Inverting Colors

beginners
#1

Using a basic slitscanner code, I am trying to get the slitscan image to change and invert colors once a key is pressed. I uploaded the ofxOpenCV library, and tried to use .invert, but that didn’t work.

Here is the code that I thought would try and change the colors of the slit scan, but it is not doing anything.
`void ofApp::keyPressed(int key){

// if (key == 32){

cout << "Key Pressed";
    ofImage slitScanInvert = slitScanImage;
    ofPixels ssPixels = slitScanInvert.getPixels();
    
    // 1. get pixels from copied ofImage
    // 2. get the color of each pixels
    // 3. invert the color from eac pixel
    // 4. write new color back to the image
    
    for (int col = 0; col < slitScanInvert.getWidth(); col++) {
        for (int row = 0; row < slitScanInvert.getHeight(); row++) {
            ofColor c = ssPixels.getColor(col, row);
            ssPixels.setColor(col, row, c.invert());
        }
    }
    
    slitScanInvert.draw(200, camHeight+40);
}`
#2

Hi,

you can try to do:

c.r = 255 - c.r;
c.g = 255 - c.g;
c.b = 255 - c.b;

to be honest, you should try to use a shader to do this, as it seems like you are doing it on the whole image at once.
Try using this addon : ofxKsmrFragmentFx .

Try modifying the example by using :
in ofApp.cpp::update():

	fx.getfxUnit(KSMR_FRAGFX_INVERT)->bEnable		= ofGetKeyPressed(" ");

Hope this helps,

++

P

1 Like
#3

as an alternative, you could draw it to an fbo and then invert it using ofx::dotfrag::InvertStrobe from my ofxDotFrag addon:

1 Like
#4

hey @npisanti, thanks for that suggestion to use ofxDotFrag.

I am building an “oled screen emulator” and It’s working nice to invert a selected text line on the screen.
19

I have an isolated class/addon for this that works fine on a clean ofApp. But when including into the main app I am building, I am getting in trouble with alpha transparency alphablending, drawings modes, arb texture or I don’t know… but the rectangle that I am inverting is just full filled black or not drawed.

I tried several combinations of enabling alpha blendings, disabling all other draws into draw() method, to used push/pop Style… without success. Maybe some of the other addons that I am using has some drawing/GL/alpha mode…)

Could be the way that I am cleaning / updating the zones in the fbo (I am really newbie with them) …
I tried too to use not alpha fbo but not worked neither…)

I put some code here to ask you or someone if there’s something strange:

//.h
ofFbo rgbaFbo; // the full lcd-screen rectangle
ofFbo fbo2; // the inverted line or pixels zone
ofx::dotfrag::InvertStrobe frag;

void ofxOled::setup()
{
	rgbaFbo.allocate(128, 64, GL_RGBA);
	rgbaFbo.begin();
	ofClear(255,255,255, 0);//white
	rgbaFbo.end();

	fbo2.allocate(128, 9, GL_RGBA);
	frag.allocate( fbo2 );
	frag.active = true;
}

// add/draw just a text line into the lcd-screen fbo
// there are other lines that I don't want to erase
// that's the reason why I don't make a ofClear( ) after the begin()
void ofxOled::gPrintln(int x, int y, int color, string txt)
{
    rgbaFbo.begin();
  
    ofPushStyle();
    ofNoFill();
    ofSetLineWidth(0.5f);
    ofSetColor(color);

    font.drawString(txt, x, y);

    ofPopStyle();

    rgbaFbo.end();
}

// invert a rectangle zone when called
void ofxOled::gInvertArea(int x, int y, int w, int h)
{
    // apply shader to invert area into a 2nd fbo
    fbo2.allocate(w, h, GL_RGBA);
    fbo2.begin();

    // get the subsection we want to invert from the lcd-screen fbo
    rgbaFbo.getTexture().drawSubsection(0, 0, w, h, x ,y, w, h);

    fbo2.end();

    // 1. apply ofxDotFrag inverter shader
    frag.apply( fbo2 );

    //-

    // 2. draw inverted zone upon in front of lcd-screen fbo
    rgbaFbo.begin(); // don't want to clear all, just add upfront
    fbo2.draw( x, y );
    rgbaFbo.end();
}

// main draw
void ofxOled::draw()
{
    ofPushMatrix();
    ofTranslate(pos);

    // draw lcd-screen with the inverted rectangle included 
    rgbaFbo.draw(oLed_x, oLed_y);

    // preview inverted rectangle line only, displaced 200px to the right
    fbo2.draw( oLed_x + 200, oLed_y );

    ofPopMatrix();
}
#5

try not allocating fbo2 again in gInvertArea, i don’t know if that is executed each frame but sometimes i have problems if i allocate the fbos too often

#6

well, the thing is that is not drawing every frame.
The testing app is working fine with the same drawing calls frequency than in the bigger app where is failing…

I tried to disabling the allocate on the method, allocating only into setup, but do not change at all: black rectangle.

Not sure if frag allocate must be done too when fbo size changes and is re-allocated… but not helps.

//    frag.allocate( fbo_inverter );
//    frag.active = true;

I suspect could be something related to drawing modes. So maybe I need to ‘force’ some blend mode when drawing the inverted-fbo into the main oled-fbo…

 //    /// \brief Blend mode is disabled.
 //    ofEnableBlendMode(OF_BLENDMODE_DISABLED);
 //    /// \brief Blend mode used for alpha blending.
 //    ofEnableBlendMode(OF_BLENDMODE_ALPHA);
 //    /// \brief Blend mode used for additive blending.
 //    ofEnableBlendMode(OF_BLENDMODE_ADD);
 //    /// \brief Blend mode used for subtractive blending.
 //    ofEnableBlendMode(OF_BLENDMODE_SUBTRACT);
 //    /// \brief Blend mode used for multiplicative blending.
 //    ofEnableBlendMode(OF_BLENDMODE_MULTIPLY);
 //    /// \brief Blend mode used for screen blending.
 //    ofEnableBlendMode(OF_BLENDMODE_SCREEN);

18

#7

frag should reallocate itself automatically when fbo size changes, so that won’t be a problem, i understand that blending mode could give some issues with fbo or shader alphas, but i don’t know what is specifically going wrong.

i know it could be time consuming, but could you please try to set up an ofApp with the minimum amount of code to reproduce the bad behavior?

#8

sorry to re-up this.
but I found that my problem was that ofxDotFrag disables the default OF mode ofEnableArbTex().

So the solution when combining some shaders/fx addons seems to enable the respective required arb mode before allocating the fbo for any addon.
If not, you can experience some black drawings of fbo’s/addons.

In this case, after allocating ofxDotFrag, I must enable arb again to avoid breaking the other allocating shaders/addons/fbo’s that requires arb tex enabled.