Drawing to FrameBuffer ignores depth test - opengl

I'm drawing my scene to a texture using FBO and reading the pixel in order to select my object.
The problem is that the drawing into the texture is ignoring the depth
On Left is scene and right is the texture (saved it to file for debug reasons).
As u can see there are 2 planes one on top of another and the one in the front is more directed up. although on the texture it's the other way around. and this makes the user pick the plane in the background when he sees the other plane.
I'v tried to enable everything i thought of but i guess i'm missing something.

Thanks to ratchet freak,
I've realized that i skipped the depth buffer :)
I just had to create a render buffer for the depth component and attach it to the frame buffer object.

Related

vertex buffer renders later vertexes in front of earlier ones

While trying to render a cube with OpenGL I noticed that the vertexes I defined later in my positions array rendered on top of the ones defined earlier:
It is hard to see in the image but if you look closely, you can see the back faces of the cube are rendering on top of the faces closest to the camera. When I move the camera to the other side of the cube it appears normally.
Do I need a check in my fragment shader to see if the face is behind others relative to the camera, and if so how do I implement it?
At first I tried enabling the depth test (I wasn't aware it was off by default) But then nothing rendered. The problem was not clearing the depth buffer every frame with glClear(GL_DEPTH_BUFFER_BIT).
Problem Solved!

Get pixel behind the current pixel

I'm coding a programm in C++ with glut, rendering a 3D model in a window.
I'm using glReadPixels to get the image of the scenery displayed in the windows.
And I would like to know how I can get, for a specific pixel (x, y), not directly its color but the color of the next object behind.
If I render a blue triangle, and a red triangle in front of it, glReadPixels gives me red colors from the red triangle.
I would like to know how I can get the colors from the blue triangle, the one I would get from glReadPixels if the red triangle wasn't here.
The default framebuffer only retains the topmost color. To get what you're suggesting would require a specific rendering pipeline.
For instance you could:
Create an offscreen framebuffer of the same dimensions as your target viewport
Render a depth-only pass to the offscreen framebuffer, storing the depth values in an attached texture
Re-render the scene with a special shader that only drew pixels where the post-transformation Z values was LESS than the value in the previously recorded depth buffer
The final result of the last render should be the original scene with the top layer stripped off.
Edit:
It would require only a small amount of new code to create the offscreen framebuffer and render a depth only version of the scene to it, and you could use your existing rendering pipeline in combination with that to execute steps 1 and 2.
However, I can't think of any way you could then re-render the scene to get the information you want in step 3 without a shader, because it both the standard depth test plus a test against the provided depth texture. That doesn't mean there isn't one, just that I'm not well versed in GL tricks to think of it.
I can think of other ways of trying to accomplish the same task for specific points on the screen by fiddling with the rendering system, but they're all far more convoluted than just writing a shader.

'Render to Texture' and multipass rendering

I'm implementing an algorithm about pencil rendering. First, I should render the model using Phong shading to determine the intensity. Then I should map the texture to the rendered result.
I'm going to do a multipass rendering with opengl and cg shaders. Someone told me that I should try 'render to texture'. But I don't know how to use this method to get the effects that I want. In my opinion, we should first use this method to render the mesh, then we can get a 2D texture about the whole scene. Now that we have draw content to the framebuffer, next we should render to the screen, right? But how to use the rendered texture and do some post-processing on it? Can anybody show me some code or links about it?
I made this tutorial, it might help you : http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/
However, using RTT is overkill for what you're trying to do, I think. If you need the fragment's intensity in the texture, well, you already have it in your shader, so there is no need to render it twice...
Maybe this could be useful ? http://www.ozone3d.net/demos_projects/toon-snow.php
render to a texture with Phong shading
Draw that texture to the screen again in a full screen textured quad, applying a shader that does your desired operation.
I'll assume you need clarification on RTT and using it.
Essentially, your screen is a framebuffer (very similar to a texture); it's a 2D image at the end of the day. The idea of RTT is to capture that 2D image. To do this, the best way is to use a framebuffer object (FBO) (Google "framebuffer object", and click on the first link). From here, you have a 2D picture of your scene (you should check it by saving to an image file that it actually is what you want).
Once you have the image, you'll set up a 2D view and draw that image back onto the screen with an 800x600 quadrilateral or what-have-you. When drawing, you use a fragment program (shader), which transforms the brightness of the image into a greyscale value. You can output this, or you can use it as an offset to another, "pencil" texture.

OpenGL 2D game question

I want to make a game with Worms-like destructible terrain in 2D, using OpenGL.
What is the best approach for this?
Draw pixel per pixel? (Uh, not good?)
Have the world as a texture and manipulate it (is that possible?)
Thanks in advance
Thinking about the way Worms terrain looked, I came up with this idea. But I'm not sure how you would implement it in OpenGL. It's more of a layered 2D drawing approach. I'm posting the idea anyway. I've emulated the approach using Paint.NET.
First, you have a background sky layer.
And you have a terrain layer.
The terrain layer is masked so the top portion isn't drawn. Draw the terrain layer on top of the sky layer to form the scene.
Now for the main idea. Any time there is an explosion or other terrain-deforming event, you draw a circle or other shape on the terrain layer, using the terrain layer itself as a drawing mask (so only the part of the circle that overlaps existing terrain is drawn), to wipe out part of the terrain. Use a transparent/mask-color brush for the fill and some color similar to the terrain for the thick pen.
You can repeat this process to add more deformations. You could keep this layer in memory and add deformations as they occur or you could even render them in memory each frame if there aren't too many deformations to render.
I guess you'd better use texture-filled polygons with the correct mapping (a linear one that doesn't stretch the texture to use all the texels, but leaves the cropped areas out), and then reshape them as they get destroyed.
I'm assuming your problem will be to implement the collision between characters/weapons/terrain.
As long as you aren't doing this on opengl es, you might be able to get away with using the stencil buffer to do per-pixel collision detection and have your terrain be a single modifyable texture.
This page will give an idea:
http://kometbomb.net/2007/07/11/hardware-accelerated-2d-collision-detection-in-opengl/
The way I imagine it is this:
a plane with the texture applied
a path( a vector of points/segments ) used for ground collisions.
When something explodes, you do a boolean operation (rectangle-circle) for the texture(revealing the background) and for the 'walkable' path.
What I'm trying to say is you do a geometric boolean operation and you use the result to update the texture(with an alpha mask or something) and update the data structure you use to keep track of the walkable area(which ever that might be).
Split things up, instead of relying only on gl draw methods
I think I would start by drawing the foreground into the stencil buffer so the stencil buffer is set to 1 bits anywhere there's foreground, and 0 elsewhere (where you want your sky to show).
Then to draw a frame, you draw your sky, enable the stencil buffer, and draw the foreground. For the initial frame (before any explosion has destroyed part of the foreground) the stencil buffer won't really be doing anything.
When you do have an explosion, however, you draw it to the stencil buffer (clearing the stencil buffer for that circle). Then you re-draw your data as before: draw the sky, enable the stencil buffer, and draw the foreground.
This lets you get the effect you want (the foreground disappears where desired) without having to modify the foreground texture at all. If you prefer not to use the stencil buffer, the alternative that seems obvious to me would be to enable blending, and just manipulate the alpha channel of your foreground texture -- set the alpha to 0 (transparent) where it's been affected by an explosion. IMO, the stencil buffer is a bit cleaner approach, but manipulating the alpha channel is pretty simple as well.
I think, but this is just a quick idea, that a good way might be to draw a Very Large Number of Lines.
I'm thinking that you represent the landscape as a bunch of line segments, for each column of the screen you have 0..n vertical lines, that make up the ground:
12 789
0123 6789
0123456789
0123456789
In the above awesomeness, the column of "0":s makes up a single line, and so on. I didn't try to illustrate the case where a single pixel column has more than one line, since it's a bit hard in this coarse format.
I'm not sure this will be efficient, but it at least makes some sense since lines are an OpenGL primitive.
You can color and texture the lines by enabling texture-mapping and specifying the desired texture coordinates for each line segment.
Typically the way I have seen it done is to have each entity be a textured quad, then update the texture for animation. For a destructible terrain it might be best to break the train into tiles then you only have to update the ones that have changed. Don't use GLdrawpixels it is probably the slowest approach possible (outside of reloading textures from disk every frame though it would be close.)

Can you render two quads with transparency at the same point?

I'm learning about how to use JOGL and OpenGL to render texture-mapped quads. I have a test program and a test quad, and I figured out how to enable GL_BLEND so that I can specify the alpha value of a vertex to make a quad with a sort of gradient... but now I want this to show through to another textured quad at the same position.
Drawing two quads with the same vertex locations didn't work, it only renders the first quad. Is this possible then, or will I need to basically construct a custom texture on-the-fly based on what I want and then draw one quad with this texture? I was really hoping to take advantage of blending in this case...
Have a look at which glDepthFunc you're using, perhaps you're using GL_LESS/GL_GREATER and it could work if you're using GL_LEQUAL/GL_GEQUAL.
Its difficult to make out of the question what exactly you're trying to achieve but here's a try
For transparency to work correctly in OpenGL you need to draw the polygons from the furthest to the nearest to the camera. If you're scene is static this is definitely something you can do. But if it's rotating and moving then this is usually not feasible since you'll have to sort the polygons for each and every frame.
More on this can be found in this FAQ page:
http://www.opengl.org/resources/faq/technical/transparency.htm
For alpha blending, the renderer blends all colors behind the current transparent object (from the camera's point of view) at the time the transparent object is rendered. If the transparent object is rendered first, there is nothing behind it to blend with. If it's rendered second, it will have something to blend it with.
Try rendering your opaque quad first, then rendering your transparent quad second. Plus, make sure your opaque quad is slightly behind your transparent quad (relative to the camera) so you don't get z-buffer striping.