vispy.visuals.transforms.transform_system module

class vispy.visuals.transforms.transform_system.TransformSystem(canvas=None, dpi=None)

Bases: object

TransformSystem encapsulates information about the coordinate systems needed to draw a Visual.

Visual rendering operates in six coordinate systems:

  • Visual - arbitrary local coordinate frame of the visual. Vertex buffers used by the visual are usually specified in this coordinate system.

  • Scene - This is an isometric coordinate system used mainly for lighting calculations.

  • Document - This coordinate system has units of _logical_ pixels, and should usually represent the pixel coordinates of the canvas being drawn to. Visuals use this coordinate system to make measurements for font size, line width, and in general anything that is specified in physical units (px, pt, mm, in, etc.). In most circumstances, this is exactly the same as the canvas coordinate system.

  • Canvas - This coordinate system represents the logical pixel coordinates of the canvas. It has its origin in the top-left corner of the canvas, and is typically the coordinate system that mouse and touch events are reported in. Note that, by convention, _logical_ pixels are not necessarily the same size as the _physical_ pixels in the framebuffer that is being rendered to.

  • Framebuffer - The buffer coordinate system has units of _physical_ pixels, and should usually represent the coordinates of the current framebuffer (on the canvas or an FBO) being rendered to. Visuals use this coordinate system primarily for antialiasing calculations. It is also the coorinate system used by glFragCoord. In most cases, this will have the same scale as the document and canvas coordinate systems because the active framebuffer is the back buffer of the canvas, and the canvas will have _logical_ and _physical_ pixels of the same size. However, the scale may be different in the case of high-resolution displays, or when rendering to an off-screen framebuffer with different scaling or boundaries than the canvas.

  • Render - This coordinate system is the obligatory system for vertices returned by a vertex shader. It has coordinates (-1, -1) to (1, 1) across the current glViewport. In OpenGL terminology, this is called clip coordinates.

Parameters
canvasCanvas

The canvas being drawn to.

dpifloat

The dot-per-inch resolution of the document coordinate system. By default this is set to the resolution of the canvas.

Notes

By default, TransformSystems are configured such that the document coordinate system matches the logical pixels of the canvas,

Examples

1. To convert local vertex coordinates to normalized device coordinates in the vertex shader, we first need a vertex shader that supports configurable transformations:

vec4 a_position;
void main() {
    gl_Position = $transform(a_position);
}

Next, we supply the complete chain of transforms when drawing the visual:

def draw(tr_sys):

tr = tr_sys.get_full_transform() self.program[‘transform’] = tr.shader_map() self.program[‘a_position’] = self.vertex_buffer self.program.draw(‘triangles’)

2. Draw a line whose width is given in mm. To start, we need normal vectors for each vertex, which tell us the direction the vertex should move in order to set the line width:

vec4 a_position;
vec4 a_normal;
float u_line_width;
float u_dpi;
void main() {
    // map vertex position and normal vector to the document cs
    vec4 doc_pos = $visual_to_doc(a_position);
    vec4 doc_normal = $visual_to_doc(a_position + a_normal) - doc_pos;

    // Use DPI to convert mm line width to logical pixels
    float px_width = (u_line_width / 25.4) * dpi;

    // expand by line width
    doc_pos += normalize(doc_normal) * px_width;

    // finally, map the remainder of the way to normalized device
    // coordinates.
    gl_Position = $doc_to_render(a_position);
}

In this case, we need to access the transforms independently, so get_full_transform() is not useful here:

def draw(tr_sys):
    # Send two parts of the full transform separately
    self.program['visual_to_doc'] = tr_sys.visual_to_doc.shader_map()
    doc_to_render = (tr_sys.framebuffer_transform *
                     tr_sys.document_transform)
    self.program['visual_to_doc'] = doc_to_render.shader_map()

    self.program['u_line_width'] = self.line_width
    self.program['u_dpi'] = tr_sys.dpi
    self.program['a_position'] = self.vertex_buffer
    self.program['a_normal'] = self.normal_buffer
    self.program.draw('triangles')
  1. Draw a triangle with antialiasing at the edge.

  2. Using inverse transforms in the fragment shader

property canvas

The Canvas being drawn to.

property canvas_transform

Transform mapping from canvas coordinate frame to framebuffer coordinate frame.

configure(viewport=None, fbo_size=None, fbo_rect=None, canvas=None)

Automatically configure the TransformSystem:

  • canvas_transform maps from the Canvas logical pixel coordinate system to the framebuffer coordinate system, taking into account the logical/physical pixel scale factor, current FBO position, and y-axis inversion.

  • framebuffer_transform maps from the current GL viewport on the framebuffer coordinate system to clip coordinates (-1 to 1).

Parameters
viewporttuple or None

The GL viewport rectangle (x, y, w, h). If None, then it is assumed to cover the entire canvas.

fbo_sizetuple or None

The size of the active FBO. If None, then it is assumed to have the same size as the canvas’s framebuffer.

fbo_recttuple or None

The position and size (x, y, w, h) of the FBO in the coordinate system of the canvas’s framebuffer. If None, then the bounds are assumed to cover the entire active framebuffer.

canvasCanvas instance

Optionally set the canvas for this TransformSystem. See the canvas property.

property document_transform

Transform mapping from document coordinate frame to the framebuffer (physical pixel) coordinate frame.

property dpi

Physical resolution of the document coordinate system (dots per inch).

property framebuffer_transform

Transform mapping from pixel coordinate frame to rendering coordinate frame.

get_transform(map_from='visual', map_to='render')

Return a transform mapping between any two coordinate systems.

Parameters
map_fromstr

The starting coordinate system to map from. Must be one of: visual, scene, document, canvas, framebuffer, or render.

map_tostr

The ending coordinate system to map to. Must be one of: visual, scene, document, canvas, framebuffer, or render.

property pixel_scale
property scene_transform

Transform mapping from scene coordinate frame to document coordinate frame.

property visual_transform

Transform mapping from visual local coordinate frame to scene coordinate frame.