Hi group,

For creating a shadow map I've set up a depth render buffer object, using
this code:

      GLuint frameBuffer = 0;
      GLuint renderBuffer = 0;

      glGenFramebuffersEXT(1, &frameBuffer);
      glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, frameBuffer);

      // Render depth values to a buffer.
      glGenRenderbuffersEXT(1, &renderBuffer);
      glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, renderBuffer);
      glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT24,
width, height);
      glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT,
GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, renderBuffer);

      glDrawBuffer(GL_NONE);
      glReadBuffer(GL_NONE);

      // Check validity of the frame buffer and go back to traditional
rendering if something failed.
      GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
      if (status != GL_FRAMEBUFFER_COMPLETE_EXT)
        canUseFBOs = false;

This works fine so far but now I want to get the depth values into a memory
buffer to make it a nice look looking, smooth shadow (blurring it,
adjusting alpha values and such). Later I want to take the result and use
this as texture for a simple quad. The problem I have is the glReadPixels
call, which throws an access violation in the ATI driver. It looks so:

        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 

        render(1);

        void* buffer = malloc(3 * width * height);
        glReadBuffer(GL_DEPTH_ATTACHMENT_EXT);
        glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, buffer);

What is it that I'm doing wrong here? It doesn't really look complicated to
read the pixels. I have already done this with the color attachment (to
save to a file), but reading the depth values causes trouble.

Thank you,

Mike
-- 
www.soft-gems.net

Mike Lischke wrote:
> Hi group,
>
> For creating a shadow map I've set up a depth render buffer object, using
> this code:
>
>       GLuint frameBuffer = 0;
>       GLuint renderBuffer = 0;
>
>       glGenFramebuffersEXT(1, &frameBuffer);
>       glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, frameBuffer);
>
>       // Render depth values to a buffer.
>       glGenRenderbuffersEXT(1, &renderBuffer);
>       glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, renderBuffer);
>       glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT24,
> width, height);
>       glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT,
> GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, renderBuffer);
>
>       glDrawBuffer(GL_NONE);
>       glReadBuffer(GL_NONE);
>
>       // Check validity of the frame buffer and go back to traditional
> rendering if something failed.
>       GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
>       if (status != GL_FRAMEBUFFER_COMPLETE_EXT)
>         canUseFBOs = false;
>
> This works fine so far but now I want to get the depth values into a memory
> buffer to make it a nice look looking, smooth shadow (blurring it,
> adjusting alpha values and such). Later I want to take the result and use
> this as texture for a simple quad. The problem I have is the glReadPixels
> call, which throws an access violation in the ATI driver. It looks so:
>
>         glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
>
>         render(1);
>
>         void* buffer = malloc(3 * width * height);
>         glReadBuffer(GL_DEPTH_ATTACHMENT_EXT);
>         glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, buffer);
>
> What is it that I'm doing wrong here? It doesn't really look complicated to
> read the pixels. I have already done this with the color attachment (to
> save to a file), but reading the depth values causes trouble.
>
> Thank you,
>
> Mike
> --
> www.soft-gems.net

Hi,

The reason for the access violation is that your buffer is too small.
The format argument to glReadPixels should be GL_DEPTH_COMPONENT
and the type argument should be GL_FLOAT, giving

void *buffer = malloc(sizeof(GLfloat) * width * height);
glReadPixels(0, 0, width, height, GL_DEPTH_COMPONENT, GL_FLOAT,
buffer);

Presumably sizeof(GLfloat) >= 4 and so that was the reason for the
access violation.

All the best,

Nicholas

Mike Lischke <newsgroups@soft-gems.net> wrote:

> This works fine so far but now I want to get the depth values into a
> memory buffer to make it a nice look looking, smooth shadow (blurring
> it, adjusting alpha values and such). Later I want to take the result
> and use this as texture for a simple quad. The problem I have is the

    If you are using shadow mapping, be aware of the fact that blurring 
the depthmap will not blur the shadow. Read up on "Percentage Closer 
Filtering" (short PCF) .. there is a pretty old paper (mid 80s I think) 
about it out there written by William Reeves et al.

    Alternatively, there was an article on either gamedev or devmaster, 
which described one (not too fast) technique for rendering soft shadows.

hth
-- 
jb

(reply address in rot13, unscramble first) 

Nicholas Nash wrote

>The reason for the access violation is that your buffer is too small.
>The format argument to glReadPixels should be GL_DEPTH_COMPONENT
>and the type argument should be GL_FLOAT, giving

Exactly that was it, thank you.

Mike
-- 
www.soft-gems.net

Jakob Bieling wrote

>If you are using shadow mapping, be aware of the fact that blurring the
>depthmap will not blur the shadow. 

Hmm, I'd think it would. If I create an image with smooth shadow and use
this as texture I have of course this shadow there too. What I'm after is
not a complete and correct dynamic scene shadowing, but rather good looking
but cheap drop shadows underneath my 2D objects. So mostly the shape is
important and stored in a texture for quick rendering, everything else is
done only when the shape changes. I have no idea how that works out with
many objects (can be 2000+, each with an own shadow)

>Read up on "Percentage Closer
>Filtering" (short PCF) .. there is a pretty old paper (mid 80s I think)
>about it out there written by William Reeves et al.

Interesting stuff. Found some pdfs (from ATI and Max-Planck-Institut f�r
Infomatik in Saarbr�cken). However, I'm afraid this will too expensive (in
terms of render time), but since I never have tried that I can't say for
sure.

>  Alternatively, there was an article on either gamedev or devmaster,
>which described one (not too fast) technique for rendering soft shadows.

Do you know a sample implementation?

Thanks for your help, Jacob.

Mike
-- 
www.soft-gems.net

Mike Lischke <newsgroups@soft-gems.net> wrote:
> Jakob Bieling wrote

>> If you are using shadow mapping, be aware of the fact that blurring
>> the depthmap will not blur the shadow.

> Hmm, I'd think it would. If I create an image with smooth shadow and
> use this as texture I have of course this shadow there too. What I'm
> after is not a complete and correct dynamic scene shadowing, but
> rather good looking but cheap drop shadows underneath my 2D objects.

    Does not sound like commonly known shadow mapping technique. In this 
case your approach should work, if I understood you correctly :)

>> Read up on "Percentage Closer
>> Filtering" (short PCF) .. there is a pretty old paper (mid 80s I
>> think) about it out there written by William Reeves et al.

> Interesting stuff. Found some pdfs (from ATI and Max-Planck-Institut
> f�r Infomatik in Saarbr�cken). However, I'm afraid this will too
> expensive (in terms of render time), but since I never have tried
> that I can't say for sure.

    It is expensive. And since you are not using the shadow mapping 
technique, where depth-value comparisons are involved, it is probably 
not of much use for you either.

>>  Alternatively, there was an article on either gamedev or devmaster,
>> which described one (not too fast) technique for rendering soft
>> shadows.

> Do you know a sample implementation?

    I think the article has code accompanying it. But it is using 
DirectX. Modifying this for OpenGL would be some work, but surely is not 
impossible.

hth
-- 
jb

(reply address in rot13, unscramble first)