I want to rotate a sprite in C and SDL2, with a set center of rotation, and without scaling or anti-aliasing.
My game resolution is 320x240, and the display is scaled up when I set the game to full screen, because I'm using SDL_RenderSetLogicalSize(renderer, 320, 240).
Using SDL2's SDL_RenderCopyEx() (or SDL_RenderCopyExF()) to rotate a SDL_Texture.
As shown in this example ( https://imgur.com/UGNDfEY ) when the window is set to full screen, the texture is scaled up and at much higher resolution. Is would like the final 320x240 rendering to be scaled up, not the individual textures.
Using SDL_gfx's rotozoomSurface() was a possible alternative.
However, as shown in this example ( https://imgur.com/czPEUhv ), while this method give the intended low-resolution and aliased look, it has no center of rotation, and renders the transparency color as half-transparent black.
Is there a function that does what I'm looking for? Are there some tricks to get around that?
What I would do is to render what you want to in a SDL_Texture, and then print this texture into the renderer, using something like :
// Set 'your_texture' as target
SDL_SetRenderTarget(your_renderer, your_texture);
// We are now printing the rotated image on the texture
SDL_RenderCopyEx(your_renderer, // we still use the renderer; it will be automatically printed into the texture 'your_texture'
your_image,
&srcrect,
&dstrect,
angle,
¢er,
SDL_FLIP_NONE); // unless you want to flip vertically / horizontally
// Set the renderer as target and print the previous texture
SDL_SetRenderTarget(your_renderer, NULL);
SDL_RenderClear(your_renderer);
SDL_RenderCopy (your_renderer, your_texture, NULL, NULL); // here the scale is automatically done
SDL_RenderPresent(your_renderer);
It works, but I don't know if it is very efficient.
Don't forget to define your_texture with a SDL_TEXTUREACCESS_TARGET access.
Hope this helps,
Durza42
Thanks to #Durza42, here's the solution to my problem:
#define kScreenWidth 320
#define kScreenHeight 240
SDL_Window* g_window = NULL;
SDL_Texture* g_texture = NULL;
SDL_Renderer* g_renderer = NULL;
SDL_Texture* g_sprite = NULL;
double g_sprite_angle = 0.0;
SDL_FRect g_sprite_frect = {
.x = 50.0f,
.y = 50.0f,
.w = 32.0f,
.h = 32.0f,
};
void
sdl_load(void)
{
SDL_Init(SDL_INIT_VIDEO);
g_window = SDL_CreateWindow(NULL, SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, kScreenWidth, kScreenHeight, 0);
g_renderer = SDL_CreateRenderer(g_window, -1, SDL_RENDERER_PRESENTVSYNC);
g_texture = SDL_CreateTexture(g_renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, kScreenWidth, kScreenHeight);
SDL_RenderSetLogicalSize(g_renderer, kScreenWidth, kScreenHeight);
}
void
sprite_load(void)
{
g_sprite = IMG_LoadTexture(g_renderer, "./sprite.png");
}
void
draw(void)
{
SDL_SetRenderTarget(g_renderer, g_texture);
SDL_RenderClear(g_renderer);
SDL_RenderCopyExF(g_renderer, g_sprite, NULL, &g_sprite_frect, g_sprite_angle, NULL, SDL_FLIP_NONE);
SDL_SetRenderTarget(g_renderer, NULL);
SDL_RenderClear(g_renderer);
SDL_RenderCopy(g_renderer, g_texture, NULL, NULL);
SDL_RenderPresent(g_renderer);
}
I have opened a window which shows two rects on the screen then using SDL_TTF to show the mouse position on the screen.
The bit I am having hard time understanding is why after rendering text the the two rects before it do not show up.
I am using SDL_RenderFillRect to draw two rects on screen
SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
SDL_RenderFillRect(renderer, rect1);
SDL_SetRenderDrawColor(renderer, 0, 0, 255, 255);
SDL_RenderFillRect(renderer, rect2);
Code for rendering the text is
// define string with mouse x, y coords
sprintf(someString, "x: %d, y: %d", mouse.x, mouse.y);
SDL_Point textPos = {10, 10};
WriteText(renderer, font, someString, textPos, (SDL_Color){255, 255, 255, 255});
SDL_Surface *fontSurface = TTF_RenderText_Blended(font, someString, COLOR_BLACK); // create font surface
SDL_Texture *fontTexture = SDL_CreateTextureFromSurface(renderer, fontSurface); // create the texture
// get clip width and height from fontsurface clip rect
SDL_Rect *fontRect = &fontSurface->clip_rect;
fontRect->x = pos.x;
fontRect->y = pos.y;
SDL_RenderCopy(renderer, fontTexture, NULL, fontRect); // copy text to the renderer
// delete surface and texture
SDL_FreeSurface(fontSurface);
SDL_DestroyTexture(fontTexture);
It to shows the mouse positon top left corner of the window. However this makes the rest of the window blank.
To prevent this my work around is having to draw something on the screen after calling SDL_RendererCopy (and weirdly before calling SDL_DestroyTexture too) For example drawing single point
...
SDL_RenderCopy(renderer, fontTexture, NULL, fontRect); // copy text to the renderer
// why is this needed??
SDL_RenderDrawPoint(renderer, 0, 0);
// delete surface and texture
SDL_FreeSurface(fontSurface);
SDL_DestroyTexture(fontTexture); // have to draw a point before this
...
This then shows the two rects rendered before the text
If I set dstRect to NULL when calling SDL_RenderCopy then the text spans the whole window but I can see what was rendered before underneath the text.
Why am I having to draw a point after calling SDL_RenderCopy to stop what was rendered before from not showing up?
NOTE: Link to full source code https://pastebin.com/tRSFT0PV
This is a bug in SDL 2.0.10. It's fixed by https://hg.libsdl.org/SDL/rev/6ee12b88beed and this fix will ship with 2.0.11. Sorry about that!
Right now my game is currently just showing an image (bmp) stretched onto a surface.
The problem I'm having is drawing text onto it.
The text is drawn correctly how I want it however, if the game is not in windowed mode, rather, if it's in fullscreen mode, the text will flicker on and off, and I can't for the life of my figure out what's wrong.
int Game::Render(){
HRESULT r;
D3DLOCKED_RECT LockedRect;//locked area of display memory(buffer really) we are drawing to
LPDIRECT3DSURFACE9 pBackSurf = 0;
if(!g_pDevice){
SetError("Cannot render because there is no device");
return E_FAIL;
}
if(g_pDevice->BeginScene()){
//clear the display arera with colour black, ignore stencil buffer
//
g_pDevice->Clear(0,0,D3DCLEAR_TARGET, D3DCOLOR_XRGB(0,0,25), 1.0f, 0);
//get pointer to backbuffer
r=g_pDevice->GetBackBuffer(0,0,D3DBACKBUFFER_TYPE_MONO, &backBuffer); //&pBackSurf
g_pDevice->StretchRect(surface, NULL, backBuffer, NULL, D3DTEXF_NONE);
if(FAILED(r)){
SetError("Couldn't get backbuffer");
}
r=backBuffer->LockRect(&LockedRect, NULL, 0);
if(FAILED(r)){
SetError("Could not lock the back buffer");
}
g_font->DrawText(NULL,fps_string,-1,&font_rect,DT_LEFT|DT_NOCLIP,0xFFAABBCC);
backBuffer->UnlockRect();
//backBuffer->Release();//release lock
g_pDevice->EndScene();
g_pDevice->Present(NULL, NULL, NULL, NULL);//swap over buffer to primary surface
}
return S_OK;
}
What's the bare skeleton to get a triangle drawn on an OpenGL window in C on OSX? I've gone through the tutorials at Nehe and tried to get it working, but the CreateGLWindow seems hopelessly tied to win32.
I want to stick to just opengl and glut, etc. I'll be eventually wrapping this in scheme, but I'd like to have a firmer understanding at the c-level beforehand.
Here's what I have thus far:
#include <OpenGL/gl.h>// Header File For The OpenGL32 Library
#include <OpenGL/glu.h>// Header File For The GLu32 Library
#include <GLUT/glut.h>// Header File For The GLut Library
#define kWindowWidth 400;
#define kWindowHeight 300;
HGLRC hRC=NULL;// Permanent Rendering Context
HDC hDC=NULL;// Private GDI Device Context
HWND hWnd=NULL;// Holds Our Window Handle
HINSTANCE hInstance;// Holds The Instance Of The Application
LRESULTCALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);// Declaration For WndProc
boolkeys[256];// Array Used For The Keyboard Routine
boolactive=TRUE;// Window Active Flag Set To TRUE By Default
boolfullscreen=TRUE;// Fullscreen Flag Set To Fullscreen Mode By Default
GLvoid ReSizeGLScene(GLsizei width, GLsizei height)// Resize And Initialize The GL Window
{
if (height==0)// Prevent A Divide By Zero By
{
height=1;// Making Height Equal One
}
glViewport(0, 0, width, height);// Reset The Current Viewport
glMatrixMode(GL_PROJECTION);// Select The Projection Matrix
glLoadIdentity();// Reset The Projection Matrix
// Calculate The Aspect Ratio Of The Window
gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,0.1f,100.0f);
glMatrixMode(GL_MODELVIEW);// Select The Modelview Matrix
glLoadIdentity();// Reset The Modelview Matrix
}
int InitGL(GLvoid)// All Setup For OpenGL Goes Here
{
glShadeModel(GL_SMOOTH);// Enables Smooth Shading
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);// Black Background
glClearDepth(1.0f);// Depth Buffer Setup
glEnable(GL_DEPTH_TEST);// Enables Depth Testing
glDepthFunc(GL_LEQUAL);// The Type Of Depth Test To Do
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);// Really Nice Perspective Calculations
return TRUE;// Initialization Went OK
}
int DrawGLScene(GLvoid)// Here's Where We Do All The Drawing
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);// Clear The Screen And The Depth Buffer
glLoadIdentity();// Reset The Current Modelview Matrix
return TRUE;// Everything Went OK
}
GLvoid KillGLWindow(GLvoid)// Properly Kill The Window
{
if (fullscreen)// Are We In Fullscreen Mode?
{
ChangeDisplaySettings(NULL,0);// If So Switch Back To The Desktop
ShowCursor(TRUE);// Show Mouse Pointer
}
if (hRC)// Do We Have A Rendering Context?
{
if (!wglMakeCurrent(NULL,NULL))// Are We Able To Release The DC And RC Contexts?
{
MessageBox(NULL,"Release Of DC And RC Failed.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION);
}
if (!wglDeleteContext(hRC))// Are We Able To Delete The RC?
{
MessageBox(NULL,"Release Rendering Context Failed.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION);
}
hRC=NULL;// Set RC To NULL
}
if (hDC && !ReleaseDC(hWnd,hDC))// Are We Able To Release The DC
{
MessageBox(NULL,"Release Device Context Failed.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION);
hDC=NULL;// Set DC To NULL
}
if (hWnd && !DestroyWindow(hWnd))// Are We Able To Destroy The Window?
{
MessageBox(NULL,"Could Not Release hWnd.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION);
hWnd=NULL;// Set hWnd To NULL
}
if (!UnregisterClass("OpenGL",hInstance))// Are We Able To Unregister Class
{
MessageBox(NULL,"Could Not Unregister Class.","SHUTDOWN ERROR",MB_OK | MB_ICONINFORMATION);
hInstance=NULL;// Set hInstance To NULL
}
}
BOOL CreateGLWindow(char* title, int width, int height, int bits, bool fullscreenflag)
{
GLuintPixelFormat;// Holds The Results After Searching For A Match
WNDCLASSwc;// Windows Class Structure
DWORDdwExStyle;// Window Extended Style
DWORDdwStyle;// Window Style
RECT WindowRect;// Grabs Rectangle Upper Left / Lower Right Values
WindowRect.left=(long)0;// Set Left Value To 0
WindowRect.right=(long)width;// Set Right Value To Requested Width
WindowRect.top=(long)0;// Set Top Value To 0
WindowRect.bottom=(long)height;// Set Bottom Value To Requested Height
fullscreen=fullscreenflag;// Set The Global Fullscreen Flag
hInstance= GetModuleHandle(NULL);// Grab An Instance For Our Window
wc.style= CS_HREDRAW | CS_VREDRAW | CS_OWNDC;// Redraw On Move, And Own DC For Window
wc.lpfnWndProc= (WNDPROC) WndProc;// WndProc Handles Messages
wc.cbClsExtra= 0;// No Extra Window Data
wc.cbWndExtra= 0;// No Extra Window Data
wc.hInstance= hInstance;// Set The Instance
wc.hIcon= LoadIcon(NULL, IDI_WINLOGO);// Load The Default Icon
wc.hCursor= LoadCursor(NULL, IDC_ARROW);// Load The Arrow Pointer
wc.hbrBackground= NULL;// No Background Required For GL
wc.lpszMenuName= NULL;// We Don't Want A Menu
wc.lpszClassName= "OpenGL";// Set The Class Name
if (!RegisterClass(&wc))// Attempt To Register The Window Class
{
MessageBox(NULL,"Failed To Register The Window Class.","ERROR",MB_OK|MB_ICONEXCLAMATION);
return FALSE;// Exit And Return FALSE
}
if (fullscreen)// Attempt Fullscreen Mode?
{
DEVMODE dmScreenSettings;// Device Mode
memset(&dmScreenSettings,0,sizeof(dmScreenSettings));// Makes Sure Memory's Cleared
dmScreenSettings.dmSize=sizeof(dmScreenSettings);// Size Of The Devmode Structure
dmScreenSettings.dmPelsWidth= width;// Selected Screen Width
dmScreenSettings.dmPelsHeight= height;// Selected Screen Height
dmScreenSettings.dmBitsPerPel= bits;// Selected Bits Per Pixel
dmScreenSettings.dmFields=DM_BITSPERPEL|DM_PELSWIDTH|DM_PELSHEIGHT;
// Try To Set Selected Mode And Get Results. NOTE: CDS_FULLSCREEN Gets Rid Of Start Bar.
if (ChangeDisplaySettings(&dmScreenSettings,CDS_FULLSCREEN)!=DISP_CHANGE_SUCCESSFUL)
{
// If The Mode Fails, Offer Two Options. Quit Or Run In A Window.
if (MessageBox(NULL,"The Requested Fullscreen Mode Is Not Supported By\nYour Video Card. Use Windowed Mode Instead?","NeHe GL",MB_YESNO|MB_ICONEXCLAMATION)==IDYES)
{
fullscreen=FALSE;// Select Windowed Mode (Fullscreen=FALSE)
}
else
{
// Pop Up A Message Box Letting User Know The Program Is Closing.
MessageBox(NULL,"Program Will Now Close.","ERROR",MB_OK|MB_ICONSTOP);
return FALSE;// Exit And Return FALSE
}
}
}
if (fullscreen)// Are We Still In Fullscreen Mode?
{
dwExStyle=WS_EX_APPWINDOW;// Window Extended Style
dwStyle=WS_POPUP;// Windows Style
ShowCursor(FALSE);// Hide Mouse Pointer
}
else
{
dwExStyle=WS_EX_APPWINDOW | WS_EX_WINDOWEDGE;// Window Extended Style
dwStyle=WS_OVERLAPPEDWINDOW;// Windows Style
}
AdjustWindowRectEx(&WindowRect, dwStyle, FALSE, dwExStyle);// Adjust Window To True Requested Size
if (!(hWnd=CreateWindowEx(dwExStyle,// Extended Style For The Window
"OpenGL",// Class Name
title,// Window Title
WS_CLIPSIBLINGS |// Required Window Style
WS_CLIPCHILDREN |// Required Window Style
dwStyle,// Selected Window Style
0, 0,// Window Position
WindowRect.right-WindowRect.left,// Calculate Adjusted Window Width
WindowRect.bottom-WindowRect.top,// Calculate Adjusted Window Height
NULL,// No Parent Window
NULL,// No Menu
hInstance,// Instance
NULL)))// Don't Pass Anything To WM_CREATE
{
KillGLWindow();// Reset The Display
MessageBox(NULL,"Window Creation Error.","ERROR",MB_OK|MB_ICONEXCLAMATION);
return FALSE;// Return FALSE
}
staticPIXELFORMATDESCRIPTOR pfd=// pfd Tells Windows How We Want Things To Be
{
sizeof(PIXELFORMATDESCRIPTOR),// Size Of This Pixel Format Descriptor
1,// Version Number
PFD_DRAW_TO_WINDOW |// Format Must Support Window
PFD_SUPPORT_OPENGL |// Format Must Support OpenGL
PFD_DOUBLEBUFFER,// Must Support Double Buffering
PFD_TYPE_RGBA,// Request An RGBA Format
bits,// Select Our Color Depth
0, 0, 0, 0, 0, 0,// Color Bits Ignored
0,// No Alpha Buffer
0,// Shift Bit Ignored
0,// No Accumulation Buffer
0, 0, 0, 0,// Accumulation Bits Ignored
16,// 16Bit Z-Buffer (Depth Buffer)
0,// No Stencil Buffer
0,// No Auxiliary Buffer
PFD_MAIN_PLANE,// Main Drawing Layer
0,// Reserved
0, 0, 0// Layer Masks Ignored
};
if (!(hDC=GetDC(hWnd)))// Did We Get A Device Context?
{
KillGLWindow();// Reset The Display
MessageBox(NULL,"Can't Create A GL Device Context.","ERROR",MB_OK|MB_ICONEXCLAMATION);
return FALSE;// Return FALSE
}
if (!(PixelFormat=ChoosePixelFormat(hDC,&pfd)))// Did Windows Find A Matching Pixel Format?
{
KillGLWindow();// Reset The Display
MessageBox(NULL,"Can't Find A Suitable PixelFormat.","ERROR",MB_OK|MB_ICONEXCLAMATION);
return FALSE;// Return FALSE
}
if(!SetPixelFormat(hDC,PixelFormat,&pfd))// Are We Able To Set The Pixel Format?
{
KillGLWindow();// Reset The Display
MessageBox(NULL,"Can't Set The PixelFormat.","ERROR",MB_OK|MB_ICONEXCLAMATION);
return FALSE;// Return FALSE
}
if (!(hRC=wglCreateContext(hDC)))// Are We Able To Get A Rendering Context?
{
KillGLWindow();// Reset The Display
MessageBox(NULL,"Can't Create A GL Rendering Context.","ERROR",MB_OK|MB_ICONEXCLAMATION);
return FALSE;// Return FALSE
}
if(!wglMakeCurrent(hDC,hRC))// Try To Activate The Rendering Context
{
KillGLWindow();// Reset The Display
MessageBox(NULL,"Can't Activate The GL Rendering Context.","ERROR",MB_OK|MB_ICONEXCLAMATION);
return FALSE;// Return FALSE
}
ShowWindow(hWnd,SW_SHOW);// Show The Window
SetForegroundWindow(hWnd);// Slightly Higher Priority
SetFocus(hWnd);// Sets Keyboard Focus To The Window
ReSizeGLScene(width, height);// Set Up Our Perspective GL Screen
if (!InitGL())// Initialize Our Newly Created GL Window
{
KillGLWindow();// Reset The Display
MessageBox(NULL,"Initialization Failed.","ERROR",MB_OK|MB_ICONEXCLAMATION);
return FALSE;// Return FALSE
}
return TRUE;// Success
}
LRESULT CALLBACK WndProc(HWNDhWnd,// Handle For This Window
UINTuMsg,// Message For This Window
WPARAMwParam,// Additional Message Information
LPARAMlParam)// Additional Message Information
{
switch (uMsg)// Check For Windows Messages
{
case WM_ACTIVATE:// Watch For Window Activate Message
{
if (!HIWORD(wParam))// Check Minimization State
{
active=TRUE;// Program Is Active
}
else
{
active=FALSE;// Program Is No Longer Active
}
return 0;// Return To The Message Loop
}
case WM_SYSCOMMAND:// Intercept System Commands
{
switch (wParam)// Check System Calls
{
case SC_SCREENSAVE:// Screensaver Trying To Start?
case SC_MONITORPOWER:// Monitor Trying To Enter Powersave?
return 0;// Prevent From Happening
}
break;// Exit
}
case WM_CLOSE:// Did We Receive A Close Message?
{
PostQuitMessage(0);// Send A Quit Message
return 0;// Jump Back
}
case WM_KEYDOWN:// Is A Key Being Held Down?
{
keys[wParam] = TRUE;// If So, Mark It As TRUE
return 0;// Jump Back
}
case WM_KEYUP:// Has A Key Been Released?
{
keys[wParam] = FALSE;// If So, Mark It As FALSE
return 0;// Jump Back
}
case WM_SIZE:// Resize The OpenGL Window
{
ReSizeGLScene(LOWORD(lParam),HIWORD(lParam));// LoWord=Width, HiWord=Height
return 0;// Jump Back
}
}
// Pass All Unhandled Messages To DefWindowProc
return DefWindowProc(hWnd,uMsg,wParam,lParam);
}
GLvoid InitGL(GLvoid);
GLvoid DrawGLScene(GLvoid);
GLvoid ReSizeGLScene(int Width, int Height);
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize (kWindowWidth, kWindowHeight);
glutInitWindowPosition (100, 100);
glutCreateWindow (argv[0]);
InitGL();
glutDisplayFunc(DrawGLScene);
glutReshapeFunc(ReSizeGLScene);
glutMainLoop();
return 0;
}
As far as I can tell, the NeHe tutorials are specifically for windows and are great for higher level stuff. However, when it comes to the basics, They can become overcomplicated. So here is a simple skeletal program for rendering a triangle using only glut and opengl functionality. If you want to be Apple specific, try using agl.
// The OpenGL libraries, make sure to include the GLUT and OpenGL frameworks
#include <GLUT/glut.h>
#include <OpenGL/gl.h>
#include <OpenGL/glu.h>
// This is just an example using basic glut functionality.
// If you want specific Apple functionality, look up AGL
void init() // Called before main loop to set up the program
{
glClearColor(0.0, 0.0, 0.0, 0.0);
glEnable(GL_DEPTH_TEST);
glShadeModel(GL_SMOOTH);
}
// Called at the start of the program, after a glutPostRedisplay() and during idle
// to display a frame
void display()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glBegin(GL_TRIANGLES);
glVertex3f(0.0, 0.0, -10.0);
glVertex3f(1.0, 0.0, -10.0);
glVertex3f(0.0, 1.0, -10.0);
glEnd();
glutSwapBuffers();
}
// Called every time a window is resized to resize the projection matrix
void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-0.1, 0.1, -float(h)/(10.0*float(w)), float(h)/(10.0*float(w)), 0.5, 1000.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
int main(int argc, char **argv)
{
glutInit(&argc, argv); // Initializes glut
// Sets up a double buffer with RGBA components and a depth component
glutInitDisplayMode(GLUT_DOUBLE | GLUT_DEPTH | GLUT_RGBA);
// Sets the window size to 512*512 square pixels
glutInitWindowSize(512, 512);
// Sets the window position to the upper left
glutInitWindowPosition(0, 0);
// Creates a window using internal glut functionality
glutCreateWindow("Hello!");
// passes reshape and display functions to the OpenGL machine for callback
glutReshapeFunc(reshape);
glutDisplayFunc(display);
glutIdleFunc(display);
init();
// Starts the program.
glutMainLoop();
return 0;
}
NeHe has a tutorial for setting up an OpenGL window on Mac OS X with GLUT here. They also provide the code for all of the lessons for various different platforms. If you scroll to the bottom of lesson 2, you can download the lesson code ready to go for a number of platforms. You probably want either the GLUT sample or the Mac OS X/Cocoa sample.
Easiest way is to use Objective-C to get everything set up, then use C for drawing. (not the only way, GLUT is also an option.)
Sample code from Apple
Here's the "hello.c" example from the OpenGL Red Book converted to R6RS Scheme:
http://gist.github.com/319363
That program runs in Ikarus Scheme and Ypsilon Scheme.
Notice that the 'import' form is referring to some 'agave' libraries. Those are available at:
http://github.com/dharmatech/agave
Agave is a project which provides a bunch of OpenGL demos and libraries for R6RS Scheme.
Ikarus and Ypsilon are both available for OS X.
Ed