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OpenGL So, you want to make a 2d OpenGL game engine (long)

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A few months ago I posted up a thread on loading models from the wavefront OBJ file format. I enjoyed doing so, and the thread got a lot of response. There are a lot of tutorials out there that cover 2d OpenGL. However, they all lack a level of completeness and style in my opinion. I feel the need to lay down another, with the following ideals: - An interface not unlike OpenGL's design, though supporting threading. - Using DevIL for image processing. - Support for 2d texture-mapped fonts based those created by LMNOpc's BitmapFontBuilder - Free of underlying wrapper code (Win, Lin, SDL, or GLFW for instance) Those first three are obvious and easy to acheive, the last though is the kicker. I'll be supplying functions that need 'filled in' for whatever wrapper is used. I need access to mutex for instance, and that is something that needs filled in by the user. First, let me prototype the functions I'm going to explain in the first post.
// ********************************************************************************
// Config
typedef struct _v2dConfig {
     int ScreenWidth, ScreenHeight;
} v2dConfig;
extern v2dConfig v2d;

// ********************************************************************************
// General Functions
void v2dEnter();
void v2dExit();
void v2dFlip();

// ********************************************************************************
// Image Functions
int v2dLoadImage(char *Filename);
void v2dFreeImage(int i);
void v2dRealizeImage(int v); // usually called internally
void v2dBindImage(int v);
void v2dImage(float x, float y,int i);

v2dEnter() initializes the 2d projection mode, readying OpenGL for drawing in 2d. v2dExit() closes up 2d drawing mode. vxdFlip() has to be filled in by the user, and flips the OpenGL buffers to show what has been done. v2dLoadImage() reads an image file from storage using DevIL and loads it into memory. It returns a unique number to identify the texture/image. v2dFreeImage() releases all the memory attached to an image object when it is no longer needed. v2dRealizeImage() pumps the loaded image into OpenGL, and is called internally as needed. I separate this from ~LoadImage() such that you may call that function from another thread. The ~RealizeImage() is always called in the main context holding thread and can use valid OpenGL calls. v2dBindImage() binds the current image for OpenGL drawing and makes sure it has been realized with ~RealizeImage(). v2dImage() draws the image i at the specified 2d coords on screen.

void v2dEnter()
    glMatrixMode(GL_PROJECTION); // select the projection matrix
    glLoadIdentity(); // clear it
    glOrtho(0,v2d.ScreenWidth,v2d.ScreenHeight,0,-1,1); // setup a new projection for 2d
    glMatrixMode(GL_MODELVIEW); // select the model matrix
    glLoadIdentity(); // clear it

    glDisable(GL_DEPTH_TEST); // we aren't going to depth test
    glDisable(GL_LIGHTING); // or light the scene
    glEnable(GL_BLEND); // we however do use alpha and blending
    glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA); // for transparency
    glColor4f(1.0f,1.0f,1.0f,1.0f); // make sure our normal color is full white

void v2dExit()
    // If we need to cleanup custom information, here is where we would
    // right now we don't need to though... (stub)

void vxdFlip()
    // call glfwSwapBuffers(), SwapBuffers(HDC hdc), SDL_GL_SwapBuffers(), etc.
    // make sure the buffers are swapped and we see what we draw...

[Edited by - Ranger_One on January 11, 2006 10:30:59 AM]

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Now I can move onto the image functions, and after which I'll talk a bit about how and why things are handled the way they are.

First, we need to define a struct to hold the information for each image/texture object.

// we need a struct to hold the information
typedef struct _v2dImg {
GLuint TexId; // the OpenGL texture name for this image
ILuint ImageId; // the DevIL image name for this image
ILinfo Info; // info about the image pulled from DevIL
char SourceFile[256]; // the source file for the image

float x,y,w,h; // rendering coords, in case we had to enlarge the canvas...
} v2dImg;

vector<v2dImg*> v2dImageStack;
vector<int> v2dFreeImage;

- The last part is important. I'm talking about the rendering coords. The load function allows the loading of images that are any dimension, but textures in OpenGL have to use dimensions that are a power of two. The function enlarges the canvas of the image to make it fit those restrictions, and the rendering coords allow the engine to draw the original image in spite of the change.
- The vector<> defines create two stacks for images. The first holds actual image data, and the second references 'free' entries in the stack. Since we are going for speed in an engine, I never actually free any allocated v2dImg. Instead I reuse objects by marking them as free.

Now lets get to the v2dLoadImage function...

int v2dLoadImage(char *Filename)
int ret = -1; // our returned value
// first, let DevIL try and load the image
ILuint DevilImageName;
ILinfo DevilImageInfo;

ilGenImages(1,&DevilImageName); // new devil image name
ilBindImage(Image[v]->ImageId); // now bound into the context
if (ilLoadImage((char*)Filename) == IL_TRUE) // did it load?
iluGetImageInfo(&DevilImageInfo); // read back the info
ilConvertImage(IL_RGBA,IL_BYTE); // make sure we are a 32-bit texture now...
int diWidth = DevilImageInfo.Width;
int diHeight = DevilImageInfo.Hieght; // get dims from info

diWidth = 1 << (int)floor((log((double)diWidth)/log(2.0f)) + 0.5f);
diHeight = 1 << (int)floor((log((double)diHeight)/log(2.0f)) + 0.5f);
// ok, the dims have been set to a next larger power of 2
// we should check for exceeding max texture size, but I don't :P

if (v2dFreeImage.size())
// we have free images, use one
ret = v2dFreeImage.back(); // get the free image number
v2dFreeImage.pop_back(); // its used so remove it from the list
} else
// allocate a new image object on the stack
v2dImg *ni = new v2dImg;
ret = v2dImageStack.size();

v2dImageStack[ret]->TexId = 0xFFFFFFFF; // not yet valid
v2dImageStack[ret]->ImageId = DevilImageName; // our image name
v2dImageStack[ret]->Info = DevilImageInfo; // our image info
strcpy(v2dImageStack[ret]->SourceName,Filename); // our filename (no length check? :P)

// here we calculate the rendering coords
v2dImageStack[ret]->w = (float)DevilImageInfo.Width / (float)diWidth;
v2dImageStack[ret]->h = (float)DevilImageInfo.Height / (float)diHeight;
v2dImageStack[ret]->x = 0.0f;
v2dImageStack[ret]->y = 1.0f - v2dImageStack[fin]->h;
return ret;

As you can see, the function either returns a new image name or the invalid -1. The code that generates the proper power of 2 dimension was pulled from the nehe IPicture base code.

[Edited by - Ranger_One on January 11, 2006 11:03:31 AM]

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The other image functions are simpler. Here they are in some detail. You should note that so far I have left a lot of source out, like full includes and such. I'll be linking to a download from everything soon that will provide full files for the project :)

v2dFreeImage() just clears the image data (if it has any) and then marks that image name as free, adding it to the free name stack.

void v2dFreeImage(int ImageName)
// since we don't actually remove any image objects
// this is an simple as making the image invalid and
// putting it's number on the free image stack/vector
if (v2dImageStack[ImageName]->TexId != 0xFFFFFFFF)
v2dImageStack[ImageName]->TexId = 0xFFFFFFFF;
if (v2dImageStack[ImageName]->ImageId != -1)
v2dImageStack[ImageName]->ImageId = -1;
v2dImageStack[ImageName]->SourceFile[0] = 0;

v2dBindImage() intelligently handles images that aren't yet bound into the OpenGL context, something purposely left out of v2dLoadImage().

// this either binds the texture in gl, or realizes the
// imsge into a texture if needed
void v2dBindImage(int ImageName)
if (v2dImageStack[ImageName]->TexId == 0xFFFFFFFF)

v2dRealizeImage() moves image data from DevIL into OpenGL. I don't use ilut, so neither does thi function (though that would have been easier).

void v2dRealizeImage(int ImageName)
// bind the image if valid, or scram back out if not
if (v2dImageStack[ImageName]->ImageId == -1) return;
else ilBindImage(v2dImageStack[ImageName]->ImageId);

// prepare OpenGL to accept the image data
glBindTexture(GL_TEXTURE_2D, v2dImageStack[ImageName]->TexId);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); // Linear Filtering
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR); // Linear Filtering
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, ilGetInteger(IL_IMAGE_WIDTH),
ilGetData()); /* Texture specification - ilGetInteger(IL_IMAGE_BPP) */

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finally, I get to the last function!

v2dImage() renders the loaded image onto the screen at coords specified by x and y.

void v2dImage(float x, float y, int ImageName)
v2dBindImage(ImageName); // make sure we have the image bound

v2dImg *pi = v2dImageStack[ImageName]; // make things clearer below

glBegin(GL_QUADS); // using quads makes this easy

glTexCoord2f(pi->x,pi->y); // upper left corner tex coord
glVertex2f(x,y); // upper left corner screen coord
glTexCoord2f(pi->x+pi->w,pi->y); // upper right tex
glVertex2f(x+pi->Info.Width,y); // upper right screen
glTexCoord2f(pi->x+pi->w,pi->y+pi->h); // lower right tex
glVertex2f(x+pi->Info.Width,y+pi->Info.Height); // lower right screen
glTexCoord2f(pi->x,pi->y+pi->h); // lower left tex
glVertex2f(x,y+pi->Info.Height); // lower right screen

glEnd(); // all done!

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Now that I have covered all the described functions, let me fill in some missing information. First, I actually forgot some functions. There is no v2dClear(), no v2d Color(), and so on. My next post will fix that minor failing.

Second, the entire interface used is rather abstract. In fact it is abstract enough to allow an entirely different renderer, like say, directx. That wasn't my intent but merely a side effect of the style I employed.

Third, I don't mean to make you think I was programming C. The code is C++ only and uses the STL vector class. The interface is meant to mirror GL, and therefore looks much like C (since OpenGL is C only). Make no C assumptions.

Fourth, I need to lay down the code that supports frames. Frames are segments of an image. Say you have a sprite image that contains 16 (4x4, or 8x2 etc) subimages. Each subimage is a frame that references the main image. Say Frame, think Sprite. :)

Any questions or comments can be posted here, PM'd to me, or sent to

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Easy on the GPU there buddy! If I was to have a particle system, or even a bunch of tiles, that system would hurt performence. And drawing quads, I understand VBO's might be too much for newbies, but what about a display list for the quad, and then a glScalef?

Otherwise - nice work, I like the api you give in it - abstracting things in tutorials can really end up confusing people, and you've avoiding that but kept the system organized. Good job :)

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Here is the missing clear function and some color support :)

// the missing function...
void v2dClear();

// **************************************************************
// Color Functions
int v2dColor(float r, float g, float b, float a); // all rgba values
int v2dColor(float r, float g, float b); // just rgb
int v2dColor(char *hex); // from a Hex string
int v2dColor(int ColorName); // 16 basic colors
void v2dColorSet(int ColorName, float r, float g, float b, float a); // redefine a color
void v2dColorGet(int ColorName, float *r, float *g, float *b, float *a); // read a color's components
void v2dFreeColor(int ColorName);
void v2dCast(int CastColor); // casts a color (blend) across all color calls
void v2dPick(int ColorName); // selects a color

The implementation for v2dClear couldn't be easier...

void v2dClear()

My color support code uses the same style of storage and the image names. It never actually releases any memory but uses it instead. Really shouldn't be a problem here anyway, since each color is only 4 floats = 16 bytes.

#define V2D_COLOR_RED 0
#define V2D_COLOR_GREEN 1
#define V2D_COLOR_BLUE 2
#define V2D_COLOR_ALPHA 3

// the sixteen named colors (standard)
#define V2D_BASIC_BLACK 0
#define V2D_BASIC_MAROON 1
#define V2D_BASIC_GREEN 2
#define V2D_BASIC_TEAL 3
#define V2D_BASIC_NAVY 4
#define V2D_BASIC_PURPLE 5
#define V2D_BASIC_OLIVE 6
#define V2D_BASIC_GRAY 7
#define V2D_BASIC_SILVER 8
#define V2D_BASIC_BLUE 9
#define V2D_BASIC_LIME 10
#define V2D_BASIC_AQUA 11
#define V2D_BASIC_RED 12
#define V2D_BASIC_FUCHSIA 13
#define V2D_BASIC_YELLOW 14
#define V2D_BASIC_WHITE 15

typedef struct _v2dColorType {
float Index[4];
} v2dColorType; // could have used a union here I guess, but whatever...
v2dColorType v2dCastColor = { 1.0f, 1.0f, 1.0f, 1.0f };
vector<v2dColorType> ColorStack;
vector<int> FreeColor;

float v2dColorBasic[] = {
0.0f, 0.0f, 0.0f, 1.0f,
0.0f, 0.0f, 0.5f, 1.0f,
0.0f, 0.5f, 0.0f, 1.0f,
0.0f, 0.5f, 0.5f, 1.0f,
0.5f, 0.0f, 0.0f, 1.0f,
0.5f, 0.0f, 0.5f, 1.0f,
0.5f, 0.5f, 0.0f, 1.0f,
0.5f, 0.5f, 0.5f, 1.0f,
0.75f, 0.75f, 0.75f, 1.0f,
0.0f, 0.0f, 1.0f, 1.0f,
0.0f, 1.0f, 0.0f, 1.0f,
0.0f, 1.0f, 1.0f, 1.0f,
1.0f, 0.0f, 0.0f, 1.0f,
1.0f, 0.0f, 1.0f, 1.0f,
1.0f, 1.0f, 0.0f, 1.0f,
1.0f, 1.0f, 1.0f, 1.0f

int v2dColor(float r, float g, float b, float a)
int NewColorName;
v2dColorType C;

C.Index[V2D_COLOR_RED] = r;
C.Index[V2D_COLOR_GREEN] = g;
C.Index[V2D_COLOR_BLUE] = b;
C.Index[V2D_COLOR_ALPHA] = a;

if (FreeColor.size())
NewColorName = FreeColor.back();
ColorStack[NewColorName] = C;
} else
NewColorName = ColorStack.size();

return NewColorName;

int v2dColor(float r, float g, float b)
int NewColorName;
v2dColorType C;

C.Index[V2D_COLOR_RED] = r;
C.Index[V2D_COLOR_GREEN] = g;
C.Index[V2D_COLOR_BLUE] = b;
C.Index[V2D_COLOR_ALPHA] = 1.0f;

if (FreeColor.size())
NewColorName = FreeColor.back();
ColorStack[NewColorName] = C;
} else
NewColorName = ColorStack.size();

return NewColorName;

int v2dColor(char *hex)
int NewColorName;
v2dColorType C;
unsigned int R, G, B, A;
R = B = G = A = 0xFF;

C.Index[V2D_COLOR_RED] = (float)R / 255.0f;
C.Index[V2D_COLOR_GREEN] = (float)G / 255.0f;
C.Index[V2D_COLOR_BLUE] = (float)B / 255.0f;
C.Index[V2D_COLOR_ALPHA] = (float)A / 255.0f;

if (FreeColor.size())
NewColorName = FreeColor.back();
ColorStack[NewColorName] = C;
} else
NewColorName = ColorStack.size();

return NewColorName;

int v2dColor(int ColorId)
int NewColorName;
v2dColorType C;

C.Index[V2D_COLOR_RED] = v2dColorBasic[ColorId*4];
C.Index[V2D_COLOR_GREEN] = v2dColorBasic[ColorId*4+1];
C.Index[V2D_COLOR_BLUE] = v2dColorBasic[ColorId*4+2];
C.Index[V2D_COLOR_ALPHA] = v2dColorBasic[ColorId*4+3];

if (FreeColor.size())
NewColorName = FreeColor.back();
ColorStack[NewColorName] = C;
} else
NewColorName = ColorStack.size();

return NewColorName;

void v2dColorSet(int ColorName, float r, float g, float b, float a)
ColorStack[ColorName].Index[V2D_COLOR_RED] = r;
ColorStack[ColorName].Index[V2D_COLOR_GREEN] = g;
ColorStack[ColorName].Index[V2D_COLOR_BLUE] = b;
ColorStack[ColorName].Index[V2D_COLOR_ALPHA] = a;

void v2dColorGet(int ColorName, float *r, float *g, float *b, float *a)
if (r) *r = ColorStack[ColorName].Index[V2D_COLOR_RED];
if (g) *g = ColorStack[ColorName].Index[V2D_COLOR_GREEN];
if (b) *b = ColorStack[ColorName].Index[V2D_COLOR_BLUE];
if (a) *a = ColorStack[ColorName].Index[V2D_COLOR_ALPHA];

void v2dFreeColor(int ColorName)

void v2dCast(int CastColor)
v2dCastColor = ColorStack[CastColor];

void v2dPick(int ColorName)
v2dColorType SelColor = ColorStack[ColorName];

glColor4f(SelColor.Index[V2D_COLOR_RED] * v2dCastColor.Index[V2D_COLOR_RED,
SelColor.Index[V2D_COLOR_GREEN] * v2dCastColor.Index[V2D_COLOR_GREEN,
SelColor.Index[V2D_COLOR_BLUE] * v2dCastColor.Index[V2D_COLOR_BLUE,
SelColor.Index[V2D_COLOR_ALPHA] * v2dCastColor.Index[V2D_COLOR_ALPHA);

The color code allows the whole screen to be blended with another color. This is set by calling v2dCast() with a color name. And yes, the v2dColorTypes internal structure isn't beautiful, but it is easy to read.

[Edited by - Ranger_One on January 11, 2006 11:27:42 AM]

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Thanks for the praise :) As far as more advanced implementations are concerned, I was going to introduce optimizations as I go. I picture this as part I of a several part tutorial. Though it should work fine for something like a blended HUD or such as is...

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      Graphics APIs have come a long way from small set of basic commands allowing limited control of configurable stages of early 3D accelerators to very low-level programming interfaces exposing almost every aspect of the underlying graphics hardware. Next-generation APIs, Direct3D12 by Microsoft and Vulkan by Khronos are relatively new and have only started getting widespread adoption and support from hardware vendors, while Direct3D11 and OpenGL are still considered industry standard. New APIs can provide substantial performance and functional improvements, but may not be supported by older hardware. An application targeting wide range of platforms needs to support Direct3D11 and OpenGL. New APIs will not give any advantage when used with old paradigms. It is totally possible to add Direct3D12 support to an existing renderer by implementing Direct3D11 interface through Direct3D12, but this will give zero benefits. Instead, new approaches and rendering architectures that leverage flexibility provided by the next-generation APIs are expected to be developed.
      There are at least four APIs (Direct3D11, Direct3D12, OpenGL/GLES, Vulkan, plus Apple's Metal for iOS and osX platforms) that a cross-platform 3D application may need to support. Writing separate code paths for all APIs is clearly not an option for any real-world application and the need for a cross-platform graphics abstraction layer is evident. The following is the list of requirements that I believe such layer needs to satisfy:
      Lightweight abstractions: the API should be as close to the underlying native APIs as possible to allow an application leverage all available low-level functionality. In many cases this requirement is difficult to achieve because specific features exposed by different APIs may vary considerably. Low performance overhead: the abstraction layer needs to be efficient from performance point of view. If it introduces considerable amount of overhead, there is no point in using it. Convenience: the API needs to be convenient to use. It needs to assist developers in achieving their goals not limiting their control of the graphics hardware. Multithreading: ability to efficiently parallelize work is in the core of Direct3D12 and Vulkan and one of the main selling points of the new APIs. Support for multithreading in a cross-platform layer is a must. Extensibility: no matter how well the API is designed, it still introduces some level of abstraction. In some cases the most efficient way to implement certain functionality is to directly use native API. The abstraction layer needs to provide seamless interoperability with the underlying native APIs to provide a way for the app to add features that may be missing. Diligent Engine is designed to solve these problems. Its main goal is to take advantages of the next-generation APIs such as Direct3D12 and Vulkan, but at the same time provide support for older platforms via Direct3D11, OpenGL and OpenGLES. Diligent Engine exposes common C++ front-end for all supported platforms and provides interoperability with underlying native APIs. It also supports integration with Unity and is designed to be used as graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. Full source code is available for download at GitHub and is free to use.
      Diligent Engine API takes some features from Direct3D11 and Direct3D12 as well as introduces new concepts to hide certain platform-specific details and make the system easy to use. It contains the following main components:
      Render device (IRenderDevice  interface) is responsible for creating all other objects (textures, buffers, shaders, pipeline states, etc.).
      Device context (IDeviceContext interface) is the main interface for recording rendering commands. Similar to Direct3D11, there are immediate context and deferred contexts (which in Direct3D11 implementation map directly to the corresponding context types). Immediate context combines command queue and command list recording functionality. It records commands and submits the command list for execution when it contains sufficient number of commands. Deferred contexts are designed to only record command lists that can be submitted for execution through the immediate context.
      An alternative way to design the API would be to expose command queue and command lists directly. This approach however does not map well to Direct3D11 and OpenGL. Besides, some functionality (such as dynamic descriptor allocation) can be much more efficiently implemented when it is known that a command list is recorded by a certain deferred context from some thread.
      The approach taken in the engine does not limit scalability as the application is expected to create one deferred context per thread, and internally every deferred context records a command list in lock-free fashion. At the same time this approach maps well to older APIs.
      In current implementation, only one immediate context that uses default graphics command queue is created. To support multiple GPUs or multiple command queue types (compute, copy, etc.), it is natural to have one immediate contexts per queue. Cross-context synchronization utilities will be necessary.
      Swap Chain (ISwapChain interface). Swap chain interface represents a chain of back buffers and is responsible for showing the final rendered image on the screen.
      Render device, device contexts and swap chain are created during the engine initialization.
      Resources (ITexture and IBuffer interfaces). There are two types of resources - textures and buffers. There are many different texture types (2D textures, 3D textures, texture array, cubmepas, etc.) that can all be represented by ITexture interface.
      Resources Views (ITextureView and IBufferView interfaces). While textures and buffers are mere data containers, texture views and buffer views describe how the data should be interpreted. For instance, a 2D texture can be used as a render target for rendering commands or as a shader resource.
      Pipeline State (IPipelineState interface). GPU pipeline contains many configurable stages (depth-stencil, rasterizer and blend states, different shader stage, etc.). Direct3D11 uses coarse-grain objects to set all stage parameters at once (for instance, a rasterizer object encompasses all rasterizer attributes), while OpenGL contains myriad functions to fine-grain control every individual attribute of every stage. Both methods do not map very well to modern graphics hardware that combines all states into one monolithic state under the hood. Direct3D12 directly exposes pipeline state object in the API, and Diligent Engine uses the same approach.
      Shader Resource Binding (IShaderResourceBinding interface). Shaders are programs that run on the GPU. Shaders may access various resources (textures and buffers), and setting correspondence between shader variables and actual resources is called resource binding. Resource binding implementation varies considerably between different API. Diligent Engine introduces a new object called shader resource binding that encompasses all resources needed by all shaders in a certain pipeline state.
      API Basics
      Creating Resources
      Device resources are created by the render device. The two main resource types are buffers, which represent linear memory, and textures, which use memory layouts optimized for fast filtering. Graphics APIs usually have a native object that represents linear buffer. Diligent Engine uses IBuffer interface as an abstraction for a native buffer. To create a buffer, one needs to populate BufferDesc structure and call IRenderDevice::CreateBuffer() method as in the following example:
      BufferDesc BuffDesc; BufferDesc.Name = "Uniform buffer"; BuffDesc.BindFlags = BIND_UNIFORM_BUFFER; BuffDesc.Usage = USAGE_DYNAMIC; BuffDesc.uiSizeInBytes = sizeof(ShaderConstants); BuffDesc.CPUAccessFlags = CPU_ACCESS_WRITE; m_pDevice->CreateBuffer( BuffDesc, BufferData(), &m_pConstantBuffer ); While there is usually just one buffer object, different APIs use very different approaches to represent textures. For instance, in Direct3D11, there are ID3D11Texture1D, ID3D11Texture2D, and ID3D11Texture3D objects. In OpenGL, there is individual object for every texture dimension (1D, 2D, 3D, Cube), which may be a texture array, which may also be multisampled (i.e. GL_TEXTURE_2D_MULTISAMPLE_ARRAY). As a result there are nine different GL texture types that Diligent Engine may create under the hood. In Direct3D12, there is only one resource interface. Diligent Engine hides all these details in ITexture interface. There is only one  IRenderDevice::CreateTexture() method that is capable of creating all texture types. Dimension, format, array size and all other parameters are specified by the members of the TextureDesc structure:
      TextureDesc TexDesc; TexDesc.Name = "My texture 2D"; TexDesc.Type = TEXTURE_TYPE_2D; TexDesc.Width = 1024; TexDesc.Height = 1024; TexDesc.Format = TEX_FORMAT_RGBA8_UNORM; TexDesc.Usage = USAGE_DEFAULT; TexDesc.BindFlags = BIND_SHADER_RESOURCE | BIND_RENDER_TARGET | BIND_UNORDERED_ACCESS; TexDesc.Name = "Sample 2D Texture"; m_pRenderDevice->CreateTexture( TexDesc, TextureData(), &m_pTestTex ); If native API supports multithreaded resource creation, textures and buffers can be created by multiple threads simultaneously.
      Interoperability with native API provides access to the native buffer/texture objects and also allows creating Diligent Engine objects from native handles. It allows applications seamlessly integrate native API-specific code with Diligent Engine.
      Next-generation APIs allow fine level-control over how resources are allocated. Diligent Engine does not currently expose this functionality, but it can be added by implementing IResourceAllocator interface that encapsulates specifics of resource allocation and providing this interface to CreateBuffer() or CreateTexture() methods. If null is provided, default allocator should be used.
      Initializing the Pipeline State
      As it was mentioned earlier, Diligent Engine follows next-gen APIs to configure the graphics/compute pipeline. One big Pipelines State Object (PSO) encompasses all required states (all shader stages, input layout description, depth stencil, rasterizer and blend state descriptions etc.). This approach maps directly to Direct3D12/Vulkan, but is also beneficial for older APIs as it eliminates pipeline misconfiguration errors. With many individual calls tweaking various GPU pipeline settings it is very easy to forget to set one of the states or assume the stage is already properly configured when in fact it is not. Using pipeline state object helps avoid these problems as all stages are configured at once.
      Creating Shaders
      While in earlier APIs shaders were bound separately, in the next-generation APIs as well as in Diligent Engine shaders are part of the pipeline state object. The biggest challenge when authoring shaders is that Direct3D and OpenGL/Vulkan use different shader languages (while Apple uses yet another language in their Metal API). Maintaining two versions of every shader is not an option for real applications and Diligent Engine implements shader source code converter that allows shaders authored in HLSL to be translated to GLSL. To create a shader, one needs to populate ShaderCreationAttribs structure. SourceLanguage member of this structure tells the system which language the shader is authored in:
      SHADER_SOURCE_LANGUAGE_DEFAULT - The shader source language matches the underlying graphics API: HLSL for Direct3D11/Direct3D12 mode, and GLSL for OpenGL and OpenGLES modes. SHADER_SOURCE_LANGUAGE_HLSL - The shader source is in HLSL. For OpenGL and OpenGLES modes, the source code will be converted to GLSL. SHADER_SOURCE_LANGUAGE_GLSL - The shader source is in GLSL. There is currently no GLSL to HLSL converter, so this value should only be used for OpenGL and OpenGLES modes. There are two ways to provide the shader source code. The first way is to use Source member. The second way is to provide a file path in FilePath member. Since the engine is entirely decoupled from the platform and the host file system is platform-dependent, the structure exposes pShaderSourceStreamFactory member that is intended to provide the engine access to the file system. If FilePath is provided, shader source factory must also be provided. If the shader source contains any #include directives, the source stream factory will also be used to load these files. The engine provides default implementation for every supported platform that should be sufficient in most cases. Custom implementation can be provided when needed.
      When sampling a texture in a shader, the texture sampler was traditionally specified as separate object that was bound to the pipeline at run time or set as part of the texture object itself. However, in most cases it is known beforehand what kind of sampler will be used in the shader. Next-generation APIs expose new type of sampler called static sampler that can be initialized directly in the pipeline state. Diligent Engine exposes this functionality: when creating a shader, textures can be assigned static samplers. If static sampler is assigned, it will always be used instead of the one initialized in the texture shader resource view. To initialize static samplers, prepare an array of StaticSamplerDesc structures and initialize StaticSamplers and NumStaticSamplers members. Static samplers are more efficient and it is highly recommended to use them whenever possible. On older APIs, static samplers are emulated via generic sampler objects.
      The following is an example of shader initialization:
      ShaderCreationAttribs Attrs; Attrs.Desc.Name = "MyPixelShader"; Attrs.FilePath = "MyShaderFile.fx"; Attrs.SearchDirectories = "shaders;shaders\\inc;"; Attrs.EntryPoint = "MyPixelShader"; Attrs.Desc.ShaderType = SHADER_TYPE_PIXEL; Attrs.SourceLanguage = SHADER_SOURCE_LANGUAGE_HLSL; BasicShaderSourceStreamFactory BasicSSSFactory(Attrs.SearchDirectories); Attrs.pShaderSourceStreamFactory = &BasicSSSFactory; ShaderVariableDesc ShaderVars[] = {     {"g_StaticTexture", SHADER_VARIABLE_TYPE_STATIC},     {"g_MutableTexture", SHADER_VARIABLE_TYPE_MUTABLE},     {"g_DynamicTexture", SHADER_VARIABLE_TYPE_DYNAMIC} }; Attrs.Desc.VariableDesc = ShaderVars; Attrs.Desc.NumVariables = _countof(ShaderVars); Attrs.Desc.DefaultVariableType = SHADER_VARIABLE_TYPE_STATIC; StaticSamplerDesc StaticSampler; StaticSampler.Desc.MinFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MagFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MipFilter = FILTER_TYPE_LINEAR; StaticSampler.TextureName = "g_MutableTexture"; Attrs.Desc.NumStaticSamplers = 1; Attrs.Desc.StaticSamplers = &StaticSampler; ShaderMacroHelper Macros; Macros.AddShaderMacro("USE_SHADOWS", 1); Macros.AddShaderMacro("NUM_SHADOW_SAMPLES", 4); Macros.Finalize(); Attrs.Macros = Macros; RefCntAutoPtr<IShader> pShader; m_pDevice->CreateShader( Attrs, &pShader );
      Creating the Pipeline State Object
      After all required shaders are created, the rest of the fields of the PipelineStateDesc structure provide depth-stencil, rasterizer, and blend state descriptions, the number and format of render targets, input layout format, etc. For instance, rasterizer state can be described as follows:
      PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
      Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
      // Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
      // 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 two samples, asteroids performance benchmark and example Unity project that uses Diligent Engine in native plugin.
      AntTweakBar sample is Diligent Engine’s “Hello World” example.

      Atmospheric scattering sample is a more advanced example. It 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 and Android platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and support for more platforms is planned.
    • By michaeldodis
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      Early version of the library
      I'd really love to hear anyone's thoughts on this, any suggestions on what features you'd want to see in a library like this? 
      Thanks in advance!
    • By Michael Aganier
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