I have a very simple OpenGL (3.2) setup, no lighting, perspective projection and a simple shader program (applies projection transformation and uses texture2D to read the color from the texture).
The camera is looking down the negative z-axis and I draw a few walls and pillars on the x-y-plane with a texture (http://i43.tinypic.com/2ryszlz.png).
Now I'm moving the camera in the x-y-plane and this is what it looks like:
http://i.imgur.com/VCrNcly.gif.
My question is now: How do I handle the flickering of the wall texture?
As the camera centers the walls, the view angle onto the texture compresses the texture for the screen, so one pixel on the screen is actually several pixels on the texture, but only one is chosen for display. From the information I have access to in the shaders, I don't see how to perform an operation which interpolates the required color.
As this looks like a problem nearly every 3D application should have, the solution is probably pretty simple (I hope?).
I can't seem to understand the images, but from what you are describing you seem to be looking for MIPMAPPING. Please google it, it's a very easy and very generally used concept. You will be able to use it by adding one or two lines to your program. Good Luck. I'd be more detailed but I am out of time for today.
Related
I'm trying to add a skybox to the world/camera/game and I don't know how to go about it. If someone could give me some guidance on how to apply it, it would be much appreciated.
I have already loaded the skybox, I just don't know how to draw it properly so it will fit around the camera as it moves.
I have managed to texture a sort of cube, which might be close to a skybox but then it's only visible from the outside. Once you enter the cube, you can't see it from the inside. Perhaps if I could invert the cube's faces, it will show when I'm inside the cube and I can make it larger?
From outside the cube looking at it
From inside looking out
I had a similar problem a few weeks back, if you are looking for some pseudo code I think I may be able to help. First of all using a cube isn't the best idea when rendering as your box won't look natural, map it to a sphere for a smooth effect.
Create a bounding sphere around your viewer that moves relative to your camera
Apply the texture on that sphere, this will give the impression that the sky is moving relative to you
When you are drawing, disable your z-buffer and frustum (assuming you're using any culling algorithm) this will allow the sky box to be drawn but will ensure terrain is drawn over the top of the sky box when depth sort algorithms are performed by OpenGL.
Note: Don't forget to re-enable the z-buffer after the sky box has been drawn, otherwise your terrain elements will appear outside of the sphere, meaning you will only see the Sky box.
I recently wrote a basic terrain engine in DirectX but the principals are fairly similar, if you'd like to view the repo you can find it here
Check out line 286 in this file to see how the Skybox is rendered, then also visit the SkyBox implementation file to see how it is constructed, and the SkyShader implementation file to see how the texture is mapped to the sphere, the main method to be concerned with in the shader file is SetShaderParameters()
In terms of moving the skybox relative to your camera, simply set the WVP matrix of your skybox to that of your camera, and then tweak the x, y, z planes of the skybox to your liking.
Extra If you are going to implement multi-player aspects, just disable back-face rendering for the sphere, then each player can see their SkyBox but opponents cannot. Alternatively you create one large sphere around the world
Hope that helps - if you need anymore help just ask, I know this stuff can be fairly dense at first:)
I'm trying to implement a multi-pass rendering method using OpenSceneGraph. However, I'm not entirely certain my problem is theoretical or due to a lack of applied knowledge of OSG. Thus far, I've successfully implemented multi-pass shading by rendering to a texture using an orthogonal projection, but I cannot seem to make a perspective projection work.
It may be that I don't quite understand how to implement multi-pass shading. Of course, I have to pre-render the entire scene with the multi-pass shaders to a texture, then use the texture in the final render. However, I'm not talking about creating a separate texture for each object in the scene, but effectively capturing a screenshot of the entire prerendered scene. Then, from that texture alone, applying the rendered effects to the individual geometries.
I assume this means I would have to do an extra conversion of the vertex coordinates for each geometry in the vertex shader. That is, after computing:
gl_Position = ModelViewProjectionMatrix * Vertex;
I would need to go a step further and calculate the vertex's screen coordinates in order to map the vertices correctly (again, given that the texture consists of an entire screen shot of the scene).
If I am correct, then I must be able to pre-render the scene in a perspective view identical to the view used in the final render, rather than an orthogonal view. This is where I have troubles. I can make an orthogonal view do what I want, but not the perspective view.
Am I correct in my approach? The only other approach I can imagine is to render everything to a screen-filling quad (in effect, the same thing as converting to screen coordinates), but that doesn't alleviate the need to use a perspective projection in the pre-render stage.
Thoughts? Links??
edit: I should also point out that in my successful attempts, I used a fragment shader only. The perspective projection worked, but, of course, the screen aligned quad I was using was offset rather than centered. I added a pass-through vertex shader and everything went blank.
As it turns out, my approach was correct. It's especially nice as it avoids having to add another camera to my scene graph to render the final output - I can simply use the main camera. Unfortunately, it means that all of my output textures are rendered at the screen resolution, rather than a resolution appropriate to the size of the object. That is, if my screen is 1024 x 1024, then so is the output texture, one for each pre-render camera in the graph. Not exactly efficient, but it'll do for now.
Well, i have a 3d scene currently with just a quad (painting) with texture on it. Between the painting and the "camera" i have places an other quad i would like to behave like a optical lens: distorting the picture "below" it
how would one achieve it preferably with a shader and some pixelbuffers?
Here is an example I found a while ago which does something very similar to what you want. http://www.paulsprojects.net/opengl/refract/refract.html
You will probably have to modify the code a bit to achieve the inversion effect you want, but this will get you started on the right track.
Edit:
By the way, you will not need the second image (the inverted small rectangle). Just use a single background image and the shader.
Between the painting and the "camera" i have places an other quad i would like to behave like a optical lens:
This is a tricky one. First one must understand that OpenGL is a so called localized rendering model rasterizer, which means in layman terms, that it works like pencils and brushes on a canvas.
It thus works in very contrast to global scene representation renderers like raytracers. A raytracer actually operates on a fully defined scene, because of that it can to things like refraction trivially.
Indeed one must treat OpenGL like an artist treats its tools. So any optical "effect" you want to create must be implemented by mastering various drawing techiques possible with the tools OpenGL offers. To create the effect you desire you must implement a multistage process.
For refraction you first render the scene as "seen" by the refracting object in all directions (you create a dynamic cube map), then you use this cube map as input data for rasterizing the "refracting" object, where a shader is used to determine the refracted direction of a ray of light hitting the rasterized fragments.
BTW: What holds for refraction holds for any other like interacting effect. Shadows are as non-trivial like refractions in OpenGL.
is it possible to create a GLSL shader to get any object to be surrounded by a glowing effect?
Let's say i have a 3d cube and if it's selected the cube should be surrounded by a blue glowing effect. Any hints?
Well there are several ways of doing this. If each object is also represented in a winged edge format then it is trivial to calculate the silhouette and then extrude it to generate a glow. This however is, very much, a CPU method.
For a GPU method you could try rendering to an offscreen buffer with the stencil set to increment. If you then perform a blur on the image (though only writing to pixels where the stencil is non zero) you will get a blur around the edge of the image which can then be drawn into the main scene with alpha blending. This is more a blur than a glow but it would be relatively easy to re-jig the brightness so that it renders a glow.
There are plenty of other methods too ... here are a couple of links for you to look through:
http://http.developer.nvidia.com/GPUGems/gpugems_ch21.html
http://www.codeproject.com/KB/directx/stencilbufferglowspart1.aspx?display=Mobile
Have a hunt round on google because there is lots of information :)
Newbie to OpenGL...
I have some very simple code (non OpenGL) for rotating a rectangle around a single axis, and projecting the result down to screen coordinates. I'm now trying to map a bitmap to the resulting shape using OpenGL. When animating the rotation, the perspective of the bitmap is quite heavily distorted. Is this to be expected? Is there something I can do about it?
I know I can use OpenGL to do the whole thing instead (and that works fine), but for my current project the approach above would suit me better, if I can just get around this perspective issue... I'm thinking maybe there's not enough information after I have projected the rotated rectangle down to 2D space for OpenGL to correctly map the bitmap with the right perspective..?
Any input would be much appreciated.
Thanks,
Daniel
To clarify:
I'm using an orthographic projection, and doing the 3D calculation and projection to 2D myself. Then I just use OpenGL for rendering the resulting shape with a texture.
If you project your coordinates yourself and do the texture mapping in 2D screen coordinates you will loose all projective information and the textures will badly distort.
You can get around this by using a perspective texture mapping. A lot of different ways to do this exist. Either by writing a real perspective texture mapper or by faking and using a plain texture mapper.
Explaining how this works is somewhat beyond the scope of a single question. I assume you read the wiki-page about perspective texture mapping first and try out the subdivision method:
http://en.wikipedia.org/wiki/Texture_mapping
Then come back and ask for detail questions..
I found the following page that explains the subdivision method in detail:
http://freespace.virgin.net/hugo.elias/graphics/x_persp.htm
It worked perfectly! Thanks Nils for pointing me in the right direction.