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    <p>As you've not shown any code, I'm going to make the assumption that what you're doing is storing a set of points and then in a paint function, using a painter to draw those points. The effect you're getting is when you draw over the area that you've already drawn. </p> <p>One method you can use to prevent this is to use a QPainterPath object. When the mouse down event occurs, use the moveTo function for the QPainterPath object. Then call the lineTo function for mouse move events.</p> <p>Finally when it comes to rendering, instead of drawing the points, render the QPainterPath object.</p> <p>---------- Edit --------------------------------------</p> <p>Since you've added the example of the effect you're wanting, I understand your problem better and you may not be able to use the QPainterPath here, but I do recommend it for the opaque lines. </p> <p>However, if you work out the gradient changes before adding the lines to a QPainterPath, it may be possible to use a gradient pen with the QPainterPath and get that working the way you want. Alternatively...</p> <p>You mentioned this in your original answer: -</p> <blockquote> <p>draw on a separate transparent buffer and render there and obtain the maximum opacity and render it back to the original buffer.</p> </blockquote> <p>This sounds more complicated than it is due to the word buffer. In actuality, you just create a separate QImage and draw to that rather than the screen. Then when it comes to drawing the screen, you copy the image instead. To 'obtain the maximum opacity' you can either scan the bits on the image and look at the alpha channel, or keep a separate struct of info that records the pressure of the pen and its location at each point. I would look to get the maximum and minimum values of when the alpha is increasing and then decreasing and linearly interpolate the values for rendering, rather than trying to map every minute change.</p> <p>When rendering the buffer image back to the main one, I think you need to set a composition mode on the QPainter, but off the top of my head, I'm not exactly sure which one. Read the documentation to see what they do and experiment with them to see what effects they produce.</p> <p>In my experience with graphics, it's often the case that I find you need to experiment to see what works and get a feel for what you're doing, especially when you find a method that you're using starts to become slow and you need to optimise it to work at a reasonable frame rate.</p>
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    1. POQt: Creating a pencil/brush tool
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    1. COthat is good for solid transparent lines. And i tried to use a path but you can't obtain a brush stroke that goes from transparent to opaque. And I want that effect. A path doesn't solve the problem because the path acts as one (so no blending can occur) and because i tried it
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