gluint buffer; glgenbuffers(1, &buffer); glbindbuffer(gl_array_buffer, buffer); glbufferdata(gl_array_buffer, sizeof(vertices), vertices, GL_STATIC_DRAW); Data in current vertex array is sent to GPU

11 Initialization Vertex arrays and associated buffer objects can be set up on init() Also set OpenGL state (such as clear color, projection, etc.) Also set up shaders as part of initialization - Read - Compile - Link

12 init() void init() { GLuint abuffer; glgenvertexarrays( 1, &abuffer ); glbindvertexarray( abuffer ); GLfloat vertices[numvertices][2] = { { -0.5, -0.5 }, { -0.5, 0.5 }, { 0.5, 0.5 }, { 0.5, -0.5 }, { -0.5, -0.5 }, { 0.5, 0.5 } }; GLuint buffer; glgenbuffers( 1, &buffer ); glbindbuffer( GL_ARRAY_BUFFER, buffer ); glbufferdata(gl_array_buffer, sizeof(vertices), vertices, GL_STATIC_DRAW); // Load shaders and use the resulting shader program GLuint program = InitShader( vshader_simple.glsl", fshader_simple.glsl" ); GLuint loc = glgetattriblocation( program, "vposition" ); glenablevertexattribarray( loc ); glvertexattribpointer(loc, 2, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(0)); } gluseprogram( program );

13 Coordinate Systems The units of in vertices are determined by the application and are called world (or object) coordinates The viewing specifications are also in world coordinates, and it is the size of the viewing volume that determines what will appear in the image Internally, OpenGL will convert to camera coordinates and later to window (or image) coordinates

14 OpenGL Camera OpenGL places a camera at the origin pointing in the negative z direction The default viewing volume is a box centered at the origin with a side of length 2 The default projection is orthographic

15 Orthographic Viewing In the default orthographic view, points are projected forward along the z axis onto the plane z = 0. z =0 z=0

16 Viewport Do not have to use the entire window for the image: glviewport(x,y,w,h) (can be used in init()) Values are in pixels (window coordinates)

17 Transformations and Viewing In OpenGL, projection is carried out by a 4x4 projection matrix 4x4 matrices are also used to specify changes in coordinate systems! p = M projection M transform p Pre 3.0 OpenGL had a set of built-in state variables (4x4 matrices) and functions (such as glrotate(), gltranslate()) to manipulate them Have been deprecated (and removed by 3.1) Three choices - Application code - GLSL functions for vector and matrix operations - vec.h and mat.h (provided with the textbook code)

18 OpenGL Primitives GL_POINTS GL_LINES GL_LINE_STRIP GL_POLYGON GL_TRIANGLES GL_LINE_LOOP GL_QUAD_STRIP GL_TRIANGLE_STRIP GL_TRIANGLE_FAN Removed...

19 Polygon Issues OpenGL will only display triangles Simple: Edges cannot cross Flat: All vertices are in the same plane Convex: All points on line segment between two points in a triangle are also in the triangle Application program must tessellate a polygon into triangles to display (triangulation) OpenGL 4.1 contains a tessellator Non-simple polygon Non-convex polygon

20 Good and Bad Triangles Long thin triangles render badly Equilateral triangles render well Maximize minimum angle

21 Attributes Attributes determine the appearance of objects Color (points, lines, triangles) Size and width (points, lines) Triangle mode Display as filled: solid color Display edges Only a few attributes (such as glpointsize, glpoygonmode) are supported by modern OpenGL functions

22 RGB(A) color Each color component is stored separately in the frame buffer Usually 8 bits per component in buffer Color values range from 0.0 (none) to 1.0 (all) using floats, or over the range from 0 to 255 using unsigned char. A channel is optional, encodes transparency.

23 Setting Colors Colors are ultimately set in the fragment shader but can be determined in either shader or in the application Application color: Pass to vertex shader as a uniform variable (next lectures) or as a vertex attribute Vertex shader color: Pass to fragment shader as varying variable (next lectures) Fragment color: Can alter color via shader code

24 Smooth Color Default is smooth shading - OpenGL interpolates vertex colors across visible triangles Alternative is flat shading - Color of first vertex determines the fill color - Handled in shader or in application