I learn opengl here: https://learnopengl.com/#!Advanced-OpenGL/Cubemaps
Did skybox. If you draw it first, then everything is fine. However, to reduce the number of pixels for its output, I try to draw it last. But when you look at the skybox through transparent objects, it is not displayed. If you draw skybox before transparent objects, then they are not displayed. How to fix it?enter image description here
Transparency is not order independent. You cannot draw something "behind" a already drawn surface. You will have to draw the skybox (at least) before you draw your transparent objects.
Note, that you also have to order your transparent objects back to front if it should be possible to correctly see through multiple of them.
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Not long ago, I tried out a program from an OpenGL guidebook that was said to be double buffered; it displayed a spinning rectangle on the screen. Unfortunately, I don't have the book anymore, and I haven't found a clear, straightforward definition of what a buffer is in general. My guess is that it is a "place" to draw things, where using a lot could be like layering?
If that is the case, I am wondering if I can use multiple buffers to my advantage for a polygon clipping program. I have a nice little window that allows the user to draw polygons on the screen, plus a utility to drag and draw a selection box over the polygons. When the user has drawn the selection rectangle and lets go of the mouse, the polygons will be clipped based on the rectangle boundaries.
That is doable enough, but I also want the user to be able to start over: when the escape key is pressed, the clip box should disappear, and the original polygons should be restored. Since I am doing things pixel-by-pixel, it seems very difficult to figure out how to change the rectangle pixel colors back to either black like the background or the color of a particular polygon, depending on where they were drawn (unless I find a way to save the colors when each polygon pixel is drawn, but that seems overboard). I was wondering if it would help to give the rectangle its own buffer, in the hopes that it would act like a sort of transparent layer that could easily be cleared off (?) Is this the way buffers can be used, or do I need to find another solution?
OpenGL does know multiple kinds of buffers:
Framebuffers: Portions of memory to which drawing operations are directed changing pixel values in the buffer. OpenGL by default has on-screen buffers, which can be split into a front and a backbuffer, where drawing operations happen invisible on the backbuffer and are swapped to the front when finishes. In addition to that OpenGL uses a depth buffer for depth testing Z sort implementation, a stencil buffer used to limit rendering to cut-out (=stencil) like selected portions of the framebuffer. There used to be auxiliary and accumulation buffers. However those have been superseeded by so called framebuffer objects, which are user created object, combining several textures or renderbuffers into new framebuffers which can be rendered to.
Renderbuffers: User created render targets, to be attached to framebuffer objects.
Buffer Objects (Vertex and Pixel): User defined data storage. Used for geometry and image data.
Textures: Textures are sort of buffers, i.e. they hold data, which can be sources in drawing operations
The usual approach with OpenGL is to rerender the whole scene whenever something changes. If you want to save those drawing operations you can copy the contents of the framebuffer to a texture and then just draw that texture to a single quad and overdraw it with your selection rubberband rectangle.
I'm trying to make simple shading for a 2D OpenGL scene by placing semi-transparent black quads over the scene, but in certain places instead of darkening the scene it completely blacks it out. I'm using glBlendFunc(GL_DST_COLOR, GL_SRC_ALPHA) for the shader quads, and glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) for all the other drawing underneath.
This normally works, dimming the area under the semi-transparent quad, but in certain situations it behaves oddly. When the area under the semi-transparent quad has only had one quad drawn behind it, it behaves normally. When another quad (or multiple quads) are drawn on top of the back quad, it sometimes behaves normally, sometimes blacks out the quad entirely, or sometimes blacks out everything beneath the last-drawn quad (e.g. if I had a red texture with a blue dot on top of it, the red beneath would be blacked out, but the blue dot would remain unchanged).
I thought this looked like the quads drawing in the wrong order, but I have GL_DEPTH_TEST disabled, so I'm not sure if this is odd behavior from the blending, misorder, or something else entirely. Any ideas as to what could be causing this?
Iam trying to render some 3d objects using opengl. Requirement is that i need to hide all the transparent objects which are z-behind another transparent object. All the triangles are in single triangle buffer and will be drawn at once. Please throw some light.
Try using glDepthMask():
//Render all opaque objects
glDepthMask(false); //disable z-testing
//Render all transparent objects*
glDepthMask(true); //enable z-testing (for the next frame)
*Technically, you should render the transparent objects from back to front, but it is rarely noticeable if you don't.
You can do this by sorting your scene, which is what you have to do anyway to get transparency working correctly.
Here's what you need to do:
Enable z-buffer writes and tests
Render all opaque objects
Render all transparent objects front to back. The z-buffer will prevent transparent objects from being displayed behind other transparent objects.
I am developing a paint-like application using C++ and Open GL. But every time i draw objects like circle, lines etc they don't ** stay ** on the page. By this I mean that every new object I draw is getting placed on a blank page. How do I get my drawn objects to persist?
OpenGL has no geometry persistency. Basically it's pencils, brushes and paint, with which you draw on a canvas called the "framebuffer". So after you drawn something and clear the framebuffer, it will not reappear in some magic way.
There are two solutions:
you keep a list of all drawing operations and at each redraw you repaint everything from that list.
After drawing something copy the image in the framebuffer to a texture and instead of glClear you fill the background with that texture.
Both techniques can be combined.
Just don't clear the framebuffer and anything you draw will stay on the screen. This is the same method I use to allow users to draw on my OpenGL models. This is only good for marking up an image, since by using this method you can't erase what you've drawn, unless your method of erasing is to draw using your background color.
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.