# DX11 Depth buffer and depth stencil state comparison function confusion

## Recommended Posts

I was wondering if anyone could explain the depth buffer and the depth stencil state comparison function to me as I'm a little confused

So I have set up a depth stencil state where the DepthFunc is set to D3D11_COMPARISON_LESS, but what am I actually comparing here? What is actually written to the buffer, the pixel that should show up in the front?

I have these 2 quad faces, a Red Face and a Blue Face. The Blue Face is further away from the Viewer with a Z index value of -100.0f. Where the Red Face is close to the Viewer with a Z index value of 0.0f.

When DepthFunc is set to D3D11_COMPARISON_LESS the Red Face shows up in front of the Blue Face like it should based on the Z index values. BUT if I change the DepthFunc to D3D11_COMPARISON_LESS_EQUAL the Blue Face shows in front of the Red Face. Which does not make sense to me, I would think that when the function is set to D3D11_COMPARISON_LESS_EQUAL the Red Face would still show up in front of the Blue Face as the Z index for the Red Face is still closer to the viewer

Am I thinking of this comparison function all wrong?

Vertex data just in case


//Vertex date that make up the 2 faces
Vertex verts[] = {

//Red face
Vertex(Vector4(0.0f, 0.0f,     0.0f), Color(1.0f, 0.0f, 0.0f)),
Vertex(Vector4(100.0f, 100.0f, 0.0f), Color(1.0f, 0.0f, 0.0f)),
Vertex(Vector4(100.0f, 0.0f,   0.0f), Color(1.0f, 0.0f, 0.0f)),
Vertex(Vector4(0.0f, 0.0f,     0.0f), Color(1.0f, 0.0f, 0.0f)),
Vertex(Vector4(0.0f, 100.0f,   0.0f), Color(1.0f, 0.0f, 0.0f)),
Vertex(Vector4(100.0f, 100.0f, 0.0f), Color(1.0f, 0.0f, 0.0f)),

//Blue face
Vertex(Vector4(0.0f, 0.0f,     -100.0f), Color(0.0f, 0.0f, 1.0f)),
Vertex(Vector4(100.0f, 100.0f, -100.0f), Color(0.0f, 0.0f, 1.0f)),
Vertex(Vector4(100.0f, 0.0f,   -100.0f), Color(0.0f, 0.0f, 1.0f)),
Vertex(Vector4(0.0f, 0.0f,     -100.0f), Color(0.0f, 0.0f, 1.0f)),
Vertex(Vector4(0.0f, 100.0f,   -100.0f), Color(0.0f, 0.0f, 1.0f)),
Vertex(Vector4(100.0f, 100.0f, -100.0f), Color(0.0f, 0.0f, 1.0f)),
};

##### Share on other sites

Screen space (post-projection / NDC) Z goes from 0 to 1 (or 0 to w in post-projection). Your vertex shader transforms your vertex data to this space, usually using a projection matrix.

Maybe your projection matrix is squashing your Z range so that both triangles end up at NDCz=0

##### Share on other sites
On 11/18/2017 at 1:33 AM, noodleBowl said:

but what am I actually comparing here?

You're comparing the value thats already in the depth buffer to the z value of the fragment you want to write to the screen.

edit - did I get that backwards...

Edited by Infinisearch

##### Share on other sites
9 hours ago, Infinisearch said:

You're comparing the value thats already in the depth buffer to the z value of the fragment you want to write to the screen.

This is what I thought, which was why I was really confused. Turns out my problem had nothing to do with my depth buffer/depth stencil state. Turns out the viewport has MinDepth and MaxDepth properties. I never set these values for my viewport after setting these everything seems to work correctly depth wise

Side question about the Z near and Z far plans for projection matrices. If I have a right handed coord system, so positive Z points towards the Viewer (or me) and negative Z goes further into the screen does this mean my Z near plan should be a positive value and my Z far plan should be negative value? Does this change between different projection matrix types such as Perspective vs Orthographic?

##### Share on other sites

That depends on your function or library that you're using for creating your project matrix. If you're using D3DX or DirectXMath functions for creating a "right-handed" perspective projection matrix, then you'll specify your near and far clipping plane parameters as the absolute distance from the camera to the plane. So they would both be positive, typically with zFar > zNear (although you can flip them if you want a reversed Z buffer, which can be be useful for working with floating point depth buffer formats). This is the same for the orthographic projections in both of those libraries. If you're using a different library, you should check the documentation to see what values they expect, although it's very likely to be the same as the D3DX/DirectXMath functions.

FYI the documentation for the D3DX matrix functions can be helpful to look at, since they show you exactly how the matrix is constructed (you can also look at the actual implementations for the DirectXMath functions, since it's all in inline header files). If  you look at the RH projection functions, you'll see that they essentially end up negating the Z value so that it ends up positive in clip space, since this is what D3D expects.

## Create an account

Register a new account

• 22
• 10
• 19
• 15
• 14
• ### Similar Content

• Hi again,  After some looking around I have decided to base my game directly on Direct X rather than using an existing game engine.  Because of the nature of the stuff I'm doing it just didn't seem to fit very well and I kept running into road blocks.  At this point I have a big blob of code for doing fractal world generation and some collision code,  and I'm trying to put it into some form that resembles a game engine.  Since I've never used one before It's a bit alien to me ..... so can someone direct me to a book, website, article, whatever... that covers this?  I'm mainly looking for stuff that covers C++ library design. I'm not adverse to using 3rd party tools for stuff I can used them for.
• By chiffre
Introduction:
In general my questions pertain to the differences between floating- and fixed-point data. Additionally I would like to understand when it can be advantageous to prefer fixed-point representation over floating-point representation in the context of vertex data and how the hardware deals with the different data-types. I believe I should be able to reduce the amount of data (bytes) necessary per vertex by choosing the most opportune representations for my vertex attributes. Thanks ahead of time if you, the reader, are considering the effort of reading this and helping me.
I found an old topic that shows this is possible in principal, but I am not sure I understand what the pitfalls are when using fixed-point representation and whether there are any hardware-based performance advantages/disadvantages.
(TLDR at bottom)
The Actual Post:
To my understanding HLSL/D3D11 offers not just the traditional floating point model in half-,single-, and double-precision, but also the fixed-point model in form of signed/unsigned normalized integers in 8-,10-,16-,24-, and 32-bit variants. Both models offer a finite sequence of "grid-points". The obvious difference between the two models is that the fixed-point model offers a constant spacing between values in the normalized range of [0,1] or [-1,1], while the floating point model allows for smaller "deltas" as you get closer to 0, and larger "deltas" the further you are away from 0.
To add some context, let me define a struct as an example:
struct VertexData { float[3] position; //3x32-bits float[2] texCoord; //2x32-bits float[3] normals; //3x32-bits } //Total of 32 bytes Every vertex gets a position, a coordinate on my texture, and a normal to do some light calculations. In this case we have 8x32=256bits per vertex. Since the texture coordinates lie in the interval [0,1] and the normal vector components are in the interval [-1,1] it would seem useful to use normalized representation as suggested in the topic linked at the top of the post. The texture coordinates might as well be represented in a fixed-point model, because it seems most useful to be able to sample the texture in a uniform manner, as the pixels don't get any "denser" as we get closer to 0. In other words the "delta" does not need to become any smaller as the texture coordinates approach (0,0). A similar argument can be made for the normal-vector, as a normal vector should be normalized anyway, and we want as many points as possible on the sphere around (0,0,0) with a radius of 1, and we don't care about precision around the origin. Even if we have large textures such as 4k by 4k (or the maximum allowed by D3D11, 16k by 16k) we only need as many grid-points on one axis, as there are pixels on one axis. An unsigned normalized 14 bit integer would be ideal, but because it is both unsupported and impractical, we will stick to an unsigned normalized 16 bit integer. The same type should take care of the normal vector coordinates, and might even be a bit overkill.
struct VertexData { float[3] position; //3x32-bits uint16_t[2] texCoord; //2x16bits uint16_t[3] normals; //3x16bits } //Total of 22 bytes Seems like a good start, and we might even be able to take it further, but before we pursue that path, here is my first question: can the GPU even work with the data in this format, or is all I have accomplished minimizing CPU-side RAM usage? Does the GPU have to convert the texture coordinates back to a floating-point model when I hand them over to the sampler in my pixel shader? I have looked up the data types for HLSL and I am not sure I even comprehend how to declare the vertex input type in HLSL. Would the following work?
struct VertexInputType { float3 pos; //this one is obvious unorm half2 tex; //half corresponds to a 16-bit float, so I assume this is wrong, but this the only 16-bit type I found on the linked MSDN site snorm half3 normal; //same as above } I assume this is possible somehow, as I have found input element formats such as: DXGI_FORMAT_R16G16B16A16_SNORM and DXGI_FORMAT_R16G16B16A16_UNORM (also available with a different number of components, as well as different component lengths). I might have to avoid 3-component vectors because there is no 3-component 16-bit input element format, but that is the least of my worries. The next question would be: what happens with my normals if I try to do lighting calculations with them in such a normalized-fixed-point format? Is there no issue as long as I take care not to mix floating- and fixed-point data? Or would that work as well? In general this gives rise to the question: how does the GPU handle fixed-point arithmetic? Is it the same as integer-arithmetic, and/or is it faster/slower than floating-point arithmetic?
Assuming that we still have a valid and useful VertexData format, how far could I take this while remaining on the sensible side of what could be called optimization? Theoretically I could use the an input element format such as DXGI_FORMAT_R10G10B10A2_UNORM to pack my normal coordinates into a 10-bit fixed-point format, and my verticies (in object space) might even be representable in a 16-bit unsigned normalized fixed-point format. That way I could end up with something like the following struct:
struct VertexData { uint16_t[3] pos; //3x16bits uint16_t[2] texCoord; //2x16bits uint32_t packedNormals; //10+10+10+2bits } //Total of 14 bytes Could I use a vertex structure like this without too much performance-loss on the GPU-side? If the GPU has to execute some sort of unpacking algorithm in the background I might as well let it be. In the end I have a functioning deferred renderer, but I would like to reduce the memory footprint of the huge amount of vertecies involved in rendering my landscape.
TLDR: I have a lot of vertices that I need to render and I want to reduce the RAM-usage without introducing crazy compression/decompression algorithms to the CPU or GPU. I am hoping to find a solution by involving fixed-point data-types, but I am not exactly sure how how that would work.

• Well i found out Here what's the problem and how to solve it (Something about world coordinates and object coordinates) but i can't understand how ti works. Can you show me some examples in code on how you implement this???

Scaling Matrix:
m_Impl->scale = glm::mat4(1.0f); m_Impl->scale = glm::scale(m_Impl->scale, glm::vec3(width, height, 0)); Verticies:
//Verticies. float verticies[] = { //Positions. //Texture Coordinates. 1.0f, 1.0f, 0.0f, 0.0f, 2.0f, 1.0f, 1.0f, 0.0f, 2.0f, 2.0f, 1.0f, 1.0f, 1.0f, 2.0f, 0.0f, 1.0f }; Rendering:
//Projection Matrix. glm::mat4 proj = glm::ortho(0.0f, (float)window->GetWidth(), 0.0f, (float)window->GetHeight(), -1.0f, 1.0f); //Set the uniform. material->program->setUniformMat4f("u_MVP", proj * model); //model is the scale matrix from the previous code. //Draw. glDrawElements(GL_TRIANGLES, material->ibo->GetCount(), GL_UNSIGNED_INT, NULL);
#shader vertex #version 330 core layout(location = 0) in vec4 aPos; layout(location = 1) in vec2 aTexCoord; out vec2 texCoord; uniform mat4 u_MVP; void main() { gl_Position = u_MVP*aPos; texCoord = aTexCoord; } #shader fragment #version 330 core out vec4 colors; in vec2 texCoord; uniform sampler2D u_Texture; void main() { colors = texture(u_Texture, texCoord); }
Before Scaling (It's down there on the bottom left corner as a dot).

After Scaling

Problem: Why does the position also changes?? If you see my Verticies, the first position starts at 1.0f, 1.0f , so when i'm scaling it should stay at that position

• Hey guys!

Ok so I have been developing some ideas to get to work on and I have one specifically that I need some assistance with. The App will be called “A Walk On the Beach.” It’s somewhat of a 3D representation of the Apple app “Calm.” The idea is that you can take a virtual stroll up and down a pier on the beach. Building the level of a pier seems self explanatory to me... but my question is this.... How could I make it so that players can leave notes on the pier for other users to read and or respond to? I was thinking something like a virtual “peg board” at the end of the pier where players can “pin up” pictures or post it’s.