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Freedom251 › Blog › Go

5 Seconds to 30 ms: Rebuilding IDW Heatmaps in Go

By Freedom251 Team · · 5 min read

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In Panth Patel’s case study, profiling—not simply switching languages or rewriting the interpolation—revealed the biggest cost: Base64 PNG encoding. Patel says it consumed about 80% of measured time. Replacing PNG output with JPEG and precomputing inverse-distance-weighting (IDW) weights produced a reported example of 20 images in about 30 ms. That is an author-reported result using fake, random heatmap data, not a general performance guarantee.

Why the original heatmap job was slow

Patel describes an hourly Python cron job that prepared heatmaps in advance. It built a grid of 1 km by 1 km boxes, interpolated device values with IDW and wind effects, colored the grid, then stored the results as Base64-encoded PNG images.

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The account gives two different descriptions of the old runtime: its title and opening frame the work as about five seconds per image, while a later passage says the job used 4 GB of RAM and two CPUs and took 10–20 seconds per image. Those are Patel’s reported figures in different parts of the account, not a single reconciled benchmark.

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Profiling found an unexpected bottleneck

Rather than assuming that IDW math or the programming language was responsible, Patel timed individual functions. He reports that Base64 PNG encoding accounted for about 80% of measured time, or roughly 1–3 ms per image. The lesson is specific but useful: measure the whole output path, including serialization and encoding, before focusing optimization on the algorithm.

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In Patel’s comparison at five requests per second, the reported average was 419 ms for Node.js, 114 ms for Go with Base64 PNG, and 29 ms for Go with JPEG. The figures describe his setup; the account does not establish a reproducible benchmark that can predict another service’s performance.

How the on-demand Go pipeline works

The redesign generates heatmaps when requested rather than preparing them only once an hour. The core backend gathers configuration, device information, and time-series values, then sends a payload to a Go service. Patel describes a pipeline that:

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  1. Maps geographic boundaries and sample positions from longitude and latitude into normalized image coordinates.
  2. Creates a target grid and uses a GeoJSON polygon to filter grid points.
  3. Precomputes the influence weights from known points to target pixels.
  4. Maps interpolated values to color levels and encodes the resulting images.
  5. Applies a geographic mask in the browser.

Reusing spatial weights can reduce repeated work when source and target locations remain the same across time snapshots. It also creates a memory and invalidation trade-off: storage depends on the number of source points and target pixels, and changed locations may require recomputation. Patel does not publish a complete memory profile or a fully reproducible benchmark for this design.

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Why JPEG helped—and what it gives up

JPEG does not carry an alpha channel, so it cannot represent transparent pixels in the way PNG can. Patel’s design handles boundaries with a browser-side mask instead. That is a reasonable option only when client-side masking fits the application and JPEG’s image-quality limitations are acceptable.

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Go’s official image/jpeg package documentation describes JPEG decoding and encoding; its encoder writes baseline JPEG and accepts a configurable quality setting. The documentation lists 75 as the default quality. That package fact does not, by itself, establish the exact quality setting or output trade-offs in Patel’s test.

What the reported load test does—and does not—show

Patel reports 20 images in about 30 ms for the rebuilt example and says past time ranges can be requested. The test’s displayed heatmap values were fake and random, so the result should not be read as evidence of production accuracy or a universal latency target.

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Reported load Go/JPEG average latency Reported failures
5 requests per second 29 ms Not stated
50 requests per second 88 ms Not stated
500 requests per second 496 ms 80 requests per second failed

These are Patel’s case-study figures. The rising latency and failures at the highest listed request rate make the load table a warning against treating the 30 ms example as a capacity promise. The account does not provide an independent accuracy comparison or enough test detail to generalize its throughput.

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Which IDW choices affect the result

IDW estimates a target value by combining nearby observations with influence that falls as distance increases. MapServer’s interpolation documentation describes weights based on inverse distance raised to a configurable power. Its implementation also exposes a search radius and discusses border extension and tile metabuffers. Those details are useful design considerations, but they do not prove that Patel’s Go service uses MapServer’s implementation.

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Patel says his service exposes resolution, distance power, wind power, and wind effect as options. When tuning an implementation, consider the following together rather than copying a default in isolation:

  • Resolution: Finer target grids mean more pixels to calculate and encode. Evaluate whether the resulting detail is justified by the density and quality of source observations.
  • Distance and coordinate system: Decide how distance is measured and how far an observation can influence a target. A geographic coordinate difference is not automatically a suitable ground-distance measure.
  • Power and neighbors: The distance power changes how quickly influence declines; neighbor selection changes which observations contribute. Validate both against the data and intended use.
  • Wind effects: If direction-dependent influence is part of the model, test its effect separately from ordinary distance weighting.
  • Edges and masking: Check how the grid behaves at boundaries and whether browser-side masking produces the intended transparent outline.

A Spatial Workflow IDW tutorial also treats neighbor count, power, distance cutoff, and output cell size as relevant quality choices. Its numerical results come from a separate sample dataset and Python workflow, so they are not recommended settings for Patel’s system.

How to apply the lesson to another heatmap service

The transferable idea is not “JPEG always beats PNG” or “Go makes IDW fast.” It is to measure the full workload, identify the dominant cost, and compare alternatives under representative conditions. A useful evaluation should include:

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  • Whether the output needs alpha transparency, and whether client-side masking is acceptable.
  • End-to-end latency and failure rate at realistic concurrency—not just time for one image or a small batch.
  • Grid resolution relative to source density, along with IDW radius, power, neighbor rules, and edge treatment.
  • Accuracy checks against known or held-out observations, not only visual inspection.
  • CPU and memory consumption, including the cost of storing precomputed weights and refreshing them when geometry changes.

Patel’s case study is most useful as an optimization story: the dominant delay was in image encoding, a stage that could easily be overlooked when the visible feature is spatial interpolation. Its timing figures remain specific to the author’s reported setup and synthetic data.

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GoHeatmapsIDWimage encodingperformance

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