How does the coordinate mapping work?

[zhaozhemin]

It seems that instead of frame-coord-map, this implementation maps the points in the unit square to the actual output device by manipulating the transformation matrix. I played a little bit, but can't grasp how it's done.

Take this painter as an example:

(define line (vects->painter (list (make-vect 0 0) (make-vect 1 1))))

The transformation matrix right before it draws the line is #(198.0 0.0 0.0 -198.0 0.0 200.0). I verify the mapping by hand using the formula. It correctly maps '(0 0) to '(0 200), and '(1 1) to '(200 0). (I use #(200.0 0.0 0.0 -200.0 0.0 200.0) for easy calculation.)

But when I run the following code:

(define bm (make-bitmap 200 200))
(define dc (new bitmap-dc% [bitmap bm]))
(send dc set-initial-matrix (vector 198.0 0.0 0.0 -198.0 0.0 200.0))
(send dc draw-line 0.0 0.0 1.0 1.0)

It just gives this picture

What am I missing?

[soegaard]

You are missing the shape and size of the pen used to draw the line. The default pen is a 1x1 square (a single pixel). The matrix transform blows this up to a size of 198x198 so your line is drawn with a huge, square pen. If you want a "hair line" you can set the pen width to 0 otherwise try 1/198.

lang racket

(require racket/draw)

(define bm (make-bitmap 200 200)) (define dc (new bitmap-dc% [bitmap bm])) (define pen (new pen% [width 1/198])) (send dc set-pen pen)

(send dc set-initial-matrix (vector 198.0 0.0 0.0 -198.0 0.0 200.0)) (send dc draw-line 0.0 0.0 1.0 1.0) bm

/Jens Axel

Den ons. 22. apr. 2020 kl. 17.48 skrev Zhao Min notifications@github.com:

It seems that instead of frame-coord-map, this implementation maps the points in the unit square to the actual output device by manipulating the transformation matrix. I played a little bit, but can't grasp how it's done.

Take this painter as an example:

(define line (vects->painter (list (make-vect 0 0) (make-vect 1 1))))

The transformation matrix right before it draws the line is #(198.0 0.0 0.0 -198.0 0.0 200.0). I verify the mapping by hand using the formula. It correctly maps '(0 0) to '(0 200), and '(1 1) to '(200 0). (I use #(200.0 0.0 0.0 -200.0 0.0 200.0) for easy calculation.)

But when I run the following code:

(define bm (make-bitmap 200 200)) (define dc (new bitmap-dc% [bitmap bm])) (send dc set-initial-matrix (vector 198.0 0.0 0.0 -198.0 0.0 200.0)) (send dc draw-line 0.0 0.0 1.0 1.0)

It just gives this picture

[image: untitled] https://user-images.githubusercontent.com/10150604/80001775-59fda000-84f1-11ea-9e46-d95c9c92386e.png

What am I missing?

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-- -- Jens Axel Søgaard

[zhaozhemin]

It works. Thank you very much.

[soegaard]

Great it worked.