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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Unexpected Y-axis translation when using `ortho()` and `resetMatrix()`","articleBody":"\u003e [!NOTE]\r\n\u003e This issue is a repost of https://github.com/processing/processing/issues/6175 by @usuallyannoyed, updated and edited for tone and clarity.\r\n\r\n## Description  \r\nThe `ortho()` function does not behave as expected. \r\n\r\n## Expected Behavior  \r\n`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).  \r\n\r\n## Current Behavior  \r\nInstead, 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.  \r\n\r\n## Steps to Reproduce  \r\n```Processing\r\nvoid setup() {\r\n  size(500, 500, P3D);\r\n}\r\n\r\nvoid draw() {\r\n  ortho(0, width, 0, height);\r\n  resetMatrix(); // clear modelview matrix\r\n  fill(255);\r\n  \r\n  // Uncomment the line below to 'fix' the behavior\r\n  // translate(0, -height); \r\n  \r\n  beginShape(TRIANGLES);\r\n  vertex(640, 640);\r\n  vertex(0, 640);\r\n  vertex(0, 0);\r\n  endShape();\r\n}\r\n```  \r\n\r\n### Observed Result  \r\nThe triangle only becomes visible when compensating for the unexpected offset by translating the y-axis.  \r\n\r\n### Expected Result  \r\nThe triangle should appear within the specified `ortho` bounds without additional transformations.  \r\n\r\n## Environment  \r\n- **Operating System**: macOS 13.4 \r\n- **Processing Version**: 4.3.1  \r\n\r\n## Possible Causes / Solutions  \r\nThe issue appears to stem from the following block of code in the Processing source:  \r\n\r\nhttps://github.com/processing/processing4/blob/937f528c1638fa96bc8fde5a67483dad9de8b5c9/core/src/processing/opengl/PGraphicsOpenGL.java#L4482-L4486\r\n\r\nDespite 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.  \r\n\r\n### Suggested Fix  \r\nTo 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:  \r\n\r\n```java\r\n// Add a pre-multiplied -1 y scale to flip the y axis in NDC.\r\n    projection.set(x,  0, 0, tx,\r\n                   0, -y, 0, -ty,\r\n                   0,  0, z, tz,\r\n                   0,  0, 0,  1);\r\n```  \r\n\r\nThis approach ensures the y-axis behaves as expected without requiring additional transformations.  \r\n\r\n### Additional comments\r\nThis 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.","author":{"url":"https://github.com/SableRaf","@type":"Person","name":"SableRaf"},"datePublished":"2024-12-17T20:59:11.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/891/processing4/issues/891"}