# OpenGL c# opentk - converting screen coords to world coords

## Recommended Posts

I'm making a windows forms application with opengl view and I'm using c#, openTK . I need to get the mouse coords converted to the opengl world coords. Well, my Y coord gets converted wrong. It's hard to explain, so here is the video: http://tinypic.com/r/23sal8k/6. I'm constantly pressing left mouse button and the red dot should be where the mouse is(but it isn't). How to fix this? Here is the code:

       private void glview_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
if (e.Button == MouseButtons.Left)
{
Point worldCoords = convertScreenToWorldCoords(e.X, e.Y);
shitx = worldCoords.X;
shity = worldCoords.Y;
}
}

// functions:
public static Point convertScreenToWorldCoords(int x, int y)
{
int[] viewport = new int[4];
Matrix4 modelViewMatrix, projectionMatrix;
GL.GetFloat(GetPName.ModelviewMatrix, out modelViewMatrix);
GL.GetFloat(GetPName.ProjectionMatrix, out projectionMatrix);
GL.GetInteger(GetPName.Viewport, viewport);
Vector2 mouse;
mouse.X = x;
mouse.Y = viewport[3] - y;
Vector4 vector = UnProject(ref projectionMatrix, modelViewMatrix, new Size(viewport[2], viewport[3]), mouse);
Point coords = new Point((int)vector.X, (int)vector.Y);
return coords;
}
public static Vector4 UnProject(ref Matrix4 projection, Matrix4 view, Size viewport, Vector2 mouse)
{
Vector4 vec;

vec.X = 2.0f * mouse.X / (float)viewport.Width - 1;
vec.Y = -(2.0f * mouse.Y / (float)viewport.Height - 1);
vec.Z = 0;
vec.W = 1.0f;

Matrix4 viewInv = Matrix4.Invert(view);
Matrix4 projInv = Matrix4.Invert(projection);

Vector4.Transform(ref vec, ref projInv, out vec);
Vector4.Transform(ref vec, ref viewInv, out vec);

if (vec.W > float.Epsilon || vec.W < float.Epsilon)
{
vec.X /= vec.W;
vec.Y /= vec.W;
vec.Z /= vec.W;
}

return vec;
}


##### Share on other sites

Try inverting your Y coordinate before using it.

I had this same problem when using my own customized routine to project vertices.  It wasn't until I set the Y coordinate to -Y that it finally worked right.  Tbh, I've never actually had to unproject a vertex as of yet, but I'm quite sure that it's similar to projecting a vertex in OpenGL w/ RH matrices.  So, given that, I'm not 100% sure how you should do it in your code, but I suggest trying this:

mouse.Y = -( viewport[3] - y );

or this:

mouse.Y = viewport[3] + y;

I assume the first one would do the trick.

Also, I'm assuming this might not apply to you, but I had to invert one component of my modelview matrix as well in order for it to work.

modelview[13] = -modelview[13];

So, like I said, I'm familiar with your problem, just not 100% sure what order the steps need to be taken to fix it.  I gave it my best shot, so let us know if this works.

Shogun.

##### Share on other sites

Thanks for reply. Strange but it doesn't work. Using both methods the dot just shows at a bigger distance of mouse:

and I can't do modelview[13] = -modelview[13] because modelview variable is Matrix4, not array of integers or something else...

Edited by zgintasz

##### Share on other sites

I just noticed one thing, it's hard to explain in words, so here it is :

Edited by zgintasz

##### Share on other sites

Okay then, my next guess would be to change this (but after reverting the previous changes I suggested).

vec.Y = (2.0f * mouse.Y / (float)viewport.Height - 1);

Try removing the '-' and see if that works.

Also, sorry I didn't better explain what I was doing with the matrix suggestion.  13 is the column based matrix's location of the Y translation coordinate.

| 1 0 0 X |
| 0 1 0 -Y |
| 0 0 1 Z |
| 0 0 0 1 |

I simply suggested you invert it.  I also had to do this to get my vertex projection code to work properly.

Hopefully, my suggestion helps a bit better this time.  Sorry for any confusion.

Shogun.

EDIT: This may or may not matter for you, but I had to invert the incoming Y coordinate and the Y component of the translation vector in my modelview matrix in order to fix the problem.

Edited by blueshogun96

##### Share on other sites

I actually understood what you meant by inverting one of the component of matrix but I don't know how to do that in opentk because modelViewMatrix variable is a Matrix4 struct. This doesn't change anything:

modelViewMatrix.M13 = -modelViewMatrix.M13;

I removed that minus in UnProject function, so now:

and if I change

mouse.Y = viewport[3] - y;

to

mouse.Y = viewport[3] - y - 130;

then it gets converted almost fine. I've implemented zooming, so when I zoom in the view, I can see that X coord isn't very accurate too.

maybe there is another way to convert that?

Also, I forgot to mention that my application is 2D if that makes any sense.

EDIT: it's also possible that unproject function is wrong, I took it from opentk forum written by someone... I see learning opengl is really difficult .

Edited by zgintasz

##### Share on other sites

Finally, I got this fixed. I just changed

mouse.Y = viewport[3] - y;

to

mouse.Y = y + (ClientRectangle.Height - glview.Size.Height);

still not very accurate but not very bad.

Edited by zgintasz

##### Share on other sites

Glad you got it fixed.  Sorry I couldn't have been more helpful.

Just curious, how "not very accurate is it"?  Are you accounting for the window's height and the title bar?

Shogun.

##### Share on other sites

I have implemented zooming, so when I zoom in I see that it isn't very accurate(sometimes dot gets in a right place, sometimes not):

When I zoom out, I see it's accurate enough. This is how I implemented zooming:

        // zoom in: zoom *= 2;
// zoom out: zoom /= 2;
public void UpdateCameraZoom()
{
Matrix4 projection = Matrix4.CreateOrthographic(ClientRectangle.Width * 5 / zoom, ClientRectangle.Height * 5 / zoom, 0.0f, 64.0f);
GL.MatrixMode(MatrixMode.Projection);
}

What I did to fix this, I just increased the width and height of projection volume(as you can see I multiply by 5) and increased other objects size(so they won't be so small) :).

## Create an account

Register a new account

• ### Forum Statistics

• Total Topics
628359
• Total Posts
2982259
• ### Similar Content

• Hi all,

I'm starting OpenGL using a tut on the Web. But at this point I would like to know the primitives needed for creating a window using OpenGL. So on Windows and using MS VS 2017, what is the simplest code required to render a window with the title of "First Rectangle", please?

• Hi, New here.
I need some help. My fiance and I like to play this mobile game online that goes by real time. Her and I are always working but when we have free time we like to play this game. We don't always got time throughout the day to Queue Buildings, troops, Upgrades....etc....
I was told to look into DLL Injection and OpenGL/DirectX Hooking. Is this true? Is this what I need to learn?
How do I read the Android files, or modify the files, or get the in-game tags/variables for the game I want?
Any assistance on this would be most appreciated. I been everywhere and seems no one knows or is to lazy to help me out. It would be nice to have assistance for once. I don't know what I need to learn.
So links of topics I need to learn within the comment section would be SOOOOO.....Helpful. Anything to just get me started.
Thanks,
Dejay Hextrix
• By mellinoe
Hi all,
First time poster here, although I've been reading posts here for quite a while. This place has been invaluable for learning graphics programming -- thanks for a great resource!
Right now, I'm working on a graphics abstraction layer for .NET which supports D3D11, Vulkan, and OpenGL at the moment. I have implemented most of my planned features already, and things are working well. Some remaining features that I am planning are Compute Shaders, and some flavor of read-write shader resources. At the moment, my shaders can just get simple read-only access to a uniform (or constant) buffer, a texture, or a sampler. Unfortunately, I'm having a tough time grasping the distinctions between all of the different kinds of read-write resources that are available. In D3D alone, there seem to be 5 or 6 different kinds of resources with similar but different characteristics. On top of that, I get the impression that some of them are more or less "obsoleted" by the newer kinds, and don't have much of a place in modern code. There seem to be a few pivots:
The data source/destination (buffer or texture) Read-write or read-only Structured or unstructured (?) Ordered vs unordered (?) These are just my observations based on a lot of MSDN and OpenGL doc reading. For my library, I'm not interested in exposing every possibility to the user -- just trying to find a good "middle-ground" that can be represented cleanly across API's which is good enough for common scenarios.
Can anyone give a sort of "overview" of the different options, and perhaps compare/contrast the concepts between Direct3D, OpenGL, and Vulkan? I'd also be very interested in hearing how other folks have abstracted these concepts in their libraries.
• By aejt
I recently started getting into graphics programming (2nd try, first try was many years ago) and I'm working on a 3d rendering engine which I hope to be able to make a 3D game with sooner or later. I have plenty of C++ experience, but not a lot when it comes to graphics, and while it's definitely going much better this time, I'm having trouble figuring out how assets are usually handled by engines.
I'm not having trouble with handling the GPU resources, but more so with how the resources should be defined and used in the system (materials, models, etc).
This is my plan now, I've implemented most of it except for the XML parts and factories and those are the ones I'm not sure of at all:
I have these classes:
For GPU resources:
Geometry: holds and manages everything needed to render a geometry: VAO, VBO, EBO. Texture: holds and manages a texture which is loaded into the GPU. Shader: holds and manages a shader which is loaded into the GPU. For assets relying on GPU resources:
Material: holds a shader resource, multiple texture resources, as well as uniform settings. Mesh: holds a geometry and a material. Model: holds multiple meshes, possibly in a tree structure to more easily support skinning later on? For handling GPU resources:
ResourceCache<T>: T can be any resource loaded into the GPU. It owns these resources and only hands out handles to them on request (currently string identifiers are used when requesting handles, but all resources are stored in a vector and each handle only contains resource's index in that vector) Resource<T>: The handles given out from ResourceCache. The handles are reference counted and to get the underlying resource you simply deference like with pointers (*handle).
And my plan is to define everything into these XML documents to abstract away files:
Resources.xml for ref-counted GPU resources (geometry, shaders, textures) Resources are assigned names/ids and resource files, and possibly some attributes (what vertex attributes does this geometry have? what vertex attributes does this shader expect? what uniforms does this shader use? and so on) Are reference counted using ResourceCache<T> Assets.xml for assets using the GPU resources (materials, meshes, models) Assets are not reference counted, but they hold handles to ref-counted resources. References the resources defined in Resources.xml by names/ids. The XMLs are loaded into some structure in memory which is then used for loading the resources/assets using factory classes:
Factory classes for resources:
For example, a texture factory could contain the texture definitions from the XML containing data about textures in the game, as well as a cache containing all loaded textures. This means it has mappings from each name/id to a file and when asked to load a texture with a name/id, it can look up its path and use a "BinaryLoader" to either load the file and create the resource directly, or asynchronously load the file's data into a queue which then can be read from later to create the resources synchronously in the GL context. These factories only return handles.
Factory classes for assets:
Much like for resources, these classes contain the definitions for the assets they can load. For example, with the definition the MaterialFactory will know which shader, textures and possibly uniform a certain material has, and with the help of TextureFactory and ShaderFactory, it can retrieve handles to the resources it needs (Shader + Textures), setup itself from XML data (uniform values), and return a created instance of requested material. These factories return actual instances, not handles (but the instances contain handles).

Is this a good or commonly used approach? Is this going to bite me in the ass later on? Are there other more preferable approaches? Is this outside of the scope of a 3d renderer and should be on the engine side? I'd love to receive and kind of advice or suggestions!
Thanks!
• By nedondev
I 'm learning how to create game by using opengl with c/c++ coding, so here is my fist game. In video description also have game contain in Dropbox. May be I will make it better in future.
Thanks.

• 10
• 9
• 13
• 24
• 11