Somehow this thread slipped off the list... Thank you Matt! Very useful! I've been given great advice on my question, thank you all. /Fredrik On Wed, 2007-04-11 at 08:49 -0500, Matt Fischer wrote: > > > On 4/11/07, Fredrik Persson <fredrik.p.persson@xxxxxxxxx> wrote: > > > On 4/11/07, Matt Fischer <mattfischer84@xxxxxxxxx> wrote: > > > On 4/10/07, Andrew Sobala <aes@xxxxxxxxx> wrote: > That's a simple maths question, not GTK. If > the user picks (x1,y1) and > (x2,y2), then the points along that line > conform to y = (y2 - y1)/(x2 - > x1) * (x - x1) + y1. (I think!) > > So plug in values for x from x1 to x2, and > pull out y (with appropriate > whole-number rounding.) > > One more wrinkle...you'll probably want to see whether > your line is x-major or y-major first, that is, in > which dimension it is the longest, and iterate across > that dimension. Otherwise you run the risk of > skipping pixels (for an exteme example, imagine > performing the above technique on a vertical line.) > So, if your line is longer in X than in Y, then use > the equation as listed above, and if it's longer in Y > than it is in X, then swap around all the X's and Y's > and iterate along Y instead. Once you have the pixel > coordinates, you should be able to pull individual > pixel colors out of a GdkPixbuf as Michael suggests. > > --Matt > > Yes, this sounds clever, thank you. > > Do you have any idea about what to do with the fractions of a > pixel? For example, the line x1,y2 = 2,1 and x2,y2 =10,6 will, > if we iterate over x, take on y values such as 1.625. Is it > wise, in that case, to round that off to 2, or to have a > combination of pixels y = 1 and y = 2 in accordance with the > fraction 0.625? (I hope you understand what I mean.) > > /Fredrik > > > Yeah, that's the next problem you'll run into. If you want it to > really look nice, you'll want to do some filtering on it, somewhat as > you suggest. However, to do that correctly, you'll probably want to > filter it in both directions, i.e., if you want to scan along a line > that has a net length of 10, but you want to draw the resulting graph > as 20 pixels wide, you'll want it to be smooth. What I would probably > do is do it parametrically: > > Assuming you want your resulting line G to essentially be an array of > g_max pixel values > Iterate g from 0 to g_max > Let t = g / g_max > x = x1 + (x2 - x1) * t > y = y1 + (y2 - y1) * t > xa = round_down(x) > xb = round_up(x) > ya = round_down(y) > yb = round_up(y) > Get all four pixels surrounding the fractional point: (xa, ya), (xb, > ya), (xa, yb), and (xb, yb) > Average them together based on the fractional components of x and y > G[g] = result of the average > > That way it will be completely blended, and look nice and smooth > regardless of how long you want the resulting line to be in relation > to the original image. It also means you don't need to worry about my > earlier post about switching around the orientation based on x- or > y-majorness....the parametric equation is completely symmetric, so it > handles this all by itself. > > --Matt _______________________________________________ gtk-list mailing list gtk-list@xxxxxxxxx http://mail.gnome.org/mailman/listinfo/gtk-list
