[AGG] Re: Re: Beginner - how to put an image at X,Y in a rendering buffer

[Maxim S. <mc...@an...>]

> double para [ 6 ] = { 0,100 ,  0,0 , 100,0 } ; // 3 points (0,100) (0,0) 
> and (100,0)
> agg::trans_affine mtx(para, 0, 0, imgwd, imght);
> agg::path_storage path; // partner rectangle
> path.move_to( x,y);
> path.line_to( x+imgwd, y );
> path.line_to( x+imgwd, y+imght);
> path.line_to( x, y+imght);
> path.close_polygon();
> agg::conv_transform<agg::path_storage, agg::trans_affine> trans(path, 
> mtx);
>
> pf.add_path(trans); // Note you add "trans"
>
> Then, if you want your image to fit exactly your parallelogram path (you 
> also may want to do differently!), you need to create a copy of the matrix 
> and invert it:
>
> agg::trans_affine img_mtx(mtx);
> img_mtx.invert();

I'm sorry, Ken, this is not correct; I have confused myself, so, please 
discard the code above. :)

So, suppose you have an image of imgwd, imght and a destination 
parallelogram. To define the parallelogram you need 3 points, x1,y1 - bottom 
left, x2,y2 - bottom right, x3,y3 - top right. The parallelogram can also 
define a 2D affine matrix: rotation, scaling, translation and skewing. You 
can rasterize your destination parallelogram directly:

agg::rasterizer_scanline_aa<> ras;
ras.move_to_d(x1,y1);
ras.line_to_d(x2,y2);
ras.line_to_d(x3,y3);
ras.line_to_d(x1 + x3 - x2, y1 + y3 - y2);

So that, you can draw a solid parallelogram (well, you can also use the 
path_storage if you want).
To map an image to it you need to create the matrix:

double para[6] = {x1,y1,x2,y2,x3,y3};
agg::trans_affine img_mtx(0, 0, imgwd, imght, para);
img_mtx.invert();

Or, which is the same:

double para[6] = {x1,y1,x2,y2,x3,y3};
agg::trans_affine img_mtx(para, 0, 0, imgwd, imght);

The first one construicts a matrix to transform a rectangle to a a 
parellelogram, the second one - parallelogram to rectangle. The image 
transformer requires namely inverse matrix, so that, you transform your 
parallelogram (destination) to rectangle (image).

Technically that's it. But you may want to apply additional transformations. 
To do that you will need two matrices:

agg::trans_affine master_mtx;
master_mtx *= agg::trans_affine_translation(. . .);
master_mtx *= agg::trans_affine_rotation(. . .);
. . .
agg::rasterizer_scanline_aa<> ras;
agg::path_storage path; // partner rectangle
path.move_to(x1,y1);
path.line_to(x2,y2);
path.line_to(x3,y3);
path.line_to(x1 + x3 - x2, y1 + y3 - y2);
path.close_polygon();
agg::conv_transform<agg::path_storage, agg::trans_affine> trans(path, 
master_mtx);

Then you prepare the image matrix:
double para[6] = {x1,y1,x2,y2,x3,y3};
agg::trans_affine img_mtx(0, 0, imgwd, imght, para);
img_mtx *= master_mtx; //!!!!!!!!!!!!! Integrate the master transforms
img_mtx.invert();

ras.add_path(trans);
. . .Render

Now, whatever transformations you use in the master_mtxÀ they will be 
synchronized with the image.

McSeem