1. #1
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    Post HDRI

    Can you use HDRI images with Zbrush? If so, how?

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    Yes, there is a way to do it. First, the image must be converted to BMP, PSD or JPG format using something like HDR Shop. You should leave it in the Light Probe format. Load it into the Texture palette.

    Next, in the Light palette, you have two patches -- one for global diffuse, and one for global specular. Click on the patch to select a texture, in this case the HDRI image. Then you simply need to change the values of the sliders below the patches to get the effect that you'd like.

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    Question

    Aurick, to clear up confusion, when you say leave it in the 'light probe format', you mean as a picture of a reflective globe, rather than as the actual .hdr file format.

    (Right?)

    Thanks
    Jay

    "Talent and all that for the most part is nothing but hogwash. Any schoolboy with a little aptitude might very well draw better than I perhaps; but what he most often lacks is the tough yearning for realization, the teeth-grinding obstinacy and saying: even though I know I'm not capable of it, I'm still going to do it." -- M.C. Escher, in a letter to his son Arthur, 12 February 1955

    http://www.ejectejecteject.com/

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    Well, that photograph of the ball is also an HDR file format. However, HDR Shop gives you several options for the map including Light Probe and Latitude/Longitude (flat mapped). The best results with ZBrush are gotten using the Light Probe, which as you said is basically just the photograph of the reflective sphere.

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    Please tell me if I am wrong, but using GDI and GSI with a light probe image does not give typical HDRI render results. Instead, it makes almost every ZBrush material look reflective or shiny. The exceptions are certain special materials that don't incorporate reflectiveness at all and highly color-bumped materials where the bump effect hides the shiny effect.

    Two examples:



    This first render represents the ideal. Shiny metal looks great, and the ground plain uses an highly color-bumped material that mostly hides the reflective/shiny effect.



    In the second render, I did not want any surface other than the blood and floor to be shiny, but it was extremely difficult to get the 'head' and bolt-head to be as subdued as they are, which is not much. I turned GSI down to nothing, set the material specular and reflectiveness to nothing, and the materials were still very shiny with any significant GDI setting.

    Is it possible to render an HDRI-type image without turning everything shiny?

    (For everyone's information, the shadows were created with normal lights, not as a function of the GDI/GSI image.)

    Thanks,
    Jay

    "Talent and all that for the most part is nothing but hogwash. Any schoolboy with a little aptitude might very well draw better than I perhaps; but what he most often lacks is the tough yearning for realization, the teeth-grinding obstinacy and saying: even though I know I'm not capable of it, I'm still going to do it." -- M.C. Escher, in a letter to his son Arthur, 12 February 1955

    http://www.ejectejecteject.com/

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    A BMP, PSD or JPG file will still be an image of a reflected sphere or fish eye lens of an environment but it will not be an HDR image. By it's nature, a HDR image contains multiple exposures or f-stops of the scene which allows some 3D apps to use the light strength of the HDR image to recreate accurate GI-lighting and caustics of the original scene in 3D. A light probe HDRi in HDRIshop can be taken up and down through a range of exposures whilst a BMP, PSD or JPG are a 'snapshot' of any one exposure in a HDRi's range.

    Grassy Noel

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    Hi Noel,

    Yes, I know all that. It is indeed a misnomer to refer ZBrush GDI renders as HDRI renders. However, using the High Dynamic Range setting in the materials, along with the GDI rendering technique above, we can almost, sort-of, approximate the look of an HDRI render, given that you want to do reflective objects.

    The question I had is do the above renders and my characterization of ZBrush's capabilities in the area of Global Diffuse and HDRI-style rendering (or lack thereof) accurately represent ZBrush's total capabilities in this area? Is there something I am missing?

    When using GDI in conjunction with a HDRI-type light-probe image, most all ZBrush materials exhibit high reflectivity or shininess, even when their settings for reflectivity is set to zero. Furthermore, no shadowing information comes from the GDI effect. Using GDI by itself does not generate any shadows, so you will still need at least a key or primary light to generate shadows in the scene. The manual describes a procedure to 'record' information about a complex light scheme in ZBrush into a captured texture using a sphere on an empty layer. In other words, a virtual light probe is used to create a light probe image that only captures or 'records' light color and intensity. When an image is used in conjunction with GDI, the color and intensity in the image seems to be mapped directly to the object(s) in the scene.

    Of course, this is what HDRI rendering does, but ZBrush seems to have a very simple implementation of this concept. It appears to transfer, or map, each pixel of the light-probe image onto each object in the scene without checking for collision (i.e. shadow-casting). That is what we see in the ring-and-ball render above.

    However, in a more accurate implementation of "global diffuse" lighting, multiple 'rays' would be 'reflected' off of, or originate from each pixel of the light-probe image. These rays would be traced to the first surface in their path. The render that would result would depend on the diffuse and specular characteristics of the objects' materials. A more accurate render would need to incorporate multiple bounces for reflective materials. The objects in the scene would prevent most rays from reaching any overhung surface, so very natural shadows would be formed. Diffuse materials would be softly lit, and reflective materials would reflect the image of the light-probe as well as the objects around them.

    From my experience, and what the manual actually states about the intended use of GDI/GSI, this feature is a very 'lite' version of the GDI/HDRI concept. I hope that it will be improved in the future to work like other high-end rendering applications, and perhaps it will be able to use truly high-dynamic-range images.
    Jay

    "Talent and all that for the most part is nothing but hogwash. Any schoolboy with a little aptitude might very well draw better than I perhaps; but what he most often lacks is the tough yearning for realization, the teeth-grinding obstinacy and saying: even though I know I'm not capable of it, I'm still going to do it." -- M.C. Escher, in a letter to his son Arthur, 12 February 1955

    http://www.ejectejecteject.com/

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    The main use of GDI/GSI according to the manual:

    Suppose you have a scene with several lights:



    You can use GDI/GSI to simulate the effect of those lights to a degree, freeing up the lights for other uses in the scene. First create a white sphere using the Basic material on a blank layer, and turn off all ambient effects (material, global lighting). Set Specular of the material to zero, render, and MRGBGrab the picture using AutoCrop and ShadedRGB:



    Now set the Diffuse of the material to zero, and set Specular back to where it was. Render and MRGBGrab the picture of the specular highlights. You may need to edit the resulting texture to get it back to the same size as the first grabbed texture:



    Pull these textures into the GDI and GSI map slots. Turn off all lights, set GDI and GSI sliders to max, and make sure the material Diffuse and Specular are back where they were originally. Render the scene, and even though the lights are off, you will have a similar result. The shadows will be absent, but the lighting will be similar to the original scene:

    Jay

    "Talent and all that for the most part is nothing but hogwash. Any schoolboy with a little aptitude might very well draw better than I perhaps; but what he most often lacks is the tough yearning for realization, the teeth-grinding obstinacy and saying: even though I know I'm not capable of it, I'm still going to do it." -- M.C. Escher, in a letter to his son Arthur, 12 February 1955

    http://www.ejectejecteject.com/

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    nice post
    jay you hdri pict is not really hdri with that methode I think.
    Because Zbrush dont import true hdri format(light reflection coded in 16 bites grey level)

    I'm right or not?

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    Talking

    Hi Fouad,

    yes, you are right in that no maps in ZBrush can be greater than the limit of .psd, .bmp, or .jpg files.

    However, remember that many materials have an HDR slider that can increase the dynamic range within ZBrush. This is what I am referring to when I say that you can achieve certain effects.

    Also, when other packages render "HDR images" they are mostly still outputing to bitmaps like ZBrush does. Internally some of them use truly HDR images. How do they render?

    "HDRI" is becoming a catch-all term for certain types of rendering. There are really two different aspects of this type of rendering. One is of course, the High Dynamic Range Image. The other is Image Based Lighting, IBL. What the GDI feature in ZBrush does is a form of IBL, no doubt. I wish it was more advanced than it is, but it is definitely an implementation of IBL. Throw in a little HDR slider action on your material and you can see why I say that a HDRI-type image can be approximated by ZBrush.

    The reason that IBL and HDRI are intertwined is that IBL works much better if you have an HDR image as your 'light source'.

    I'm sure there is more than one technique and more than one implementation of a given technique. There is more than one way to skin a cat. In general, the light probe image, whether it is HDRI or not, is used as an environment map during the rendering process. In human terms, it is as if the render engine builds a hemishpere or sphere around the scene, maps the lightprobe to the inside of the sphere, and then uses each pixel of the image on the inside of the sphere as a light source. From this point, implementations will diverge greatly. How many rays will be cast from each pixel-light-source? How will they be traced and how will collisions be handled? How many times should they bounce before you give up tracing a given ray? (The current ZBrush implemenation appears to be relatively simple.)

    Maybe ZBrush will allow the use of HDR images in the future, or maybe it will allow us to take the raw light-probe images (several at different exposure levels, for example, to give a high dynamic range), and then it would render an "HDRI/IBL" render from these multiple images.
    Jay

    "Talent and all that for the most part is nothing but hogwash. Any schoolboy with a little aptitude might very well draw better than I perhaps; but what he most often lacks is the tough yearning for realization, the teeth-grinding obstinacy and saying: even though I know I'm not capable of it, I'm still going to do it." -- M.C. Escher, in a letter to his son Arthur, 12 February 1955

    http://www.ejectejecteject.com/

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    Cool

    A very interesting thread indeed you guys!
    I'm enjoying this interchange, and I hope Pixologic is watching, because these discussions are a great 'Flight Plan' for future upgrades...

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    quote:
    Can you use HDRI images with Zbrush? If so, how?


    Jeesh.. The answer to this simple question is: "no"

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    Thanks for all the information. I guess full implementation of HDRI will have to wait.

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