Clarify relationship between transform and registration#11
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Net improvement that was needed. Thanks @maxrjones. A few inline suggestions to tighten wording.
Non-inline items:
spatial:bboxfield description (above the diff) doesn't mention registration. Worth adding: "Interpretation depends onspatial:registration; see that section."- The Python conversion snippet (
list(affine_transform)[:6]) is a half-pixel trap under"node"— rasterio/GDAL are corner-anchored. Either scope the snippet to"pixel"or branch on registration. - Consider keeping one line of the removed "Important considerations": "Misidentifying registration shifts data by half a cell; producers SHOULD set
spatial:registrationexplicitly for non-imagery grids."
| - `a`: pixel width (w-e pixel resolution) | ||
| - `b`: row rotation (typically 0) | ||
| - `c`: x-coordinate of the upper-left corner of the upper-left pixel | ||
| - `c`: x-coordinate of pixel (0, 0) |
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pixel (0, 0) is ambiguous (array element vs. continuous point). Anchor it to the formula's variables:
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| - `c`: x-coordinate of pixel (0, 0) | |
| - `c`: x-coordinate of the point `(col_index, row_index) = (0, 0)` in coordinate space |
| - `d`: column rotation (typically 0) | ||
| - `e`: pixel height (n-s pixel resolution, negative value for north-up images) | ||
| - `f`: y-coordinate of the upper-left corner of the upper-left pixel | ||
| - `f`: y-coordinate of pixel (0, 0) |
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| - `f`: y-coordinate of pixel (0, 0) | |
| - `f`: y-coordinate of the point `(col_index, row_index) = (0, 0)` in coordinate space |
| The meaning of pixel (0, 0) depends on `spatial:registration`: | ||
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| This follows the GDAL geotransform and Python's Affine library convention. | ||
| - **`"pixel"` (default):** Pixel (0, 0) refers to the **top-left corner** of the top-left pixel. The pixel value fills the area from (0, 0) to (1, 1). An N×M image covers the bounds (0, 0) to (N, M). Coefficients `c` and `f` give the corner coordinates. |
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| - **`"pixel"` (default):** Pixel (0, 0) refers to the **top-left corner** of the top-left pixel. The pixel value fills the area from (0, 0) to (1, 1). An N×M image covers the bounds (0, 0) to (N, M). Coefficients `c` and `f` give the corner coordinates. | |
| - **`"pixel"` (default):** The point `(col_index, row_index) = (0, 0)` is the **top-left corner** of the top-left pixel. The pixel value covers the index-space area from (0, 0) to (1, 1). An N×M image covers index-space bounds (0, 0) to (N, M). Coefficients `c` and `f` give the coordinates of that corner. |
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| This follows the GDAL geotransform and Python's Affine library convention. | ||
| - **`"pixel"` (default):** Pixel (0, 0) refers to the **top-left corner** of the top-left pixel. The pixel value fills the area from (0, 0) to (1, 1). An N×M image covers the bounds (0, 0) to (N, M). Coefficients `c` and `f` give the corner coordinates. | ||
| - **`"node"`:** Pixel (0, 0) refers to the **point location** of the top-left pixel. The pixel value is a point measurement at that location. An N×M image covers the bounds (0, 0) to (N-1, M-1). Coefficients `c` and `f` give the point coordinates. If displayed with pixel cells, the top-left corner of the display would be at (-0.5, -0.5) in index space. |
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Clearer to call it a node/center and state both the node extent and the footprint together:
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| - **`"node"`:** Pixel (0, 0) refers to the **point location** of the top-left pixel. The pixel value is a point measurement at that location. An N×M image covers the bounds (0, 0) to (N-1, M-1). Coefficients `c` and `f` give the point coordinates. If displayed with pixel cells, the top-left corner of the display would be at (-0.5, -0.5) in index space. | |
| - **`"node"`:** The point `(col_index, row_index) = (0, 0)` is the **node/center** of the top-left pixel. The pixel value is a point measurement at that location. The N×M grid of nodes spans index-space (0, 0) to (N-1, M-1); the displayed cell footprints extend to (-0.5, -0.5)–(N-0.5, M-0.5). Coefficients `c` and `f` give the coordinates of that node. |
| - Commonly used in images to prevent edge pixels from being cut in half | ||
| - A global grid will touch the edges of the poles without covering their centers | ||
| - Each cell in one registration overlaps quadrants of four cells in the corresponding node-registration | ||
| - An N×M image covers bounds (0, 0) to (N-1, M-1) in index space |
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Pin down what spatial:bbox reports under node registration (this is the common reader trap):
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| - An N×M image covers bounds (0, 0) to (N-1, M-1) in index space | |
| - An N×M image covers bounds (0, 0) to (N-1, M-1) in index space; `spatial:bbox` reports the extent of node coordinates (outermost cell *centers*), not the half-cell-padded footprint |
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The transform section was written before the registration field was added. This PR makes the two sections consistent. This content is aligned with the GeoTIFF raster space model, where the transform origin meaning depends on registration type.