OpenGL ES 2.0 Multiple Instances of Textured Object - iphone

Question

I am using Blender to create a 3D cow and obtain the correct vertices (optimized afterwards to remove duplicates). I have created a Cow Class to be able to draw multiple instances of this cow on screen.
Cow 1 is drawn at (-2,0,-10) and Cow 2 is drawn at (2,0,-10). When I render cow 1 only I see Cow 1 at (-2,0,-10). When I render either both cows or only cow 2 (comment render cow 1 out), I only get cow 2 (do not see cow 1) at (2,0,-10). I am making a game where I will have many enemies running around and I need to be able to draw multiple instances of these objects and render them independently at different locations. Can anyone tell me what I am doing wrong? Thanks
//
// Cow.m
//
#import "Cow.h"
#import "cow_verts.h"
#implementation Cow
- (id)initWithContext:(EAGLContext *)aContext {
context = aContext;
effect = [[GLKBaseEffect alloc] init];
[EAGLContext setCurrentContext:context];
sharedResourceManager = [ResourceManager sharedResourceManager];
UIImage *textureImage = [UIImage imageNamed:#"cowUV.png"];
texture = [sharedResourceManager addTexture:textureImage];
position = GLKVector3Make(0,0,0);
rotation = GLKVector3Make(0,0,0);
scale = GLKVector3Make(1,1,1);
[self setupGL];
return self;
}
- (void)setupGL {
glGenVertexArraysOES(1, &_vao);
glBindVertexArrayOES(_vao);
glGenBuffers(1, &_dynamicVBO);
glBindBuffer(GL_ARRAY_BUFFER, _dynamicVBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(CowDynamicVertexData), CowDynamicVertexData, GL_DYNAMIC_DRAW);
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(dynamicVertexData), (void *)offsetof(dynamicVertexData, position));
glEnableVertexAttribArray(GLKVertexAttribPosition);
glGenBuffers(1, &_staticVBO);
glBindBuffer(GL_ARRAY_BUFFER, _staticVBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(CowStaticVertexData), CowStaticVertexData, GL_STATIC_DRAW);
glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, sizeof(staticVertexData), (void *)offsetof(staticVertexData, texCoord));
glEnableVertexAttribArray(GLKVertexAttribTexCoord0);
glGenBuffers(1, &_indexBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(CowIndices), CowIndices, GL_STATIC_DRAW);
glBindVertexArrayOES(0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
}
- (void)updateWithDelta:(float)aDelta {
float aspect = fabsf([[UIScreen mainScreen] bounds].size.width/ [[UIScreen mainScreen] bounds].size.height);
GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(65.0f), aspect, 4.0f, 20.0f);
effect.transform.projectionMatrix = projectionMatrix;
rotation.y += 90 * aDelta;
}
- (void)render {
[EAGLContext setCurrentContext:context];
glClearColor(0, 1, 1, 1);
glClear( GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
GLKMatrix4 xRotationMatrix = GLKMatrix4MakeXRotation(GLKMathDegreesToRadians(rotation.x));
GLKMatrix4 yRotationMatrix = GLKMatrix4MakeYRotation(GLKMathDegreesToRadians(rotation.y));
GLKMatrix4 zRotationMatrix = GLKMatrix4MakeZRotation(GLKMathDegreesToRadians(rotation.z));
GLKMatrix4 scaleMatrix = GLKMatrix4MakeScale(scale.x, scale.y, scale.z);
GLKMatrix4 translateMatrix = GLKMatrix4MakeTranslation(position.x, position.y, position.z);
GLKMatrix4 modelMatrix = GLKMatrix4Multiply(translateMatrix,
GLKMatrix4Multiply(scaleMatrix,
GLKMatrix4Multiply(zRotationMatrix,
GLKMatrix4Multiply(yRotationMatrix,
xRotationMatrix))));
effect.transform.modelviewMatrix = modelMatrix;
//GLKMatrix4 viewMatrix = GLKMatrix4MakeLookAt(0, 0, 3, 0, 0, 0, 0, 1, 0);
//effect.transform.modelviewMatrix = GLKMatrix4Multiply(viewMatrix, modelMatrix);
effect.texture2d0.name = texture.name;
effect.texture2d0.target = texture.target;
effect.texture2d0.envMode = GLKTextureEnvModeReplace;
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
glCullFace(GL_FRONT);
glBindVertexArrayOES(_vao);
[effect prepareToDraw];
glDrawElements(GL_TRIANGLES, sizeof(CowIndices)/sizeof(CowIndices[0]), GL_UNSIGNED_INT, (void *)0);
glBindVertexArrayOES(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glDisable(GL_DEPTH_TEST);
glDisable(GL_CULL_FACE);
}
#end
//
// Cow.h
//
#interface Cow : NSObject {
EAGLContext *context;
GLuint _vao;
GLuint _staticVBO;
GLuint _dynamicVBO;
GLuint _indexBuffer;
GLKTextureInfo *texture;
GLKBaseEffect *effect;
GLKVector3 position;
GLKVector3 rotation;
GLKVector3 scale;
ResourceManager *sharedResourceManager;
}
- (id)initWithContext:(EAGLContext *)aContext;
- (void)updateWithDelta:(float)aDelta;
- (void)render;
#end
//
// Scene which creates these objects and calls the functions
//
#import "Cow.h"
#implementation GameScene {
NSMutableArray *cows;
}
- (id)init {
if(self = [super init]) {
sharedDirector = [Director sharedDirector];
[EAGLContext setCurrentContext:[sharedDirector context]];
cows = [[NSMutableArray alloc] init];
AbstractEnemy *cow1 = [[Cow alloc] initWithContext:[sharedDirector context]];
cow1.position = GLKVector3Make(-2, 0, -10);
[cows addObject:cow1];
AbstractEnemy *cow2 = [[Cow alloc] initWithContext:[sharedDirector context]];
cow2.position = GLKVector3Make(2, 0, -10);
[cows addObject:cow2];
AbstractEnemy *cow3 = [[Cow alloc] initWithContext:[sharedDirector context]];
cow3.position = GLKVector3Make(0, 0, -15);
[cows addObject:cow3];
}
return self;
}
- (void)updateWithDelta:(GLfloat)aDelta {
for (int i = 0; i < cows.count; i++)
{
[cows[i] updateWithDelta:aDelta];
}
}
- (void)render {
for (int i = 0; i < cows.count; i++)
{
[cows[i] render];
}
}
#end

Answer 1

From the code you gave, you are calling a glClear() every time before you draw the cow. With all those clear calls, only the last cow you draw will be visible.
Move this call into the scene's render function so you only clear the scene once at the start of each frame.