I like to write a function that gets 4 colors sitting in the corners of an image. The colors are 16 bit RGB. My job is now to output an 16x9 image that represents the gradient of these colors. The simplest case would be someone passes the color red for the topleft and bottomleft corner and green for the topright and bottomright corner. The image should now show the smooth transition from red to green :) Which algorithms would you suggest ? Help is really appreciated :)
Basically I need these operations.
createLinearGradient(horizontal)
createLinearGradient(vertical)
createLinearGradient(diagonal)
createBilinearGradient() there is a good picture about this in wikipedia
http://en.wikipedia.org/wiki/Bilinear_interpolation
But someone told me I could do everything with just one algorithmus
Related
I am working on a project for my thesis and I am building my own path tracer. Afterwards, I have to modify it in such a way to be able to implement the following paper:
https://mediatech.aalto.fi/publications/graphics/GPT/kettunen2015siggraph_paper.pdf
Of course I DO NOT want you to read the paper, but I link it anyway for those who are more curious. In brief, instead of rendering an image by just using the normal path tracing procedure, I have to calculate the gradients for each pixel, which means: if before we were shooting only rays through each pixel, we now shoot also rays for the neighbouring pixels, 4 in total, left, right, top, bottom. Let me explain in other words, I shoot one ray through a pixel and calculate its final colour as for normal path tracing, but, moreover, I shoot rays for its neighbour pixels, calculate the same final colour for those and, in order to calculate the gradients, I subtract their final colours from the main pixel. It means that for each pixel I will have 5 values in total:
colour of the pixel
gradient with right pixel = colour of the right pixel - colour of the pixel
gradient with left pixel = colour of the left pixel - colour of the pixel
gradient with top pixel = colour of the top pixel - colour of the pixel
gradient with bottom pixel = colour of the bottom pixel - colour of the pixel
The problem is that I don't know how to build the final image by both using the main colour and the gradients. What the paper says is that I have to use the screened Poisson reconstruction.
"Screened Poisson reconstruction combines the image and its
gradients using a parameter α that specifies the relative weights of
the sampled image and the gradients".
Everywhere I search for this Poisson reconstruction I see, of course, a lot of math but it comes hard to apply it to my project. Any idea? Thanks in advance!
I am looking to reproduce the glow effect from this tutorial, if I understand well, we convert the first image to an "alpha texture" (black and white), and we blur the (rgb * a) texture.
How is it possible to create this alpha texture, so that some colors go to the white, and the other go to the black? I found this : How to render a texture with alpha? but I don't really know how to use these answers.
Thanks
It appears you are misunderstanding what that diagram is showing you. It is actually all one texture, but (a) shows the RGB color and (b) shows the alpha channel. (c) shows what happens when you multiply RGB by A.
Alpha is not actually "black and white", it is an abstract concept and amounts to a range of values between 0.0 and 1.0. For the human brain to make sense out of it, it interprets that as black (0.0) and white (1.0). In reality, alpha is whatever you want it to be and unrelated to color (though it can be used to do something to color).
Typically the alpha channel would be generated by a post-process image filter, that looks for areas of the texture with significantly above average luminance. In modern graphics engines HDR is used and any part of the scene with a color too bright to be displayed on a monitor is a candidate for glowing. The intensity of this glow is derived from just how much brighter the lighting at that point is than the monitor can display.
In this case, however, it appears to be human created. Think of the alpha channel like a mask, some artist looked at the UFO and decided that the areas that appear non-black in figure (b) were supposed to glow so a non-zero alpha value was assigned (with alpha = 1.0 glowing the brightest).
Incidentally, you should not be blurring the alpha mask. You want to blur the result of RGB * A. If you just blurred the alpha mask, then this would not resemble glowing at all. The idea is to blur the lit parts of the UFO that are supposed to glow and then add that on top of the base UFO color.
So I have no idea how I should be doing what I want do so I'll explain as best as I can.
http://i.stack.imgur.com/j65H8.jpg
So imagine that entire image is a 2d square 128x128 and each color I want to apply a texture to that part of the 2d square. Also I want it to stretch as well so Red, Aqua, Green and Purple never stretch in any direction but Pink stretches all directions and then Grey, Yellow, Black and Orange stretch in the longest direction (grey/orange = width expands, yellow/black = height expands). When stretched it should look like this:
http://i.stack.imgur.com/wJiKv.jpg
Also I am using C++.
I'm trying to show only a part of a background image (game scenenario in the future). The basic way to work is for example, first I draw a background image, after that i need to "hide"/cover the image with some dark or darness (no light, don't know what option must be chosen) and use the mouse click to using a circle or a triangle (my options) show only the part of the image background over with the circle/triangle centered on mouse position. I called this "lantern effect".
First Option: Play with the alpha channel, creating a an square covering all the window size and after that trying to substract the circle area over the alpha square over the image.
Second Option: Play again with a black square covering all the image background and trying to substract a circle/triangle. Try with glLogicOp but this method only plays mixing colors. Don't know how to do operation with 2D polygons with OpenGL.
...
Any other idea or easy example to learn how to do something similar.
Image example:
That's quite easy to achieve actually:
Create black texture with your lantern light shape in Alpha channel. (Texture could be animated)
Render background.
Render Lantern texture centered at your in-game mouse cursor.
Render black padding around the lantern texture to hide everything around till screen edges.
There's no need to use any special blending modes, just an overlay.
So we have an image. We want to draw a line that must definitely be seen. So how to draw a lines with colors that are inverted relatively to the surface it should be drawn on in each point?
The XOR trick is trivially different. It's not visually the most distinct, though, if only because it entirely ignores how human eyes work. For instance, on light greys, a saturated red color is visually quite distinct.
You might want to convert the color to HSV and check the saturation S. If low (greyscale), draw a red pixel. If the saturation is high, the hue is quite obvious, and a white or black pixel will stand out. Use black (V=0) if the the original pixel had a high V; use white if the original pixel had a low V (dark saturated color)
You can use the LineIterator method as suggested earlier.
(BTW, the XOR trick has quite bad cases too. 0x7F ^ 0xFF = 0x80. That's bloody hard to see)
Use a LineIterator and XOR the colour values of each pixel manually.
This is from the top of my head and I'm not a c++ dev, but it should be possible
to draw the line into a separate image and then mimic an invert blend mode...basically you need to get the 'negative'/inverted colour behind a pixel, which you get by subtracting the color bellow your line from the maximum colour value.
Something like:
uint invert(uint topPixel, uint bottomPixel) {
return (255 - bottomPixel);
}
Not sure how if colours are from 0 to 255 or from 0.0 to 1.0, but hopefully this illustrates the idea.