getting a crash while using glReadPixels to map texture in memory

Question

I'm trying to use glReadPixels to map a texture in memory to be able to use openGL's immutable storage feature.

I'm using deferred shading so I prepare my G-Buffers as following:

glGenFramebuffers(1, &_gBuffer);
glBindFramebuffer(GL_FRAMEBUFFER, _gBuffer);
glGenTextures(1, &_pos3dTex);
glBindTexture(GL_TEXTURE_2D, _pos3dTex);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F, width, height, 0, GL_RGBA, GL_FLOAT, NULL);
GLuint layout = glGetFragDataLocation(_progID, "positionTexture");
...
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + layout, GL_TEXTURE_2D, _pos3dTex, 0);
...
unsigned int attachments[1] = { GL_COLOR_ATTACHMENT0 + layout };
glDrawBuffers(1, attachments);

glGenRenderbuffers(1, &_rboDepth);
glBindRenderbuffer(GL_RENDERBUFFER, _rboDepth);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, width, height);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, _rboDepth);

And then I want to map my texture in memory so I can use immutable storage:

//Bind Data
glGenBuffers(1, &_pos3dPbo);
glBindBuffer(GL_PIXEL_PACK_BUFFER, _pos3dPbo);
//w*h*4 (Channels)*4(size of float)
glBufferData(GL_PIXEL_PACK_BUFFER,width*height*4* 4, 0, GL_STREAM_READ);
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);

glBindFramebuffer(GL_FRAMEBUFFER, _gBuffer);
glReadBuffer(GL_COLOR_ATTACHMENT0 + layout);
glPixelStorei(GL_PACK_ALIGNMENT, 4);//4 channels

glReadPixels(0, 0, width, height, GL_RGBA,GL_FLOAT,NULL);
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);

//Reading
glBindBuffer(GL_PIXEL_PACK_BUFFER, _pos3dPbo);
void* outPos3d = glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_ONLY);
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);

//Releasing 
glBindBuffer(GL_PIXEL_PACK_BUFFER, _pos3dPbo);
glUnmapBuffer(GL_PIXEL_PACK_BUFFER);
glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);

I'm getting an "Access Violation" just after the glReadPixels function. What Am I doing wrong plz?

I'm not sure what you are actually trying to do (deferred shading does not need a read-back and no mapping of gbuffers to CPU memory, glReadPixels has nothing to do with immutable storage), but passing NULL as last argument to glReadPixels is certainly wrong. The last pointer tells OpenGL where it should write the pixel data to. But if you pass a null-pointer, every write operation on that pointer will produce a access violation. — BDL, Aug 02 '19 at 13:45
`glPixelStorei(GL_PACK_ALIGNMENT, 4);//4 channels` is also strange since the 4 in the command is not related to the number of channels. It defines at which *byte* alignment rows will start. — BDL, Aug 02 '19 at 13:48
What I'm doing is creating some g-buffers, go through the rendering and then read-back data of textures into my CPU application. using glBufferData with GL_STATIC_DRAW is working fine but I want faster reading so I'm trying immutable storage (glBufferStorage) .So as I've seen in some codes they prepare the texture pbo for Asynchronus Buffer Access. — user6138759, Aug 02 '19 at 14:01

score 2 · Accepted Answer · edited Aug 02 '19 at 15:04

2

You are unbinding your GL_PIXEL_PACK_BUFFER before the glReadPixels, so the NULL pointer will be interpreted relative to your client memory address space, and trying to copy the data to there will almost certainly result in a crash.

edited Aug 02 '19 at 15:04

Nicol Bolas

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answered Aug 02 '19 at 15:00

derhass

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getting a crash while using glReadPixels to map texture in memory

1 Answers1