# OpenGL invert colors

## Recommended Posts

hi, I'm new in opengl and in this forum... is it possible to define a color that for the next shape drawed its color is the inverse of background color??? what I wanna do is: I have a list of objects. The user draw the objects whith the mouse(click and drag) e.g. a rectangle. What may I put on the mouseMove function?? I was intending to draw the rectangle inverting the colors of the background... at the next time the moseMove execute it draw again the rectangle at the previous position returning the background('erasing' the object) and then drawing in the new position. All this because it is expensive to draw all the list at mouseMove. can you help me?? Thanks SassÃ¡

##### Share on other sites
XOR is possible see the imagesubset part of gl added in opengl1.2

though
Quote:
 All this because it is expensive to draw all the list at mouseMove

most likely this is the quicker way

##### Share on other sites
This can be done with subtractive blending, so you'll have to use an extension for it:

Check out the specs for GL_EXT_blend_subtract at

http://oss.sgi.com/projects/ogl-sample/registry/EXT/blend_subtract.txt

Obviously, this means that your card/driver combo will have to support this extension. If you need to know if your card/driver will support this, use glGetString(GL_EXTENSIONS) and find out whether GL_EXT_blend_subtract is a part of this returned string.

##### Share on other sites
Quote:
 Original post by FReYThis can be done with subtractive blending, so you'll have to use an extension for it:Check out the specs for GL_EXT_blend_subtract athttp://oss.sgi.com/projects/ogl-sample/registry/EXT/blend_subtract.txtObviously, this means that your card/driver combo will have to support this extension. If you need to know if your card/driver will support this, use glGetString(GL_EXTENSIONS) and find out whether GL_EXT_blend_subtract is a part of this returned string.

yes, my card/driver support this extension.
Is BlendEquationEXT a function? if yes, do I need any aditional library to use it??

if you post a code or a link for one it would help a lot.

remembering, I'm new in opengl... so sorry

Thanks for the help

SassÃ¡

##### Share on other sites
Quote:
 Is BlendEquationEXT a function? if yes, do I need any aditional library to use it??

Yes, this is an OpenGL function, but you can either setup the function pointer yourself or have an external library to do it for you. I highly recommend 'GLee' by Benjamin_Bunny. You can find it at:
http://elf-stone.com/glee.php

Download the 'GLee/GLee4_00.zip' file. Open the zip file and include the source files in your project. Then in the initialization of your program simply call GLeeInit();

Quote:
 if you post a code or a link for one it would help a lot.

okay, I've personally never actually used this extension, but I think I can figure it out from the blend_minmax and the blend_subtract extension definitions.

The blend_minmax extension will give you the glBlendEquationEXT function. The blend_subtract extension allows you to change your blending equation from the form:
C' = (Cs * S) + (Cd * D)
to
C' = (Cs * S) - (Cd * D)

Using GLee you can easily query support for extensions and get hold of the functions required. However, you have to test if they're supported first.
You will want to do something like this:

if (GLEE_EXT_blend_subtract){  // subtractive blending is supported. Yay!  glBlendEquationEXT(GL_FUNC_SUBTRACT_EXT);}else{  // subtractive blending is NOT supported. don't even try!}// do your subtractive blending rendering here. Do your quad rendering, but make sure your quad is white. //This will give you 'white' - backgroundcolour which should be the inverse.glBlendFunc(GL_ONE, GL_ONE);glColor3f(1.0f, 1.0f, 1.0f);glBegin(GL_QUADS)...glEnd();// change back to additive blending. this is important otherwise it will screw up the rest of your blend-modesif (GLEE_EXT_blend_subtract){  // subtractive blending is supported. Yay!  glBlendEquationEXT(GL_FUNC_ADD_EXT);}

Seriously, get GLee. It saves *so* much time it's not even funny.
Hope that helped you.

## Create an account

Register a new account

• ## Partner Spotlight

• ### Forum Statistics

• Total Topics
627636
• Total Posts
2978331
• ### Similar Content

• By xhcao
Before using void glBindImageTexture(    GLuint unit, GLuint texture, GLint level, GLboolean layered, GLint layer, GLenum access, GLenum format), does need to make sure that texture is completeness.
• By cebugdev
hi guys,
are there any books, link online or any other resources that discusses on how to build special effects such as magic, lightning, etc. in OpenGL? i mean, yeah most of them are using particles but im looking for resources specifically on how to manipulate the particles to look like an effect that can be use for games,. i did fire particle before, and I want to learn how to do the other 'magic' as well.
Like are there one book or link(cant find in google) that atleast featured how to make different particle effects in OpenGL (or DirectX)? If there is no one stop shop for it, maybe ill just look for some tips on how to make a particle engine that is flexible enough to enable me to design different effects/magic
let me know if you guys have recommendations.
• By dud3
How do we rotate the camera around x axis 360 degrees, without having the strange effect as in my video below?
Mine behaves exactly the same way spherical coordinates would, I'm using euler angles.
Tried googling, but couldn't find a proper answer, guessing I don't know what exactly to google for, googled 'rotate 360 around x axis', got no proper answers.

References:
Code: https://pastebin.com/Hcshj3FQ
The video shows the difference between blender and my rotation:

• By Defend
I've had a Google around for this but haven't yet found some solid advice. There is a lot of "it depends", but I'm not sure on what.
My question is what's a good rule of thumb to follow when it comes to creating/using VBOs & VAOs? As in, when should I use multiple or when should I not? My understanding so far is that if I need a new VBO, then I need a new VAO. So when it comes to rendering multiple objects I can either:
* make lots of VAO/VBO pairs and flip through them to render different objects, or
* make one big VBO and jump around its memory to render different objects.
I also understand that if I need to render objects with different vertex attributes, then a new VAO is necessary in this case.
If that "it depends" really is quite variable, what's best for a beginner with OpenGL, assuming that better approaches can be learnt later with better understanding?

• Hello all,

On my Windows 7 x64 machine I wrote the code below on VS 2017 and ran it.
#include <glad/glad.h>  #include <GLFW/glfw3.h> #include <std_lib_facilities_4.h> using namespace std; void framebuffer_size_callback(GLFWwindow* window , int width, int height) {     glViewport(0, 0, width, height); } //****************************** void processInput(GLFWwindow* window) {     if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)         glfwSetWindowShouldClose(window, true); } //********************************* int main() {     glfwInit();     glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);     glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);     glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);     //glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);     GLFWwindow* window = glfwCreateWindow(800, 600, "LearnOpenGL", nullptr, nullptr);     if (window == nullptr)     {         cout << "Failed to create GLFW window" << endl;         glfwTerminate();         return -1;     }     glfwMakeContextCurrent(window);     if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))     {         cout << "Failed to initialize GLAD" << endl;         return -1;     }     glViewport(0, 0, 600, 480);     glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);     glClearColor(0.2f, 0.3f, 0.3f, 1.0f);     glClear(GL_COLOR_BUFFER_BIT);     while (!glfwWindowShouldClose(window))     {         processInput(window);         glfwSwapBuffers(window);         glfwPollEvents();     }     glfwTerminate();     return 0; }
The result should be a fixed dark green-blueish color as the end of here. But the color of my window turns from black to green-blueish repeatedly in high speed! I thought it might be a problem with my Graphics card driver but I've updated it and it's: NVIDIA GeForce GTX 750 Ti.
What is the problem and how to solve it please?

• 10
• 12
• 22
• 13
• 33