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OpenGL Everything renders upside down in Perspective

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So I just recently realized I had been using glOrtho when initializing Opengl to render 3D objects. You guys pointed out to me that was wrong, and now I've been converting all my 3d projects to using perspective via gluPerspective(). Everything has been going fine, and I've been sieving through tutorials and code on the subject, but it would seem that I have done something mundane terribly wrong. Whenever I render an object it is upside down from what I expect (even textures are mapped upside down). Moving an object in the negative Y direction moves it down (from top to bottom) on the screen, however moving in positive X moves the object to the right (left to right) as I would expect. Can any of you see something terribly wrong with my opengl initialization?

private void openglInit() {


glViewport(0, 0, 640, 480);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(45f, 640 / 480, .1f, 100.0f);
glMatrixMode(GL_MODELVIEW);

// Shade the model
glShadeModel(GL_SMOOTH);

glClearColor(0f, 0f, 0f, 0f); // Set the background?
glClearDepth(1f); // Enables clearing of the depth buffer?

// Sets the z buffer testing
glEnable(GL_DEPTH_TEST);

glDepthFunc(GL_LEQUAL); // Type of depth test
glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST); // Pretty perspective calculations

// Enables culling (only drawing polygons facing the correct way)
glEnable(GL_CULL_FACE);
// Cull the frontside
glCullFace(GL_FRONT);

glEnable(GL_LIGHTING);

glEnable(GL_LIGHT0);

glEnable(GL_TEXTURE_2D);

// Light up the world
getLit();
}

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Negative Y-axis is commonly pointing down. You have probably set up your orthographic projection with positive Y-axis going down which is why you expect the opposite. You just have to learn how axes are set up and design your application from there.

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You have probably set up your orthographic projection with positive Y-axis going down which is why you expect the opposite.


So I am supposed to be using both glOrtho and gluPerspective when I initialize opengl in the function above?

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So I just recently realized I had been using glOrtho when initializing Opengl to render 3D objects. You guys pointed out to me that was wrong, and now I've been converting all my 3d projects to using perspective via gluPerspective()...

Well, using orthogonal projection isn't wrong in itself. It is at most wrong for your desire.


... Everything has been going fine, and I've been sieving through tutorials and code on the subject, but it would seem that I have done something mundane terribly wrong. Whenever I render an object it is upside down from what I expect (even textures are mapped upside down). Moving an object in the negative Y direction moves it down (from top to bottom) on the screen, however moving in positive X moves the object to the right (left to right) as I would expect. Can any of you see something terribly wrong with my opengl initialization?

The OP doesn't show how the MODELVIEW matrix is set-up. I assume that you don't expect the mistake to be buried therein!?

Maybe it is okay for the used programming language (C# ?), but in C/C++ the expression 640/480 as is used as argument for gluPerspective is an integer expression and computes to 1. That wouldn't cause the reported issue but may nevertheless be not what was intended. (Further, e.g. gcc wouldn't be happy with 45f, 0f, and 1f, but I assume that your compiler doesn't reject that.)

Besides that, AFAIS the shown code snippet is okay. I would expect the mistake to be located elsewhere.

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[quote name='Brother Bob' timestamp='1307642987' post='4821408']
You have probably set up your orthographic projection with positive Y-axis going down which is why you expect the opposite.


So I am supposed to be using both glOrtho and gluPerspective when I initialize opengl in the function above?
[/quote]
Mixing orthogonal and perspective projection for the same rendering would not be meaningful. Brother Bob meant that your previous set-up using glOrtho was wrong (in OpenGL's typical use), and that you have compensated that mistake at another place (presumable the MODELVIEW matrix) instead of correcting it.That would support my assumption, too, that the mistake is located elsewhere.

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Brother Bob meant that your previous set-up using glOrtho was wrong (in OpenGL's typical use), and that you have compensated that mistake at another place (presumable the MODELVIEW matrix) instead of correcting it.

Not saying it's wrong, just that it's different and that you have to learn to handle the differences. Many people prefer Y-down for 2D.

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The OP doesn't show how the MODELVIEW matrix is set-up. I assume that you don't expect the mistake to be buried therein!?


As far as I know this is all the setting-up of the matrices I have done (besides a gltranslatef and two glRotatef calls right before I draw the model). The code in question is 3 classes, I didn't really want to flood the post with several hundred lines of code as it might be rather tedious to sort through.


Maybe it is okay for the used programming language (C# ?), but in C/C++ the expression 640/480 as is used as argument for gluPerspective is an integer expression and computes to 1. That wouldn't cause the reported issue but may nevertheless be not what was intended. (Further, e.g. gcc wouldn't be happy with 45f, 0f, and 1f, but I assume that your compiler doesn't reject that.)


This was a good point, I wasn't thinking about integer division when I wrote that line, thanks for pointing that out. By the way my code is in Java using the LWJGL.

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[quote name='haegarr' timestamp='1307643833' post='4821415']
Brother Bob meant that your previous set-up using glOrtho was wrong (in OpenGL's typical use), and that you have compensated that mistake at another place (presumable the MODELVIEW matrix) instead of correcting it.

Not saying it's wrong, just that it's different and that you have to learn to handle the differences. Many people prefer Y-down for 2D.
[/quote]
Yep, that's the reason I've added "in OpenGL's typical use" with the meaning in comparison to how tutorials and books AFAIK commonly use the y axis.


[quote name='haegarr' timestamp='1307643584' post='4821412']
The OP doesn't show how the MODELVIEW matrix is set-up. I assume that you don't expect the mistake to be buried therein!?


As far as I know this is all the setting-up of the matrices I have done (besides a gltranslatef and two glRotatef calls right before I draw the model). The code in question is 3 classes, I didn't really want to flood the post with several hundred lines of code as it might be rather tedious to sort through.
[/quote]
Of course, you're right. However, the relevant lines would be just a hand full. E.g. the formerly set-up using glOrtho ... do you have it still at hand? Can you post it for comparison?

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Of course, you're right. However, the relevant lines would be just a hand full. E.g. the formerly set-up using glOrtho ... do you have it still at hand? Can you post it for comparison?


Yeah I used to set up the orthogonal projection as such:
glOrtho(0, 640, 480, 0, 0.1f, 100.0f);

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Yeah I used to set up the orthogonal projection as such:
glOrtho(0, 640, 480, 0, 0.1f, 100.0f);

So you've used bottom=480 and top=0, hence bottom>top, hence an "upside down" (term not to be interpreted as "wrong" but compared to the common use in tutorials and the like ;)) projection as Brother Bob has guessed.
(See e.g this manual page. The usual set-up with glOrtho looks something like glOrtho(-width/2, +width/2, -height/2, +height/2, near, far), so that bottom<top.)

Your MODELVIEW matrix (or the model itself) is defined to match the upside down projection, i.e. you previously used a double "upside down" and hence "correct" view. But because the now used gluPerspective is not upside down, the same MODELVIEW set-up (or the model itself) causes the model to appear upside down.

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So you've used bottom=480 and top=0, hence bottom>top, hence an "upside down" (term not to be interpreted as "wrong" but compared to the common use in tutorials and the like ;)) projection as Brother Bob has guessed.
(See e.g this manual page. The usual set-up with glOrtho looks something like glOrtho(-width/2, +width/2, -height/2, +height/2, near, far), so that bottom<top.)

Your MODELVIEW matrix (or the model itself) is defined to match the upside down view. But because the now used gluPerspective is not upside down, the MODELVIEW set-up (or the model itself) causes the model to appear upside down.


Ahh alright, thanks for all the help guys. I've never run across any graphical programming where bottom<top so this one stumped me. Anyway I flipped all the y's and everything looks great.

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...I've never run across any graphical programming where bottom<top so this one stumped me. ...

Many windowing systems (i.e. systems responsible for GUIs) use a co-ordinate system where y runs from top to bottom. glOrtho is used typically to render a GUI, and your selection of arguments (I mean the 0, 640, 480, 0) hints at that you've copied from such a use case. However, in the given use case you are rendering a 3D scene. In a 3D scene it is typical that y runs from bottom to top. Although you may have not seen that because most tutorials use perspective projection when rendering 3D scenes. That's probably the whole mystery here...

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      // Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
      Binding Shader Resources
      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains tutorials, sample applications, asteroids performance benchmark and an example Unity project that uses Diligent Engine in native plugin.
      Atmospheric scattering sample demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows, Linux, Android, MacOS, and iOS platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and Metal backend is in the plan.
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