GL_ERROR: 0x0502 - glkit

Question

I hope to create an offScreen FBO, and use glReadPixels() to get the value of RGBA in this offScreen FBO. unfortunately, I got the error: GL_ERROR:0x0502
#import "ViewController.h"
#define BUFFER_OFFSET(i) ((char *)NULL + (i))
#interface ViewController ()
{
GLuint backFBO;
GLuint defaultFBO;
int fbo_width;
int fbo_height;
//
GLuint _vertexArray;
GLuint _vertexBuffer;
GLuint _texCoordBuffer;
GLuint depthBuffer;
//
GLKMatrix4 _modelViewProjectionMatrix;
GLKMatrix3 _normalMatrix;
float _rotation;
}
#property (nonatomic, strong) GLKBaseEffect *baseEffect;
#property (nonatomic, strong) EAGLContext *context;
- (void)setupGL;
- (void)tearDownGL;
- (void)setupFBO;
#end
the vertex data and normal data of object in onScreen FBO
GLfloat gVertexData[] =
{
0.5f, 0.5f, 0.0f, 0.0f, 0.0f, 1.0f,
-0.5f, 0.5f, 0.0f, 0.0f, 0.0f, 1.0f,
0.5f, -0.5f, 0.0f, 0.0f, 0.0f, 1.0f,
-0.5f, -0.5f, 0.0f, 0.0f, 0.0f, 1.0f
};
#implementation ViewController
- (void)viewDidLoad
{
[super viewDidLoad];
//1. setup context
self.context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2
];
if (!self.context) {
NSLog(#"Failed to create GLES Context");
}
GLKView *view = (GLKView *)self.view;
view.context = self.context;
view.drawableDepthFormat = GLKViewDrawableDepthFormat24;
[self setupGL];
}
- (void)viewDidUnload
{
[super viewDidUnload];
[self tearDownGL];
if ([EAGLContext currentContext] == self.context) {
[EAGLContext setCurrentContext:nil];
}
self.context = nil;
}
- (void) setupGL
{
[EAGLContext setCurrentContext:self.context];
self.baseEffect = [[GLKBaseEffect alloc] init];
glEnable(GL_DEPTH_TEST);
glGenVertexArraysOES(1, &_vertexArray);
glBindVertexArrayOES(_vertexArray);
glGenBuffers(1, &_vertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, _vertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(gVertexData), gVertexData, GL_STATIC_DRAW);
glEnableVertexAttribArray(GLKVertexAttribPosition);
glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, 24, BUFFER_OFFSET(0));
glEnableVertexAttribArray(GLKVertexAttribNormal);
glVertexAttribPointer(GLKVertexAttribNormal, 3, GL_FLOAT, GL_FALSE, 24, BUFFER_OFFSET(12));
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArrayOES(0);
// initialize FBO
[self setupFBO];
}
- (void) tearDownGL
{
[EAGLContext setCurrentContext:self.context];
glDeleteBuffers(1, &_vertexBuffer);
}
- (void)update
{
float aspect = fabsf(self.view.bounds.size.width / self.view.bounds.size.height);
GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(65.0f), aspect, 0.1f, 100.0f);
self.baseEffect.transform.projectionMatrix = projectionMatrix;
// Compute the model view matrix for the object rendered with ES2
GLKMatrix4 modelViewMatrix = GLKMatrix4MakeTranslation(0.0f, 0.0f, -2.0f);
modelViewMatrix = GLKMatrix4Rotate(modelViewMatrix, _rotation, 1.0f, 1.0f, 1.0f);
self.baseEffect.transform.modelviewMatrix = modelViewMatrix;
_rotation += self.timeSinceLastUpdate * 0.5f;
}
- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
// render FBO tex
[self renderFBO];
// reset to main framebuffer
[((GLKView *) self.view) bindDrawable];
glViewport(0, 0, self.view.bounds.size.width, self.view.bounds.size.height);
// render main
glClearColor(0.65f, 0.65f, 0.65f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBindVertexArrayOES(_vertexArray);
[self.baseEffect prepareToDraw];
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}
#pragma mark - FBO
// intialize FBO
- (void)setupFBO
{
fbo_width = 512;
fbo_height = 512;
glGetIntegerv(GL_FRAMEBUFFER_BINDING, &defaultFBO);
NSLog(#"default FBO: %d", defaultFBO);
glGenFramebuffers(1, &backFBO);
glGenRenderbuffers(1, &depthBuffer);
glBindFramebuffer(GL_FRAMEBUFFER, backFBO);
glBindRenderbuffer(GL_RENDERBUFFER, depthBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24_OES, fbo_width, fbo_height);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthBuffer);
// FBO status check
GLenum status;
status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
switch(status) {
case GL_FRAMEBUFFER_COMPLETE:
NSLog(#"fbo complete");
break;
case GL_FRAMEBUFFER_UNSUPPORTED:
NSLog(#"fbo unsupported");
break;
default:
/* programming error; will fail on all hardware */
NSLog(#"Framebuffer Error");
break;
}
glBindFramebuffer(GL_FRAMEBUFFER, defaultFBO);
}
// render FBO
- (void)renderFBO
{
glBindFramebuffer(GL_FRAMEBUFFER, backFBO);
glViewport(0,0, fbo_width, fbo_height);
glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
**//in here, I hope to get the value of RGBA in offScreen FBO**
Byte pixelColor[4] = {0,};
glReadPixels(50, 50, fbo_width, fbo_height, GL_RGBA, GL_UNSIGNED_BYTE, pixelColor);
glBindFramebuffer(GL_FRAMEBUFFER, defaultFBO);
}
when I comment glReadPixels(), it's works well. Any help or advice would be much appreciated.

Answer 1

in your offScreen FBO, you should bind a GL_RENDERBUFFER, so you should add below code:
//create a new FBO
glBindFramebuffer(GL_FRAMEBUFFER, backFBO);
// add
GLuint _colorBuffer;
glGenRenderbuffers(1, &_colorBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, _colorBuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8_OES, fbo_width , fbo_height);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER, _colorBuffer);
I hope this will fix your error. good luck !!!