Accumulative Motion blur with modern OpenGL - opengl

Question

I am trying to implement Accumulation Motion Blur with modern OpenGL (Yes, I know it is slow and arguably realistic,in case Nicol Bolas is going to question...But that's what I need). My source of reference is OpenGL SuperBible-5.
It doesn't work for me.There is no blur in the output.I am passing 6 textures into fragment shader and it seems like they all have the same frame.
Also, I am getting debug log from OpenGL with the following message:
Severity:Medium, Message: Pixel-patch performance warning:Pixel transfer is synchronized with 3D rendering.
To me it looks like the PBO fails to blit pixels from backbuffer into the texture,or to acquire those from backbuffer.
Here is my setup:
GLint dataSize = _viewportW * _viewportH * 4 * sizeof(GLfloat);
void* data = (void*)malloc(dataSize);
memset(data, 0x00, dataSize);
glGenBuffers(1,&_pbo1);
glBindBuffer(GL_PIXEL_PACK_BUFFER,_pbo1);
glBufferData(GL_PIXEL_PACK_BUFFER,dataSize,data,GL_DYNAMIC_COPY);
glBindBuffer(GL_PIXEL_PACK_BUFFER , 0);
_blurTexs.resize(mbSamplesNum);
for(GLint i = 0; i <mbSamplesNum; ++i) {
glGenTextures(1, &_blurTexs[i]);
glActiveTexture(GL_TEXTURE0 + i);
glBindTexture(GL_TEXTURE_2D, _blurTexs[i]);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST );
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexStorage2D(GL_TEXTURE_2D,1, GL_RGBA8, _viewportW, _viewportH);
glTexSubImage2D(GL_TEXTURE_2D,0,0,0,_viewportW,_viewportH,GL_RGBA,GL_UNSIGNED_BYTE,data);
glBindTexture(GL_TEXTURE_2D,0);
}
And here is the render loop:
// HERE DRAW STUFF INTO BACKBUFFER.....
glBindBuffer(GL_PIXEL_PACK_BUFFER, _pbo1);
glReadPixels(0, 0, _viewportW, _viewportH, GL_RGBA, GL_UNSIGNED_BYTE, BUFFER_OFFSET(0));
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
// Next bind the PBO as the unpack buffer, then push the pixels straight into the texture
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, _pbo1);
// Setup texture unit for new blur, this gets imcremented every frame
GLuint curIndex =GetBlurTarget0();
glBindTexture(GL_TEXTURE_2D,_blurTexs[curIndex]);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, _viewportW, _viewportH, 0, GL_RGBA, GL_UNSIGNED_BYTE, BUFFER_OFFSET(0));
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, 0);
// Draw full screen quad with blur shader and all blur textures:
_progManager->BindPipeline(ACCUM_PIPELINE);
GLuint fragProg =GetProg();
GLuint t0= GetBlurTarget0();
GLuint t1= GetBlurTarget1();
GLuint t2= GetBlurTarget2();
GLuint t3= GetBlurTarget3();
GLuint t4= GetBlurTarget4();
GLuint t5= GetBlurTarget5();
glActiveTexture(GL_TEXTURE0 );
glBindTexture(GL_TEXTURE_2D,_blurTexs[t0]);
glActiveTexture(GL_TEXTURE1 );
glBindTexture(GL_TEXTURE_2D,_blurTexs[t1]);
glActiveTexture(GL_TEXTURE2 );
glBindTexture(GL_TEXTURE_2D,_blurTexs[t2]);
glActiveTexture(GL_TEXTURE3 );
glBindTexture(GL_TEXTURE_2D,_blurTexs[t3]);
glActiveTexture(GL_TEXTURE4 );
glBindTexture(GL_TEXTURE_2D,_blurTexs[t4]);
glActiveTexture(GL_TEXTURE5 );
glBindTexture(GL_TEXTURE_2D,_blurTexs[t5]);
_screenQuad->Draw();
glBindTexture(GL_TEXTURE_2D,0);
AdvanceBlurTaget();
_progManager->UnBindPipeline();
Here is my fragment shader :
#version 420 core
// This is the output color
out vec4 color;///layout (location = 0)
const int numFrames =6;
layout(binding=0)uniform sampler2D textureUnit0;
layout(binding=1)uniform sampler2D textureUnit1;
layout(binding=2)uniform sampler2D textureUnit2;
layout(binding=3)uniform sampler2D textureUnit3;
layout(binding=4)uniform sampler2D textureUnit4;
layout(binding=5)uniform sampler2D textureUnit5;
void main(void)
{
vec2 texelSize = 1.0 / vec2(textureSize(textureUnit0, 0));
vec2 screenTexCoords = gl_FragCoord.xy * texelSize;
// 0 is the newest image and 5 is the oldest
vec4 blur0 = texture(textureUnit0, texelSize);
vec4 blur1 = texture(textureUnit1, texelSize);
vec4 blur2 = texture(textureUnit2, texelSize);
vec4 blur3 = texture(textureUnit3, texelSize);
vec4 blur4 = texture(textureUnit4, texelSize);
vec4 blur5 = texture(textureUnit5, texelSize);
vec4 summedBlur = blur0 + blur1 + blur2 + blur3 + blur4 + blur5;
color = summedBlur / numFrames;
}
UPDATE
Ok,Ok, MY BAD....
Silly mistake in the fragment shader:
vec2 screenTexCoords = gl_FragCoord.xy * texelSize;
// 0 is the newest image and 5 is the oldest
vec4 blur0 = texture(textureUnit0, screenTexCoords );
vec4 blur1 = texture(textureUnit1, screenTexCoords );
vec4 blur2 = texture(textureUnit2, screenTexCoords );
vec4 blur3 = texture(textureUnit3, screenTexCoords );
vec4 blur4 = texture(textureUnit4, screenTexCoords );
vec4 blur5 = texture(textureUnit5, screenTexCoords );
I mistakenly passed texelSize for UV instead of screenTexCoords .