Description
Note
This issue is a repost of processing/processing#6175 by @usuallyannoyed, updated and edited for tone and clarity.
Description
The ortho()
function does not behave as expected.
Expected Behavior
ortho(0, width, 0, height)
with a cleared modelview matrix (resetMatrix
) should allow users to draw within the specified bounds (e.g., 0
to width
on the x-axis and 0
to height
on the y-axis).
Current Behavior
Instead, the drawing area behaves incorrectly, requiring adjustments (e.g., translating -height
on the y-axis) to achieve the expected results. This behavior makes it unintuitive for users who expect the specified bounds to align with the visible viewport.
Steps to Reproduce
void setup() {
size(500, 500, P3D);
}
void draw() {
ortho(0, width, 0, height);
resetMatrix(); // clear modelview matrix
fill(255);
// Uncomment the line below to 'fix' the behavior
// translate(0, -height);
beginShape(TRIANGLES);
vertex(640, 640);
vertex(0, 640);
vertex(0, 0);
endShape();
}
Observed Result
The triangle only becomes visible when compensating for the unexpected offset by translating the y-axis.
Expected Result
The triangle should appear within the specified ortho
bounds without additional transformations.
Environment
- Operating System: macOS 13.4
- Processing Version: 4.3.1
Possible Causes / Solutions
The issue appears to stem from the following block of code in the Processing source:
processing4/core/src/processing/opengl/PGraphicsOpenGL.java
Lines 4482 to 4486 in 937f528
Despite the comment saying that The minus sign is needed to invert the Y axis.
, this implementation does not fully invert the y-axis as intended. Instead, it flips the y-axis around zero, resulting in all the y-coordinates being offset into negative space.
Suggested Fix
To invert the y-axis properly in normalized device coordinates (NDC), pre-multiply the projection matrix with a -1
scale for the y-axis. This can be achieved by modifying the projection matrix calculation as follows:
// Add a pre-multiplied -1 y scale to flip the y axis in NDC.
projection.set(x, 0, 0, tx,
0, -y, 0, -ty,
0, 0, z, tz,
0, 0, 0, 1);
This approach ensures the y-axis behaves as expected without requiring additional transformations.
Additional comments
This behavior stems from Processing's choice to have the Y-axis increase downward, consistent with traditional computer graphics. While reasonable, the implementation relies on ad-hoc adjustments throughout the code. Centralizing it at a lower level (NDC or viewport) and exposing it as an optional flag would provide more clarity and flexibility.